Digital Camera Lenses

 Choosing DSLR Lenses

One of the benefits of choosing a Digital SLR is the large range of lenses and other accessories you are able to use with it if you want. As far as lenses are concerned size and weight are important factors, and there are substantial differences in both depending on the lenses you choose. Below is a small selection representing just a few of the lenses we have, illustrating the difference in size that can occur. These are actually all older 35mm film design lenses although many of the newer dedicated digital designs aren't really any smaller, and some are quite a bit bigger, which isn't really what you would expect, given the smaller sensor sizes.

There's a small 19mm prime wide angle at the front, standard 35-80mm and 80-200mm zooms behind, a 28-200 zoom in the rear middle, 100mm macro to the left rear and, unmistakably, a large 135-400mm APO telephoto zoom. All the lenses are shown without their hoods and are AF lenses apart from the 19mm wide angle.

As we have stated elsewhere lenses is where camera makers generally make their money. Fast aperture lenses are always more expensive than slower one's, and will also be much bigger and thus heavier, although they will give far higher image quality. Whilst this does not matter too much with the relatively small standard prime lenses, it becomes increasingly important with zooms and long telephoto types. A standard f2.8 28-70mm zoom will cost perhaps 3-4 times that of a f3.5-5.6 28-70mm version, and be probably 2-3 times the size and weight. When you are dealing with the shorter focal lengths, the weight of a camera and lens combination can be very important, especially if you prefer to walk around the place you are shooting, which is we believe, the only real way to do it, especially with a city. Lugging a large and heavy camera and lens around on your shoulder in this case can be very tiring and ultimately counterproductive. 

There are many differences in the way lenses can be designed to work on a camera.  Some of these are minor and mainly cosmetic in nature, others are major and clearly influence how easy the lens is to use and operate on the camera. Here is a shot of the 80-200mm lens and alongside it that of a 28-200mm at both ends of the zoom range. The 80-200 is a lens that has internal zooming. What this means is that the lens stays the same size whatever the focal length it is set to. Now compare this to the 28-200mm. This has internal focusing but external zooming as have most of the lenses made today. Set at 28mm it is fairly small, but zoom the lens to 200 and look what happens, it doubles in size. The 80-200mm is a nice lens to use because you can cradle it in your hand and support it well, even though it is very light. By contrast you can only hold the 28-200 at the base because the barrel moves when you zoom, and the focusing ring is located on the middle zoom sleeve. The reason it has internal focusing is so the focus mount does not revolve, to enable a dedicated petal lens hood to be provided to suit the 28mm end which need all the help it can get with regard to the avoidance of stray light.

Magnification Ratio's

Macro Lenses

Although most lenses don't alter much in length whatever the distance they are focused to, the length generally altering according to the focal length setting used, one that can is the macro. Here are a couple of views showing the difference between the 100mm macro focused at infinity [left] and at it's closest focus distance of 30cm [right] where it provides a reproduction ratio of 1:1 - lifesize. This is the normal way of describing the magnification ratio of a lens, not how close it can focus. The 30cm measurement is from the front end of the lens to the subject being shot.  As you can see there is a substantial difference between the two focused distances as regards the length that the focusing barrel extends to. This is why, as we have already said, they are often called 'long throw' lenses. A 50mm macro lens which can provide a 1:1 ratio has a closest focus distance of 20cm. Increasing the minimum focus distance to a subject by 50% is the big benefit of a 100mm macro.

The description lifesize means that the lens will provide an image on the sensor which is the same size as in real life. In other words on a APS-C sized sensor of the type fitted in the *ist-D we use, which has a size of 23.5mm x 15.7mm that is the size of the subject that will fit on it at the 1:1 ratio. Here is a shot of a steel ruler at 1:1 ratio. As you can see if you look along the bottom edge of it and count the divisions there is just a fraction over 23mm of the ruler showing. 

Although on the DSLR the lens has an equivalent focal length of 150mm in 35mm terms using the 1.5x Lens multiplication factor which is applicable, the maximum reproduction ratio stays the same because the sensor size is smaller than a 35mm frame. However compared to using the lens on a 35mm camera the image produced is still larger, because of course a 35mm frame is 36mm x 24mm so 36mm of the ruler would be captured at the 1:1 ratio. To take the same sized image as with a 35mm camera you need to be further away. 

