Digital Camera Lenses

 Digital SLR Lenses

Most DSLR's made today have sensors that are smaller in size than the 35mm film frame size used in most past film SLR's. As a consequence one of the problems many Digital SLR owners face at some stage, unless they have one of the few models using a 35mm full frame sensor, is that of working out and coming to terms with the changes in lens focal lengths and subsequent fields of view that have accompanied their arrival. This aspect isn't just confined to when existing 35mm lenses are used on them however, because even the new designs specifically made to suit the smaller sensor sizes still use different focal lengths to those of the past. 

We have dealt with the basics of lens design and the reasons why lens focal lengths differ according to the sensor size here  Lens Basics  This doesn't however totally explain the conundrum that many face when they find that they can use the same lens on two different cameras and get different results. Use for example a 50mm lens on a 35mm full frame sensor DSLR and the field of view will be at odds with that if the lens is then used on a small sensored DSLR. The reason this can occur is two-fold. It is firstly a result of the difference in sensor size, but it is also as a historical consequence of how DSLR's first arose, and the requirement that early models used existing lenses.

The first DSLR's to arrive were intended mainly for use by newspapers. They were at the time very experimental in nature and newspapers were just about the only one's who could afford the enormous costs involved, which was considered part of the consequence of the move into desktop publishing which was then taking place. As a result of the experimental nature, for no one really knew at the time the extent to which digital cameras might succeed, they were designed to be used with the existing SLR camera lens mounts and lenses that newspapers used. So even though the sensors were smaller, they were designed around the standard 35mm system. As a direct result the cameras produced shots that were the equivalent to cropping a normal 35mm sized image.

This wasn't considered a particular problem because all cameras crop the circular image that a lens produces to get the rectangular image formats camera users are familiar with, and this more severe cropping overcame a problem that existed, (and still does), with sensor image quality towards the outer field. This is that digital sensors need the image light paths to hit them at right angles to get the best response, and in general most 35mm film lenses are not designed to do this because film doesn't have this requirement. It's not so much of a problem with longer focal lengths, but as the focal lengths shorten, the angle at which the light paths strike the sensor become increasingly oblique, resulting in reduced image quality. This generally shows as lower contrast and less sharpness. Using smaller sensors which cropped the central portion and gave better overall image quality at the expense of a narrower field of view for any particular lens used was considered a reasonable trade off. So the situation arose that a lens used on one camera could produce a different field of view, to that when it was used on another.

This light ray angle requirement for sensors is why almost all digital cameras produce better images at longer focal lengths than they do at the shorter ones despite the technological developments in sensor design. It's not the only reason however, pixel frequency/pattern frequency is another, and that is also tied to the image view, with wide angle lenses producing the wrong mixture of low pixel and high pattern frequency.  Please refer to  Sensors  Another problem is the very large front elements that are involved with wide and ultra-wide angle lenses and the difficulty that can result in using them in anything other than 'flat' light conditions. Avoiding flare is a constant problem, and while its a pain using a prime wide-angle, with a zoom wide-angle its worse because the element can still collect and bounce the flare around inside the lens, even if your using another focal length on the lens. 

A further difficulty is that digital camera sensors are more highly reflective than film and this can have an adverse effect on an image if a poor quality lens is used. If you look inside a camera with the lens removed you'll see all component surfaces inside are dark matt coloured to prevent as much stray light bouncing around as possible when the shutter is open. Reflective light is very degrading to image quality.

So a welcome development has been the arrival of an increasing number of lenses made specifically to suit digital cameras. A distinction needs to be drawn between those lenses made specifically for a certain sensor size, and those which have been modified 'to suit digital cameras'.  Lenses which fall into the latter category are ones which are still designed for use with a 35mm sized sensor but where the light paths have been corrected to fall at a better angle on the digital sensor, and use extra coatings to improve image quality. Lenses built to suit the correct sensor size also of course have these qualities, but the advantage is, or should be, that smaller, lighter lenses result. The focal length quoted will still be the same, but as the lens only has provide an image which covers a smaller area - the size of the smaller sensor - the optics used, and especially the front element, can be made smaller.

However, even if you own a camera that uses dedicated lenses, those matched to the size of the sensor, you still feel at odds as the lenses are marked with the correct focal length for the size of the sensor - as do all compact cameras - which has little or no meaning for the average user used to the 35mm designations and the corresponding angle of view. So the convention has arisen whereby the lens focal lengths are also given as the equivalent to 35mm sizes. Digicams usually have this equivalent marked on them, DSLR lenses don't because they can be used on cameras with different sized sensors.

Most DSLR's using smaller sensors are given as having a focal length multiplier of a certain size in order that it can be worked out roughly what the focal length/fields of view are in 35mm terms, and this is relative to the size of sensor used. Thus Canon's 1D is 1.3x, all Nikon, Pentax and Fuji's 1.5x, Canon's 1.6, whilst Sigma's is 1.7. The 4/3rds sensor is half the area of a 35mm frame so a 2x factor gives an equivalent focal length. The smaller the sensor, the larger the multiplier factor will be. 

Whilst the use of lenses made to suit the sensor size has its advantages, so does the method of using 35mm lenses with smaller sensors. Apart from the obvious one of increasing apparent focal lengths by in-camera 'cropping' of the image produced, it has the added advantage that as the image comes from the best portion of the lens it is less likely to exhibit the common lens faults of barrel and pincushion distortion, and corner shading.  Although corrective lens elements are used to reduce these faults as much as is possible, as lenses produce their best image quality at the centre which progressively reduces towards the outer edges, it follows that any image cropped from a smaller central portion will have higher image levels and lower distortion. It also means that smaller, lighter lenses can be used. For example, a 400mm f5.6 lens becomes the equivalent of a 600mm f5.6 lens when used on a DSLR with a APS 'C' size sensor, yet is lighter, smaller, and considerably cheaper.

