Articles & Tutorials
Depth of Field and Compact Cameras
7/06/10The almost universal response and indeed the photography marketing response is that the DSLR wins, with most citing far better image quality and lower image noise as the core benefits.
Now I have many cameras of all types so I have no vested interest in picking a winner as such, I feel all of them are perfect for certain jobs and not for others, but I would like to perhaps do a bit of debunking on some of the issues just to help put things in better perspective for readers. Please note, what I am discussing here is taken from a real world shooting viewpoint rather than a lab measurement perspective.
It is true that the great majority of professional photographers use DSLR’s on most jobs, what is not true is that the DSLR is the best tool for the job in all instances. If the pro-shooter were to be really candid they may admit they know how to get great results from compacts but importantly are also a bit worried about creating the professional impression whilst doing so, which of course requires gear that has to look at least a little bit “pro–like”. One of the real problems with compacts is the often slow operation, but in terms of image quality this is of little consequence , but it is certainly a deal killer for many pros.
Funnily enough, a lot of the pros I know shoot much of their personal stuff with compact cameras and often wax lyrical at just how much better the shots can look straight out of the camera and how much less trouble they have with the compact when they just need snaps.
In my case, because I teach photography I will often take all sorts of cameras with me on paying jobs, including film, and shoot the same stuff in different formats, basically so I can compare the results for class and students benefits. You know what, for my work the DSLR probably only clearly outshoots the compact about 60% of the time. Now I know that sounds like hearsay but I stand by the statement and I will add that it outshoots film close to 100% of the time unless I am either after really fine grained monochrome results or a certain film look.
So what is it that the DSLR really does better when stripped of all the marketing hype and the hysterical rants of those who have spent enormous amounts of money on DSLR systems and now have to justify the expenditure via various internet forums.
First the most important issue as hinted, is speed, there is simply no compact I have ever used that can focus and shoot as rapidly as a DSLR, but by the same token none of my DSLR’s seem to hold a candle to my Canon EOS 1n film camera when it comes to focus accuracy and speed of operation.
Second the DSLR’s have a much better viewing system which is vastly easier to use in bright light, where as, a compacts’ LCD screen becomes quite unreadable, and don’t even talk to me about electronic viewfinders on compacts, so far I have never found one that is even half acceptable, though the new Panasonic G1 sounds promising.
The DSLR also has great options for alternative lenses and these days there are some really tasty optics available for pretty much any purpose.
Under low light the DSLR will generally outperform the compact, providing both lower image noise and better clarity.
The above is a pretty comprehensive list but you’ll note I said nothing about image quality in general and that is because I firmly believe that under good light in real world situations many compacts can actually chew up DSLR images and spit them out, so lets explore why this may occur and when this might happen.
I must preface all of the following by saying that I am referring here to the better compacts of the last couple of years, there is no doubt that compacts of 5 or 6 years ago were generally quite poor performing compared to DSLR’s, but technology has moved on so if you haven’t used a new model compact maybe it is time for a reappraisal.
Depth of field and diffraction are the great levellers in real world photography and can have a large impact on the appearance of final print results. For many images we want shallow depth of field, portraits are a prime example of this, and indeed many amateurs equate a shallow depth of field with the concept of a professional photograph.
Equally there are many times when “great depth of field” is required and I find myself mostly in this camp for the type of ‘non-portraiture’ images that I take for money and personal use.
I would contend it is actually far harder to shoot images with great depth of field successfully than the shallow DOF images because the blurring present in shallow DOF images can hide a multitude of compositional sins and issues. The DOF differences might partially explain why many feel their DSLR images look better, as the extensive DOF of compacts’ lay bare any slackness in terms of composition, framing and attention to picture details.
On the other hand I must mention that many folk are actually going so shallow on the depth of field they are getting photos which are terminally soft and out of focus and then calling it art! Professionally I have little time for this, frankly, calling a portrait with nothing but the eye lashes sharp, a success is utterly daft. I could never sell such a print in good consceince and few clients would find it satisfying once printed to a 16 by 20 and hanging on the wall. I suspect much of the ultra shallow DOF lunacy is really a result of people spending far too much time looking at small “screen resolution” images on the NET, rather than looking at real world prints.