The use of the smaller sensor sizes and the resultant LMF's has a big benefit for several areas where macro lenses are employed. Medical, Dental, and forensic photography are some. There is often a need to get high magnification shots, but at the same time stay clear of the work being carried out. In the past special and very expensive 200mm macro lenses have been used. Usually these have a maximum aperture of f 4.0 rather than the f 2.8 of the 100mm macro's. Illumination is usually provided by means of a ring-flash. Because of the LMF's and the increased magnifications there now exists the chance of using the cheaper and faster 100mm macro's and get the same magnifications needed.

Magnification with standard lenses

You must be very careful when comparing magnifications ratio's of lenses and Digicam macro modes. Quite often the Digicam macro modes don't actually produce very high magnification ratios despite their apparent close focusing abilities, because of the smaller focal lengths they use. It is not un-common to find that most Digicam macro modes have a maximum reproduction ratio of about 1:4 or a quarter lifesize, about the same as many SLR lenses produce at their closest focused distance. 

As we have said this is quite a common figure to find. The 80-200mm telephoto lens for example has a minimum focusing distance of 1.1metre, and at the 200mm end at this distance has a ratio of 1:4. The 135-400mm APO has a minimum focus of 2 metres and also, at the 400mm end, a 1:4 ratio. This is the basic idea of using a telephoto lens, to be able to get the same sized shot from further away. 

Using Large Telephoto Lenses

One of the problems with using large focal length lenses is holding them steady enough to get a sharp shot. As focal lengths increase so do the size and weight of the lenses. Most camera users don't really need or use lenses with focal lengths much above 200mm [equiv], although there seems a big push from lens and digicam makers to make the 300mm [equiv] length the norm, and as we have said previously there can be problems with camera/lens shake when you do. Many makers now try to help by introducing different methods to offset camera shake with long lenses but the biggest help is understanding where the problems are and what you can do to overcome them, and where the limits lie. 

A guide to comparing Lenses

A camera's lens is a very important part of the image taking process. There are some that would argue that it is the most important part and in some respects it is. The first part of producing an image is capturing it through a lens. Everything from then on is reliant upon the initial quality of image capture. Poor subsequent processing can reduce final image quality, but without a good quality image being taken in the first place no amount of post capture processing, either carried out by the camera, or subsequently using software, is going to turn a poor quality image into a good one.

Obviously, as far as digital cameras are concerned, the sensor used in the camera, and the processing applied by the camera's firmware to the image also have a vital role to play, and in this respect getting a lens to deliver the right kind of information to the sensor is thus all the more important. 

Lens design

When Digital SLR's first arrived on the scene it was not immediately apparent that enhanced lens designs, incorporating extra coatings to counter flare and ghosting and to ensue the light paths hit the sensor at or near 90° [telecentric], were required to get the best image quality from them. Indeed one of the original boasts of then new DSLR cameras were that you could continue to use all your current SLR system lenses with them, making due allowance for the lens LMF factor, which many did not fully understand or appreciate the significance of to begin with. 

So although many users continue to use older lenses on their DSLR's and get good image quality, it has become apparent that this is very much dependant on the optical quality and the type of lens it is. Longer focal length lenses and those built to the best optical standards can produce good quality, whilst those of poor optical quality and most types of wide angle design tend not to.

Recently it has become a concern that the new type of telecentric digital lenses might also have drawbacks as well as advantages as regards lower contrast images, and less accurate auto focusing as a result. Indeed it is becoming clear as time goes by that the quality of the optics used on a DSLR have a much greater impact on final image quality than has been seen in the past on film cameras. 

So the idea that all new digital lens designs produce superior image quality to the original 35mm designs is proving not to be totally accurate, and the truth appears to be that, as in the past, it is the best quality lenses, giving the highest resolution, irrespective of type, that continue to give the best images.

Lens makes

Another point we are going to make at this stage regards buying camera makers lenses versus independents. There are those that will say that sticking with the camera makers own lenses is the best policy as the lenses are made to suit the cameras and will work better. They will also be forwards and backwards compatible with other camera bodies. There is some truth in this especially as regards the silicon chips fitted in the lenses and how well they communicate with the camera body. This can affect metering and auto focus. There has been in the past, and continues to be today, problems with independent lenses sometimes not working properly, and sometimes not at all, with some cameras. Optically it's the case that the better lenses cost more, and this applies whatever source the lens comes from. Build quality is also an issue that raises problems. 

