Digital Camera Lenses

4. Lens Standards

Digicam and DSLR Zooms, Sensor formats, Focal length difference

Basic Lenses

Most Digicams and DSLR's come supplied with zoom lenses as standard and these are usually quite good enough for general use. The range of focal lengths and subsequent fields of view they cover vary quite considerably, with digicams generally tending to have zooms that cover a wider focal range than DSLR's.

Digital SLR

The present trend amongst DSLR makers is to provide a 3x times zoom, usually an 18-55mm, which covers a range from moderate wide angle to short telephoto - roughly 27-82mm [equiv] at a LMF of 1.5x - and is a reasonably useful lens to have. It gives decent coverage of fields of view whilst keeping the actual lens size and weight down by using fairly standard maximum apertures of normally, f3.5-f5.6. Keeping the wide angle coverage to a maximum of 28mm [equiv] - 75degrees - is sensible. This is quite a wide field of view but not extremely so, distortions can be kept under reasonable control at minimal cost, and the front element is not too large. Producing lenses with wider fields of view than this incurs penalties in terms of size and cost, larger front elements are involved so size and weight increase, and keeping the distortions under control is more difficult requiring higher quality internal elements. Basic zooms are mostly about providing acceptable image quality and reasonable zoom range at the cheapest possible cost. 

Nikon have often supplied a slightly more useful standard zoom in the shape of a 18-70mm, and others such as Sony are now following suit with similar or wider focal lengths. The extra mm's of focal length at the telephoto end give it a range of 27-105mm [equiv] which makes it quite useful for portraits, the field of view obtained at the 'long' end - 23.5 degrees - being regarded as quite useful for portraiture due to the generally flattering perspective that results, and that for face or head/shoulders shots the photographer will be at a reasonable distance from the sitter. Close up photography using a standard zoom is usually restricted to a minimum distance of about 0.4metre across the whole zoom range, with a 1:4 [quarter life size] magnification reached at the telephoto end. This is a fairly standard magnification ratio that a lot of lenses of all focal lengths are capable of at their minimum focusing distance. 

Of course one of the benefits of owning a DSLR is the ability it gives to change the lens for another should you so desire, and makers always hope to encourage you to purchase additional lenses for your camera - it's where they usually make their money - so standard zoom lenses are never going to be anything more than base average.

Digicam

Whilst there are a few exceptions, most digicams don't have zoom lenses that provide as wide an angle of view as those used with DSLR's. As digicam lenses are an integral part of the camera and can't be changed there is no standard zoom type as such because the lenses are made to suit each particular type of digicam and what it's expected the purchaser requires of it. So on the one hand a very small compact design might just have a 2x times 35-70mm [equiv] zoom, whilst at the other end of the scale, one of the larger 'prosumer' types might well have a 7x times 28-200mm [equiv] or even a 38-380mm [equiv] 10x times zoom. As a rule the shortest focal length most have is in the 34mm to 38mm [equiv] range which can sometimes make it more difficult to frame a shot and get in all you want, although there are one or two that go as wide as 28mm [equiv]. 

This is mainly down to the difficulty of producing wide angle view lenses with large zoom ranges and keeping distortion under control, at reasonable cost, due to the small size of the optics involved. Even the physically much larger SLR/ DSLR 28-200mm [equiv] and 28-300mm [equiv] 'super zooms' are pretty poor when it comes to the matter of distortion control at both ends of the zoom range and image quality is often disappointing as well.

Although digicams often lose out at the wide angle end, this is usually offset by the focal lengths available at the telephoto end. Most go to 105-115mm [equiv], many go to 200mm [equiv], whilst there are a few that go to what are really long lengths, anywhere between 300mm [equiv] and 380/420mm [equiv]. 

Another area where some digicam lenses seem to have an advantage over standard DSLR zooms is in the use of 'macro' modes. This is a greatly misused term in any case, but especially with digicams where the magnifications produced vary greatly from camera to camera, both in the level of magnification produced, and whether it is over the whole zoom range or just at a fixed focal length. Unlike DSLR's there is no general standard here, some only producing the same kind of magnification ratio in macro mode as DSLR lenses can at minimum focused distance. 