As a side issue, shallow DOF images normally need very low image noise to be fully effective as the noise becomes so much more obvious in the smooth featureless tones of the “out of focus areas” and especially if those tones are darker. This being the case, the low noise results from DSLR’s are clearly superior for shallow DOF images, but your compact basically cannot normally create such shallow DOF images anyway so the slightly higher image noise is not normally the issue you may first think.
In fact, a little image noise is not a bad thing at all in photographs that have a lot of detail, minor noise (especially luminosity noise which is tonal rather than colour based) can increase the impression of texture and fine detail whilst at the same time serving to blend “plastic like” tonal areas in such a way as to make the image look “less digital”. Very low image noise is an advantage when dealing with smooth toned objects, for example clear blue skies, the painted surfaces of motor vehicles and such like, so a commercial photographer shooting glassware, cars, and small glossy objects is unlikely to find true happiness with a compact anytime soon, (although low ISO compact images cleaned of noise in programs like Noise Ninja can be amazingly smooth.) On the other hand gritty subjects with lots of texture and fine detail might just look better with the compacts higher noise.
As a side issue it is actually image noise that makes film look the way it does, all that film grain (noise) does lovely things to the gradation of tones and helps give film images that rather nice look.
DSLR’s should produce the sharpest images when the object being photographed is flat, this is simply because depth of field is not an issue, all you need is enough DOF to cover focus inaccuracies and an aperture that is small enough to render lens aberrations a non- issue. Typically in this situation you could use f5.6 which is bang on the money for optimal sharpness with most top drawer lenses, so if you are shooting ‘flat art’ for example the DSLR should be the weapon of choice, especially considering you don’t want to add any noise to such images.
Trends come and go and many folk will cite the shallow DOF effects as their reason for choosing the DSLR, but in the past many photographers fought against such characteristics and the prevailing large format technology limitations of the day to achieve images that were sharp from foreground to background. One well known group was the California based f64 group of imaging professionals (the two most notable group members being Edward Weston and Ansel Adams). F 64 referring to the use of the small f64 aperture needed to obtain deep “depth of field” results with the large format camera they used.
Shallow DOF is really more of a current trend or fashion than some great fixed photographic ideal and is probably a bit of a reaction to the deep DOF of the compact digicam, in the same way that the f64 groups deep DOF image methods were a reaction to the limitations of the prevailing technology of their time, which with its much larger formats meant most photographic images lacked clarity and reasonable DOF.
If you doubt the above at all, just go to your old family albums and dig a few of those “box brownie” photos, taken on 6 by 9 cm negs, you will find there is little detail in the images that will truly be in sharp focus.
I realise quite well, that lots of photographers will maintain that you can get the exact same results with a DSLR in terms of extended/deep DOF as compact cameras, but with better image quality every time. Many clever folk, smarter than I, are able to quote theories to support these “facts” but my experience is that actual test images and real life images I have taken over many years on many different cameras do not bear witness to these concepts being irrefutable photographic laws.
The compact camera is inherently able to produce images with very extensive DOF. As an example lets say we take a compact and use a field of view equal to 35mm lens on a 35mm full frame camera and then compare the same image when actually shot on a full frame DSLR.
Lets run some numbers.
Using a well-known on-line DOF calculator and a compact digital camera with a 1 1/8 sensor (one of the most common sizes on advanced digicams), we find that when focused at 3 metres using a 7.4 mm focal length, the compact will render sharp detail from 1.1 metres to infinity at f4! The full frame 35mm lens focused at the same distance with the same angle of view will be sharp from 2.7 to 4.4. In fact to get all the way to infinity you will need an aperture of just fractionally under f16, so a difference of 4 stops is needed to get the same basic DOF look. (I must add the look will not be exactly the same as the focal length of the lens has some impact on real world DOF as well and the full frame DSLR is using a far longer lens as standard.)