Our advice is, whenever possible, to see and inspect the lens you wish to buy, whether it's from an independent lens maker or a camera maker, and test it on the camera before purchase. It is in our experience the only way of ensuring that the lens will work properly and reliably with your camera. If you buy an independent make of lens be aware that whilst it may work with your current camera there are no guarantees that it will work with a new one in the future. We have in the past tested lenses that would work with one camera model from a maker but not another.

So our advice now is to be very careful about the make of lens you buy for your camera. 

Buying a lens

When you buy a DSLR today you usually have the option of purchasing it as kit which includes a basic lens, or as a body only. Buying a camera with a lens in the form of a kit is a good way to do it if you have never owned a DSLR before because it gives you a camera and lens combination at a cheaper price than it would cost if you bought them separately. It also provides you with a easy to use package on which to base any further lens purchases you might make in the future. The lens will also be a digital type designed to work optimally with the camera's sensor. 

Buying a DSLR as the body only option is useful for those who already have lenses to suit and are just upgrading to another body, or those who want to purchase a different lens, perhaps a higher quality one, to that which comes as part of the kit option. Considering the quality of the standard kit lenses now supplied with current DSLR's, buying body only and purchasing a better quality lens is quite a good idea. The standard 18-50/18-55/18-70mm lenses are made to enable a DSLR to be sold with a basic lens at the cheapest price possible and provide acceptable image quality, no more. Often they use plastic lens mounts instead of metal one's to keep down costs.

Whatever the case, when you want or need to buy a lens to suit a DSLR you have, and depending on what you want to use the lens for, there are a number of points to bear in mind because sometimes lenses don't always deliver what you might be expecting from them, either in terms of image quality, what they are capable of being used for, or in their constructional standards.

First Considerations

When you first think about getting a lens for a camera there are a number of questions you should ask yourself. What do I want to take shots of with it?  What image quality do I need or want? How heavy a lens can I carry around without causing problems? They might seem obvious, but to ignore them is to ask for trouble. You should really only ask yourself how much you are prepared to pay for a lens once you have a good idea what standards you want. It is often said that the maxim 'you get what you pay for' holds true as regards camera lenses, and to a certain extent it does, but that maxim is usually quoted in regard to image quality and lens construction, and whilst important there are other factors that also need to be taken into account. 

For example, it is of little benefit to go down the road of acquiring a big, heavy, expensive lens on the grounds that you want the very best optical quality, if you spend a lot of time engaged on say hill or footpath walking only to find the burden of carrying the camera and lens combination for more than a short distance just too great. Or buying a wide ranging 'superzoom' with a slow maximum aperture if a lot of your photography is carried out in low light levels. Again there is little point buying a lens that exhibits too much distortion if you like shots of buildings. There are also points concerning how a lens works, and what you can or cannot achieve with it, that might not seem obvious on a casual observation of the specifications, and which if you are not careful, you might only discover the significance of once the purchase has been made.

Deciding what type of lens you want

As we have said it's important to have a good idea of the type of lens you want for your camera before you start looking around to see what lens makers have to offer. These are some of the questions you should ask yourself, there are probably others that will be allied to your own particular circumstances.

1. Do I need a general purpose lens, or one to carry out a specific task?

2. If it's for a specific task, do I want a lens made solely for the purpose, or a more general use one that can cover the task, but perhaps not quite as well?

3. When a choice of suitable lenses is available, does absolute image quality or all round usefulness matter most?

4. Does weight and size play a part. Do I need lenses that are not too big or heavy?

5. What kind of light levels do I take shots in. All types, summer and winter, from dawn to dusk, and night time too. Or only in good light on nice days, when the wind isn't blowing or pouring with rain?

All of these questions require choice decisions which help to establish a set of criteria against which you can judge the specifications of lenses you are interested in. The higher the criteria the narrower the choice will be.

For example, say you want to take shots of really small animals and insects, or small electronic assemblies, postage stamps etc or anything that's smaller than perhaps 7cm/3 inches across. They need a good magnification to show up well in a shot and a macro lens is probably what you need. But if the objects are a bit larger and you never shoot anything quite this small, well then a standard prime or zoom lens with a good close minimum focusing distance might be all you need.