There are some however that are capable of quite big magnifications, some on the scale of proper DSLR macro lenses, but there is also then the matter of how this magnification is obtained. Mostly it is by moving the camera closer and closer to the subject. Generally with digicams the magnification is classed as to how close to a subject they can focus. On the surface this seems great but there are practical problems to consider.  As you move the camera closer it over-shadows the subject and will end up eventually blocking out the light falling on it. A 1cm [10mm] closest magnification may seem wonderful, but when the front of a lens is this close to a subject very little light, if any, will be able to fall on it, so how do you get a decent exposure of it?

One area where most digicam lenses do have a distinct advantage is with regard to maximum aperture. Generally, as we have said, standard DSLR zoom lenses have maximums of about f3.5-f4.0 at the wide end and f5.6 at the telephoto. Although some of the very small digicams also have maximum apertures of this size - so the lens is as small as possible in size - most others are between 1and 2 stops faster than a comparable DSLR lens, until the longer focal lengths are reached, i.e. over 200mm [equiv]. For example we have a digicam that has a 34m -102mm [equiv] lens that has maximum apertures of f 2.0 at the wide end and f 2.5 at the telephoto. Several others have 28-200mm [equiv] zooms in the f 2.0/f 2.8 - f 3.5 range. These are fast apertures for zoom lenses and combined with the depth of field these lenses produce at these apertures, it gives a distinct advantage, especially in poor light levels.

In General use

Although digicams and DSLR's have several common features as regards lenses, there are major differences in the way many of these operate, and in their quality, and these can often have a direct impact on the quality of the image obtained.

Zooming

Digicam lenses are zoomed in and out, from wide angle to telephoto and vice versa, using electric motors which are controlled by pressing buttons. This can often be slow and tedious to use, difficult to control, especially if the zoom range is large, while often no indication of the focal length used is given, nor usually of the distance being focused to. By contrast DSLR zoom lenses are mechanical in operation, zooming being controlled by twisting a ring on the lens body. This is quick and easy to do, no power is involved, so no battery drain occurs, there are marks indicating the focal lengths so you have a rough guide as to the focal length in use, and most, but not all, also indicate focusing distance. There are also some prosumer digicams that have mechanical zooms, and the number is growing as the makers try to cut digicam power consumption and increase speed of use for users.

Autofocus

All cameras use some form of lens autofocus these days, it's very much taken for granted as the norm these days, although in photography terms it's fairly new have first arisen in the 1980's. In the past two basic systems have been used, Infrared and Contrast/Phase Detection. Infrared is very reliable and works in all light levels, even total darkness, but is not terribly accurate. It works by throwing out an Infrared beam and measuring how long it takes to return and then tells the camera the distance to focus at. 

Contrast/Phase Detection is highly accurate, but as the name implies needs good light levels and contrast to work best, as it measures the difference in contrast between subject surfaces. It can have difficulties with surfaces that have no discernable difference in contrast. A small amount of image light entering the cameras lens is diverted to a small CCD, which measures it and tells the camera the distance to focus the lens at. Originally only a very small portion of the light from the central area of the image was used, the AF point, but more recent advanced systems now use 'wide area' or multiple point AF, still mostly from a larger area in the middle of an image, to help improve AF speed and accuracy. There are even those that offer a choice of which AF point to use. 

Infrared has mostly found use in film compact camera and digicams which use focusing 'steps' and relies to some extent on depth of field to overcome any discrepancies in focusing accuracy. Contrast Detection has been used in some autofocus film cameras and all digicams, but Phase Detection always been used in SLR's and DSLR's.

The method most cameras use to focus the lens is similar, a electric motor moving the lens elements at the cameras command, but the implementation often differs. Most digicams and DSLR's use motors in the camera body, but there are some, most notably Canon DSLR's, which have motors in the lens instead. 

Manual Focusing

Although autofocus is usually pretty reliable and works well for most of the time, many cameras also offer manual focusing for the  times when it doesn't, and it 'hunts' backwards and forwards over the lens focusing range. The way manual focusing works with DSLR lenses is by turning a focusing ring similar to the zoom ring on the lens body. Again this is mechanical except in the case of the 4/3rds system lenses and most prosumer digicams where the movement of the ring is converted into an electrical signal which is sent to the lens focusing motor. Many digicams also offer manual focusing but this usually differs quite considerably in nature being a way of focusing the lens to a set distance. Some only offer a few distance steps, others many. How well this works depends, it can be a very hit and miss affair, especially if you are unable to ascertain the distance you want to focus to, whilst manual focusing with a DSLR lens is usually as least as accurate as AF mode, and sometimes more so. Being able to 'pre-focus' a lens at a certain subject or distance is a worthwhile benefit and is a technique often used by pro photographers in fast moving situations where a AF system might be caught out by multiple subjects in the same frame or a subject moving very fast diagonally across the frame. Sport is one such area.