Lens focal length comparisons

To help you work out the comparable focal lengths for the different DSLR sensor formats, below are two tables of lens focal lengths and their equivalents at the different multipliers. One for prime lenses, one for zooms. While not every single length is covered we hope there is enough of a spread to cover the vast majority of sizes.

*In all cases the figures are rounded to the nearest whole number.

A very important factor to bear in mind is that depth of field, as we explained on page 1, is related to focal length. So if you have a 28mm focal length lens, and on a 1.5x APS-C sensor camera it thus becomes the equivalent of a 42mm lens in respect of field of view, remember, the depth of field is still that for a 28mm lens over the aperture range and focusing distance. In other words it's greater than a 42mm focal length lens would produce on a 35mm full frame DSLR. This is an especially important point with regards to telephoto lenses. A 400mm focal length might be the equivalent of a 600mm in field of view terms on a 1.5x APS-C DSLR, but it's still a 400mm for depth of field. This is why APS-C sensor DSLR's have such an advantage with DOF over 35mm DSLR's. And 4/3rds DSLR's have an even greater one.  This is all explained on page 1,  Digital Camera Lens basics

Prime Lenses.

Zoom Lenses.

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Camera Lens Multiplication Factors - LMF.

• 1.3x :- Canon 1D series

• 1.5x :- All Pentax, all Fuji, all Konica-Minolta, most Sony, all Nikon DX series

• 1.6x :- Canon D30, D60, 10D, 20D, 30D, 40D, 50D, 300D, 350D, 400D, 450D, 500D.

• 1.7x :- All Sigma.

• 2.0x :- All 4/3rds system cameras.

Lens quality and resolution

Although it has always been the fact that some lenses are better than others, that the best lenses use the best quality optical glass and design and cost more, nowhere has this been more apparent than when a digital SLR is used. Digital sensors are much more demanding in terms of the quality of light that they are supplied with, much more sensitive to small changes in light, and there is a marked difference in the quality of images using an 'ordinary' lens, and one made to the highest standards.

At one time prime lenses were regarded as providers of the best quality images. More recently zooms have become almost universal in use, their image quality and convenience being seen as more than acceptable. Currently however, thanks to the arrival of DSLR's, prime lenses are staging something of a comeback as users re-discover the superior image quality they deliver with digital sensors because of their higher resolving powers.

It is also becoming clear that a high resolution lens is more important than the number of pixels on a sensor, and that as pixel counts rise, so must the resolution of the lenses used in order to make the most of the increase in pixel count. Replacing the lens on your DSLR with one of higher resolution will be more beneficial in terms of the increase in image quality, than changing the DSLR for one with a higher pixel count, and then using the same lens on it. However as DSLR sensor counts rise above the 10mp mark these remarks need some further qualification. 

The first is that increasing the resolving power of a lens can be achieved two ways. One is by using better quality optical design, more expensive materials, better quality elements. Another is by increasing the contrast the lens provides rather than increasing the glass quality. The second method is used with cheaper lenses since it costs less. Whilst either method was acceptable to a degree with past film camera use, using high contrast lenses with digital sensors leads to reduced image quality. Sensors need high resolution, but not high contrast, which can materially reduce the tonal range produced.

The second point is that as sensor counts have risen past that 10mp figure, so it has become apparent that even the best quality lenses made to date struggle to produce the level of resolving power needed. This is not a factor that only affects the smaller sensor DSLR's, for it is the relationship between sensor count and the resulting pixel size in percentage terms that matters here. Not the actual pixel size, but the percentage of space each pixel occupies on the sensor in relation to the image circle the lens produces.

Anyway, the bottom line is that lenses with ever higher resolving powers are required if the increase in sensor resolution is to be of benefit by leading to finer image detail capture. But the conundrum is that using these new very high resolving lenses which the camera makers are now introducing, with older DSLR's with lower resolution sensors of the past, 6/8mp/10mp, tends to lead to reduced image quality. 

It's nothing that is cut and dried, with a distinct separation, but instead a blurred dividing line, and that's part of the problem. You can't say for example that past film design made lenses just don't resolve enough, but that all later digital design lenses do. A recent lens review of a camera makers entry level lenses found that an early digital type just wasn't good enough for the latest 14mp sensor camera to arrive, and if used would result in an image no better than that from a 10mp sensor. We ourselves found back in 2003/2004 with the purchase of our first 6mp DSLR, that a digital designed lens bought specifically to be used with it performed far worse than the best quality film lens designs. Users of Canon's full frame cameras, the 5D and 1Ds series, have discovered that each and every lens type tends to perform differently on the cameras and the only way to find which one's are the best to use is simply one of trial and error.

What the upshot of all this will be is unclear at present, but we would suggest the following. That when you are buying a lens to use with a DSLR you consider very carefully all the points we have raised. If it is an older camera with a relatively low pixel count, be cautious with regard to the latest revised lens designs with high resolution. And if you are intending to buy a new high resolution DSLR, whatever the sensor format, APS-C or Full Frame, you discount the idea of using any current lenses you have with it, older film types or newer digital versions, and make your buying decisions around the premise that you will be purchasing the latest new design lenses to use with it. You may find that your current lenses are good enough, which will then be a bonus, but not the huge disappointment it would be if they weren't, but that you had assumed that they would be.