To take another example, say you like shooting cityscapes, churches, skyscrapers, old buildings, modern one's etc. Do you want a really good prime wide-angle lens with low distortion, or a more versatile zoom wide-angle with some distortion? How wide do you need a lens to go? Could a more useful standard general purpose wide-angle to telephoto lens cover what you need?

One lesson we have learnt over the years is to look at lens specifications closely and not to make general assumptions based on what we expect a lens should be capable of. With lenses there are optical 'rules' that have to be obeyed. Often overcoming one problem creates another, and many lenses are a compromise between what is desired and what is possible.

Another is not to dismiss cheaper lenses out of hand. Sometimes cheaper lenses can deliver performances that are high in relation to their cost and not that far off very much more expensive options. If you are on a budget and photography is a hobby and not your living, choices like this can be important. It's important even if it is your living. Treat each lens from a lens maker as an individual in it's own right. Don't assume each and every lens from a well known and highly respected source of quality lenses will deliver great image quality and those from a maker of cheap lenses will always be average to poor. We know of several real stinkers produced over the years by renowned sources, and several bargains from cheap makers that outshone all others, irrespective of  price.

Looking at lens specifications

There are a number of points to consider when looking at a lens's specification. Some are quite obvious and most makers will point out or highlight lens specifications they want to promote as a particular lens's advantages. Which leaves you the task of discovering any disadvantages that might exist. And over the years we have discovered quite a few, in one or two cases being so bad they made the lens not worth buying.

So what should you look for?

• Weight:-  This can sometimes, but not always, be a clue as to lens build quality. The better the basic construction, the more elements a lens has, and the faster it's apertures, the heavier it usually is. However as more lenses increasingly use greater amounts of moulded plastic in their construction rather than metal, in order to keep weight down to reasonable levels and reduce manufacturing costs, this is not such a good guide as it has been in the past. But comparing like with like, the better quality lenses are still generally heavier. Some lenses are very heavy. This is okay so long as you understand this and the restrictions this might place on you as to how you handle and carry them. It's important to ensure they have been properly designed not to place undue strain on a camera's lens mount. Heavy lenses should not rely on the camera for support when tripod mounted but have their own tripod mount. No damage will occur to a camera that hangs by it's mount on the back of a lens that has it's own tripod mount. Great strain, and probable damage, can occur to a very heavy lens hung on the front of a camera and reliant on the lens camera mount alone for support.

• Size:-  Lenses that use faster apertures will always be larger overall than those that have slower one's, as will those that have a constant maximum aperture throughout their focal range.  When a lens's size is quoted it is usually given at it's smallest, most compact size. Although a lens's diameter cannot change when it is zoomed and focused, it's length will. Some use internal zooming or focusing which means their length doesn't alter when either zoomed or focused. There are those whose length all but doubles when they are zoomed. A lens which has internal zooming hardly alters in size. A lens with internal focusing can still double in length when zoomed. 

• Maximum Aperture:-  The faster a lens's maximum aperture is the better. Fast apertures mean the viewfinder image is brighter and the autofocus will work better. It also means that it is easier to work in lower light levels. Whether a lens is classed as 'fast' or 'slow' is relative.  Generally it's related to focal length and lens type, prime or zoom. f2.8 is considered fast for zooms in the 28mm to 200mm focal range. For primes in the 24mm to 85mm range f1.4 to f2.0 is fast. Wide angle lenses generally have slower maximum apertures as do long telephoto's. A 600mm f4 lens is classed as fast for it's focal length whereas a 50mm f2.0 lens is looked upon as slow, a 50mm f1.7 average, and a 50mm f1.4 fast. 

• Minimum Focused Distance - How close you can focus a lens from a subject usually determines the magnification factor. How large the object will appear in a shot, how much of the frame it will fill. On average most lenses are capable of giving a maximum reproduction ratio of about 1:4 lifesize. In the case of zoom lenses this will be at their longest focal length. As a general rule the longer the focal length the larger the minimum focusing distance will be. Zoom lenses aren't often capable of focusing down as close as prime lenses of similar focal length. This is important when comparing prime wide angle lenses with the popular wide angle zooms that cover a number of focal lengths. It used to be rare to find a wide angle zoom that can focus to anywhere near the minimum distance that a prime can.  But it is becoming more common these days. Those that do often exhibit far greater levels of optical distortion and this then makes them unsuitable for the close up work that can be undertaken with prime wide angle's. Another danger lies with long telephoto's. Some have very large minimum focus distances, 3 and 5 metres is not uncommon. Which restricts what they can be used for and their usefulness in comparison to a shorter focal length lens that focuses closer. 