Image Stabilization

Because of the difficulties that can occur with getting sharp shots due to camera/ lens shake and low shutter speeds, there are increasing numbers of lenses that feature stabilization. This first appeared in long focal length telephoto lenses which really need high shutter speeds for best results, but has now spread to lenses of all focal lengths. With this a motor in the lens moves the lens elements in an effort to offset lens movement/vibration. This is used by both Canon and Nikon in many of their DSLR lenses and by a few others. Quite a number of digicams also use optical image stabilization as this method is often referred to.

Konica-Minolta tried a different approach by using a system where the image sensor moves to offset camera/lens movement. Originally fitted into their A1/A200 digicams different implementations of the basic idea have now appeared in DSLR's not only from themselves and their successors Sony, but Pentax and Olympus as well. They are known as anti-shake sensors. The advantage is that it works with any lens, so is backwards compatible with older lenses.

It has been proved that when used, image stabilization in one form or another can often allow a shutter speed up to three stops slower to be used then would normally be the case. The only downside is that it uses quite a lot of power, so it is provided as something that can be turned on and off as required. Second guessing when you need to turn it on is not supplied!

Sensor Format Differences

As we explained on page 1 the different sizes of sensors used in digital cameras have a profound effect on lens focal lengths and the subsequent fields of view and depth of field that results. The circular image that lenses produce is cropped to give the rectangular images that we are all familiar with. There are a number of different format ratios that are used in photography the main ones being 3x2, 4x3 and 1x1. 

3x2 is the 35mm format ratio used in most film cameras and nearly all Digital SLR's. 4x3 is the format used by some medium format film cameras, most Digicams, and the 4/3rds DSLR system. 1x1 is the square format used by medium format film cameras such as Hasselblad's and included purely to illustrate the difference in aspect ratios.

The different format ratios that the sensors have also has an effect on the field of view obtained. A lens field of view or angle of coverage is measured across the diameter of the image circle it produces. This means the field of view figure actually refers to the coverage across not the height or width but the diagonal of the image frame. As a result lenses which have identical fields of view produce different looking images in respect of perspective proportions due to the way the formats crop the image. As a result comparison between the different formats is difficult as either the height or width do not correspond. 

On the left is a 4x3 format with a 3x2 format rectangle overlaid. On the right a 3x2 with a 4x3 rectangle overlaid.

To take the same width of shot with a 4x3 ratio camera in the normal 'landscape' mode where you hold the camera horizontally you must be further away than with a 3x2 ratio type. When you do the format covers greater height, which may or may not be a good thing depending on what you are taking a shot of. By contrast to take the same height of image as a 3x2 ratio type with a 4x3 ratio design means less width is covered. Generally speaking the 3x2 format gives slightly greater options as there is a bigger difference is aspect between landscape and portrait orientation.

Lens Focal Length Differences

When you are comparing cameras and the lenses they have, the focal lengths that the lens covers is important, especially with the fixed lens digicams. Although 5-10mm [equiv] focal length difference at the telephoto end of a zoom lens might not mean much in area coverage and image magnification terms, especially once your above 100mm [equiv], at the wide angle end it does and there is quite a difference in the scope of coverage between lenses having  28mm, 34mm or 38mm [equiv] focal lengths. Whilst this can sometimes be overcome simply by moving further back from the subject, this isn't always possible. Although there is always the option of taking several shots and stitching them together later using software this doesn't work in all situations.

Here are some shots taken at three focal lengths, 28mm, 34mm and 38mm [equiv]. These are followed by some composite shots showing the area of coverage for the focal lengths in relation to each other. The whole frame was taken at 28mm [equiv]. The first frame is the subsequent shot at 34mm [equiv], whilst the smallest is the 38mm [equiv].  

As sometimes just producing smaller frames in the centre of an image doesn't always show clearly the differences that exist we have also included two other composite shots where the frames have been offset both to the left and right edges. 

As you can clearly see there is a marked difference between all three focal lengths, but especially between 28mm and 38mm [equiv]. The 28mm [equiv] length producing a much greater coverage. Although these images were shot using a DSLR with a 3x2 format sensor, the differences apply equally within each particular sensor format.