• Handling and build quality - Like image quality this is something you can really only investigate by a hands on trial. But there are some things to look out for when you do. Zoom creep is one. Most modern zooms have a zoom movement that uses a form of construction known as helical movement. It's designed to offer the least resistance whichever way the lens is being zoomed. Unfortunately this can have a side effect if the lens front barrel element is heavy in relation to the rest of the lens or the barrel construction is a loose/sloppy fit. The zoom creeps from it's focal length position when either pointed vertically upwards or downwards unless you hold the zoom barrel. This is a common and annoying problem and some lenses are worse than others. And of course it will happen while the camera and lens are hanging around your neck or on your shoulder, and can lead to the lens being knocked. It has led to makers adding a zoom lock so the lens can be locked in position at the shortest focal length. Plastic lens mounts are another point to watch. There are some lenses now that use plastic mounts rather than metal. It is said they are just as good. If you don't often change lenses maybe they are. Having some knowledge of engineering we are far from convinced and it simply seems like a cost cutting measure to us, and indicative of the makers attitude to the lens. If you think you might need to do a bit of manual focusing consider where the focusing ring is and how big it is, how big the zoom ring is, and where it is in relation to it. Some lens designs are quite poor with one or both rings in positions that are not comfortable to use or access. With a well designed lens you should be able to cradle the lens in the hand and at the same time be able to use that hand to zoom and manually focus the lens. If you can't do that consider whether the lens is worth having. A lens that can't be used easily and fluidly, almost without thinking, as you concentrate on taking the shot, isn't in our opinion worth having.

• Lens elements - the number and type of elements a lens has can be indicative of the kind of image quality it will be able to supply. But this has to be considered with the understanding that simple design lenses don't need the number of corrective elements that a complicated design does. Prime lenses - single focal length - are easier to design and make and image quality can be very high as a result. Zoom lenses, and especially those covering a wide range are far more complicated to design and build, and getting good image quality is much harder because there are more optical faults to be overcome and corrected. Often it's the degree to which they are able to be corrected that is important.

Getting Lens information

When you have a rough idea what type of lens you want to buy, the first task is to find as much information as you can on those available from the different makers. Whatever make of camera you have, as well as those lenses made by the camera maker, there will also be others available that might suit your needs better, made by one of the independent lens manufacturers, Sigma, Tamron, Tokina, etc.

This gathering of information is now much easier than it used to be thanks to the internet, and most lens makers have website's from which you can download details and specifications on the lenses they produce. The amount of information provided varies greatly, but It's also quite easy to do a search on a specific lens for details, and find reviews and opinions on any lens you might think would suit your needs. There are plenty of sites that encourage consumers to review products they have bought and share their views on them with others. As always, reviews and comments on products must be treated with a bit of caution, but general trends will soon emerge. The odd faulty product will always be reported and might not reflect the norm, but if something is generally really good, or just the opposite, then there will be many comments supporting that view.

In order to illustrate what you need to look out for we are going to take a look at and compare three DSLR lenses from the same maker, Sigma, that could all be considered as suitable as a standard general purpose lens for a DSLR, all offering the same focal length at the wide angle end, yet with quite different specifications.

This does not involve us coming to a conclusion that one lens is better than the another, because each user has different needs, but rather pointing out the different advantages and drawbacks to look for with each lens and what this means when making comparisons. Nor does it infer that we think Sigma lenses are a better purchase than other independent makes or camera makers own lenses. We don't. Just that they have three lenses that bear easy comparison for the purposes that we require.

The lenses we are going to take a look at are :-

• Sigma 18-50 f3.5/5.6 DC

• Sigma 18-50 f2.8  EX DC

• Sigma 18-200 F3.5/6.3 DC

Most of the basic information on these was obtained from the Sigma UK website. One of the main problems with looking at specification sheets alone is that no idea of image or build quality is obtained so we also read some magazine reviews and did some web searches on these lenses. 

And that is the first basic point to bear in mind. Once you have an idea of the lens you want to buy, testing it first, on your camera, is in our considered opinion, the only way to buy a lens and be sure you are getting what you want. It's the only way to discover lens build and image quality and the way it handles on your camera. Others might say it's a great lens at a good price,  but you could find you don't agree. In these days of web purchase and internet searches to find a product at the cheapest possible selling price, it's easy to overlook the problems that can occur if the product is not what you expected, or is faulty.

Nor is it wise to expect each lens a particular maker produces to be made to the same standards, either optically or in build quality. There are always those that are better or worse than others, either due to cost considerations or design.

Here is a table listing the basic specifications of the three lenses we are going to look at. Have a good look at them and see if there is anything that you feel might make a difference, that doesn't seem right, or looks too good to be true. This is what you should do when you look at any lens specification. 

Here are the diagrams of the Lens Optical configurations. 

and here are shots of the lenses side by side for basic size comparison. All are shown at their smallest size. For the small 18-50 this means set to the 28mm focal length focused to infinity, for the other two, set to 18mm focal length at infinity

                                             18-50 DC                  18-50 EX DC                 18-200 DC

Initial thoughts

Lets start by taking a brief look at each lens individually before trying to make any comparisons.

18-50 DC f3.5/5.6.  

This is a small, light, cheap and compact lens that is quite basic in design. It doesn't have internal focusing or zooming so the inner front element barrel moves when zooming and rotates as it is focused. As a result it uses a circular lens hood which fits on the lens body, not the front element housing. The actual increase in length that results from zooming or focusing is in fact quite small. With the lens hood in place the movement involved in both is contained within it so there is no alteration in the lenses overall size, very neat.  Optically it seems a simple telecentric design using just one aspherical element to help offset distortion with 8 elements in 8 groups. There is no special low dispersion glass. It's use is not warranted since the focal lengths are short and because of the modest maximum aperture's the front element is not overlarge, using a 58mm filter size, so flare risk is quite low even at the widest focal length setting. Despite it's modest specification the lens can focus as close as 25cm over the whole focal range giving a maximum image reproduction at the 50mm focal length of 1:3.5. 

18-50 EX DC f2.8

Although this lens shares the same focal range as the previous one that is all it has in common with it. It is designed as a high performance lens using a constant maximum aperture of f2.8. and is quite a bit bigger and a lot heavier as a result. Optically it uses two aspherical and one special low dispersion elements in it's telecentric design and has 15 elements in 13 groups. Two aspherical elements are used to keep distortion and aberration to the minimum and a SLD element is included in it's design because of the large maximum aperture and resulting large front element, 67mm filter size, and the flare collecting possibilities that result. In order to give the best possible protection against flare internal focusing is used so the front element does not rotate and a dedicated petal shaped lens hood can be employed. As it uses internal focusing it's length only increases as it is zoomed from 18mm to 50mm. It can focus down to 28cm at all focal lengths which is quite good for a fast aperture design, giving a maximum reproduction ratio of 1:5 at the 50mm mark.

18-200 DC f3.5/6.3

This lens covers a wide focal range yet is smaller when set at the 18mm focal length than the previous lens, and slightly lighter, thanks to it's very modest maximum apertures and 62mm filter size. This lens also uses 15 elements in 13 groups and includes two aspherical and two low dispersion elements in the telecentric design. Again internal focusing and a dedicated petal hood is employed in the design to help control flare and the lens is able to focus down to 45cm at all focal lengths. At the 200mm mark at minimum focused distance the lens gives a magnification ratio of 1:4.4. Although the lens uses internal focusing, zooming is external, and the lens size increases by quite a large amount when the lens is zoomed to the 200mm length. The modest 78mm length increases to no less than 152mm, quite an increase.

Making comparison

There are quite a number of differences between these lenses, mainly because they are designed for different types of user. Obviously if you were comparing two  or three similarly specified lenses from different makers the differences might be small and come down purely to a preference for a particular design and style of lens, or price. If there are two or three lenses that all give roughly equal image and build quality, it is often cost, or preference for one individual maker over another, that decides it.

It's more difficult when the maker is the same or lenses are dissimilar. In a sense the choice between the lenses is more stark, and due attention has to be paid to all the lenses specifications to see if there is something that will count for or against them in comparison with the others. So what are the points of difference that might give one lens an advantage or disadvantage over another?

Size and Weight

Obviously size and weight are two factors that stand out. The small 18-50 is light and compact, and even with it's hood doesn't reach the length of the large one before it's zoomed or is fitted with it's large petal hood. The 18-200 is compact until it is zoomed from the 18mm end up to 200mm, then it becomes much larger than either of the others, although still not as heavy as the large 18-50. None of the lenses could be classed as particularly heavy or large for what they are. The drawback with the 18-200 is the size it becomes when zoomed, and the advantage of the small zoom is just that, it's small and light.

Maximum Aperture

It is often said that a fast lens is always better than a slow one and, all other factors being equal, it is true. There are advantages. We have also made the case in other pages on this site, that a fast lens with a short focal range is better than a slower one with a larger range, and again this is generally the case. But a faster aperture on it's own does not guarantee that a lens will produce superior image quality to a slower one. Usually it does, but not always. The main advantage of the f2.8 18-50 is that it will be more usable in lower light levels than the other two. The difference is not so much at the wide angle end, where it has just a half-stop advantage, but at the 50mm length where it is two stops faster. But of course this advantage is at the expense of depth of field which will be much shallower. If you need to use an aperture smaller than f2.8 at the 18mm end, or f4.0 at the 50mm end to ensure depth of field then the only advantage this lens has in aperture terms is in delivery of a brighter viewfinder image.

Minimum focusing distance 

The first point of note is minimum focusing distance at the wide angle end.  A wide angle perspective has two main uses. To provide a wide view of a general scene, a landscape/cityscape or a building perhaps. This is how it is most often used and the focused distance is usually quite large, sometimes perhaps infinity. But it has another use. At minimum focusing distance a wide angle view can be used to provide a close up of a particular subject whilst also including the background, placing the subject in it's surrounding's. To do this effectively, especially with smaller subjects, requires a wide angle lens that focuses to a close distance. Most prime 18mm wide angle's focus down to, usually, about 20cm. 

If you look at the minimum focusing distances you'll see there is only one lens that comes anywhere near this figure. The 18-50 f3.5/5.6 DC at 25cm. Next is the 18-50 f2.8 EX DC at 28cm. The 18-200mm comes in at 45cm. In wide angle terms these differences are huge particularly between the short zooms and the long one.  At a focused distance of 45cm and a focal length of 18mm the magnification is only around 1:15, pretty poor, so only the small 18-50 f3.5/5/6 DC and to a lesser degree the larger 18-50 EX DC f2.8 are really capable of undertaking this particular kind of work. If you never carry out this kind of shot, well then it doesn't matter, but it's one point to bear in mind if you think you might.

Then there's the other end of the focal range, this is also very interesting.

Once again it is the 18-50 f3.5/5.6 DC lens that gives the largest magnification, 1:3.5, simply because the minimum focusing distance - 25cm - is available at all focal lengths. Next is the 18-200mm at 1:4.4. Then the 18-50 f2.8 at 1:5 The fact that the minimum focusing distance is available at all focal lengths for all lenses and that the 18-200 has a greater magnification than the f2.8 18-50 has repercussions for the 18-200mm lens in respect of focal length which we will now look at in greater detail.

When you use a telephoto lens it is with the expectation that you will be able to use the lens from further away than a shorter focal length lens and get the same size of image reproduction. It's the whole point of a telephoto lens. So the fact that the 18-200mm lens can focus down to 45cm when set at it's longest focal length yet only produce a maximum magnification ratio of 1:4.4 means something isn't quite what it seems. Why? Well if most 200mm focal length lenses have minimum focusing distances of usually around 100cm/36inches and yet can produce magnifications of 1:4 at this distance then something just doesn't add up. 

The actual reason is quite simple. The 18-200mm has a  variable maximum focal length. In other words the maximum focal length isn't constant. In actual fact the maximum focal length is dependant on the focused distance and the shorter the distance the shorter the focal length. This is a result of internal focusing and it's the price that has to be paid optically for a lens that is able to cover such a wide focal range and have such a close minimum focusing distance across that range. Tests carried out seem to indicate that the maximum focal length of lenses like this is only available when they are focused at or close to infinity. A recent review of the 18-200 stated that at a focused distance of 50 metres the focal length reduces to 182mm and at 25 metres, 175mm. In other words the focal length progressively reduces in line with the focused distance. 

There was no indication of what the focal length is at the 200mm focal length when focused to the minimum distance, but that's where the magnification ratio gives a clue. At the minimum distance the lens can focus to at the 200mm end, 45cm, the actual focal length is we think, probably around the 125mm mark. As the whole point of buying a lens with telephoto capabilities is to be able to take advantage of the ability to take a shot from further away, and still get the same size image as one take closer with a shorter focal length, this is another aspect to be taken into account when making comparisons. Especially as the maximum aperture stays the same.

Lens elements/image quality

It's impossible to judge image quality for yourself without testing a lens on a camera. But you can get an idea of what to expect from specifications and lens reviews carried out by others. It's clear from the various reports we have read and the lens specifications that the f2.8 18-50 is optically better than the f3.5/5.6 18-50 and 18-200 f3.5/6.3. The only question is by how much, and whether the difference is worth the extra cost. If it consistently produces sharper images across the focal range and aperture settings then it might be worth the extra weight and cost if you value image quality above all else. The 18-200 like the F2.8 18-50 has almost twice as many elements as the small 18-50, but like the f2.8 needs them as it also has more to overcome optically, the f2.8 because of it's aperture size, the 18-200 because of it's focal range. If the f2.8 produces much sharper images with less distortion than the small 18-50 then the choice will come down simply to one of cost. The 18-200 is a slightly different matter. The number of elements are there to help produce good image quality for the focal range, not exceptional, and we doubt the quality is any better than the small 18-50. Here the difference between the others is focal range. 

Lens reviews of these lenses we have since read seem to confirm our initial views and suggest the following; The f2.8 18-50 is optically better with slightly higher resolution and sharpness across the image frame. Distortion is tightly controlled across the focal range with much less distortion at the 18mm setting than the small 18-50. The small 18-50 is quite good for what it is, better it seems than other similar lenses from other makers, both in resolution and distortion, and isn't as far behind the f2.8 18-50 as you might think. The 18-200 produces good resolution for the focal range it covers, and seems roughly on a par with the small 18-50. Distortion is more of a problem and quite severe at the 18mm end in comparison with the small 18-50 let alone the f2.8. 

Handling and build quality

This is something that can only really be assessed by personal inspection. We have seen and handled, but not tested, all three lenses and gained the impression that build quality is decent throughout.  Metal lens mounts are used and zooming and focusing seems smooth and jerk free. The f2.8 is built to a higher standard optically, which is why it has the EX designation, and this needs to be taken into account. 

Conclusions

It's always difficult making decisions on lenses, particularly as these days there are not as many poor designs around as there have been in the past. A lot of this is down to CAD [computer aided design] which also accounts for the fact that so many lens designs from different makers are so similar as regards the focal length, number and type of elements, focal range etc.

The lenses we have featured here are all capable of covering a range of work, depending on what that work is. Our intention here has been to illustrate where to look for differences and what those might mean in image terms and lens usability.

So let's give you our assessment of these lenses.

18-50 DC f3.5/5.6. This is a fair lens for what it is. Image quality is acceptable at small image sizes. A very useful standard lens able to cover most work. It's absolute advantage has to be it's size and weight. Coupled with a small entry level DSLR it produces a combination that's no bigger than several of the prosumer digicams.

18-50 EX DC f2.8. This is a better quality lens. It might cost over three times more than it's smaller brother but most users seem to think it's worth it. If you want better image quality and A4/A3 size prints this is the lens you need. It may not have quite the image quality of the very best and highest resolution lenses it's been tested against, but apparently it's not far off and you do have the versatility of a zoom range that covers basic day to day needs. The tighter control over distortion means it's more suitable for shooting objects with angular lines than the small 18-50, but it is bigger and heavier.

18-200 DC f3.5/6.3. This is the lens to buy if you prize convenience and versatility above all else and just want to take shots wherever you are with the minimum of fuss and bother. It won't give you the highest image quality but it makes up for that by being able to cover such a wide range of general situations you might encounter. A fair description would be 'jack of all trades, master of none'. It would be a great travel and holiday lens, especially in bright conditions, but not perhaps best for any object involving straight lines due to the relatively high level of distortion it produces at all focal lengths. 

There are always decisions to be made when a choice between lenses is being made. 

It's often far from easy, but we  hope what we have written here will be of help when you do.