Now if your DOF needs to remain the same and you were shooting at 1/500sec at f4 on the compact you are now going to need f 16 at 1/30 assuming that you don’t raise the ISO.
Consider that if you choose to handhold the camera, the softening effect of the 1/30sec speed will likely completely negate any DSLR image superiority abilities, unless you are unusually steady.
Of course you may argue that many cameras have image stabilization so you would use this, true enough, but that applies to compacts and DSLR’s. It should be noted however that “IS” usually adds a little of its own blur, it does not fully replace the gains that can be made by keeping the camera steady in the first place. In any case all things being equal both cameras will benefit in much the same way.
If you do raise the ISO then frankly the 100 ISO digicam images will likely cream the DSLR in terms of noise and clarity because you are now going to need to use 1600 iso for true equivalence. But here is the controversial kicker, even if you choose to maintain the ISO and mount the camera on a tripod the results may still tip in favour of the compact in this instance.
Right about now after that statement I can hear a thousand voices screaming abuse at me, how so?
There are many reasons for this.
The first is diffraction, which in simple terms means that due to the light having to pass through a smaller aperture it becomes bent or distorted by the aperture blades, leading to an overall loss of clarity to the image. As the aperture becomes smaller diffraction becomes greater, there is no getting around this, it’s straight physics.
Diffraction is not something that can be fixed through clever lens design, though some lenses are worse than others, it is simply a limit within the optical system and as the format size increases, longer focal length lenses are more likely to become diffraction limited (note 1) before adequate DOF is obtained. Large format cameras shooting 8 by10 or 5 by 4 inch film are hopelessly diffraction limited at regular shooting apertures and need lens tilts and shifts to overcome the limitations while using larger apertures than is ideal for the needed DOF. The ultimate diffraction limited system is large format pinhole cameras, the diffraction effect of the really small aperture being so great it renders a very soft image (without a glass lens of course).
The advanced movements which help compensate for limited DOF in large format cameras have serious limitations for many compositions and they are not at all easy to use properly, rendering the view camera the tool of either the very dedicated or the well paid working professional.
Don’t get me wrong, large format images are a thing of great beauty and the extremely shallow DOF really suits some subjects. BUT....Most of the really sharp shots of landscapes you have seen from large format cameras are only successful due to the subject matter being quite distant, in other words close to the infinity setting. In these cases the apertures were likely quite wide (maybe f11) and possibly combined with a small degree of camera tilt and shift, in these specific instances the image quality will be superb.
Most really well designed lenses perform at their best at around two stops down from wide open, which normally means somewhere between f4 and 5.6. In some cases f 8 may be better but it is rare indeed to find a lens of standard focal length for small or even medium format cameras that is ultimately best at f11 or smaller. F11 is roughly the optimum for large format lenses.
The diffraction limit is determined by the focal length of the lens, and the longer the lens the higher the f-stop before diffraction becomes a lens performance issue. Lets take the example of a 35 mm lens on a full frame DSLR. The limit according to most charts is f16, which you might think, is OK as in the example above that was enough to give almost the same DOF as the compact camera at the equivalent focus point.
But it’s not that simple as digital cameras have turned things on their head quite a bit, let me explain. These days digital capture is far higher quality-wise than film and far less tolerable of variables. Lens manufactures became painfully aware of this, and as the megapixel count grew they found their previously stellar performing lenses for film cameras started to look pretty average resolution wise. Basically with any good DSLR the issues of diffraction really start to show about a stop earlier than was the accepted norm with film, so the real limit with our 35mm lens on the full frame DSLR is more likely f11, (repeated testing I have done on several lenses bear this out), which means you are not going to reach full DOF before diffraction cuts into the potential image quality.
This earlier limit is something I have noticed very clearly with my on my Canon 40D DSLR, even using very good lenses, for example I regularly use a Tamron XR 28 to 75 f2.8 zoom, a cheap but excellent performer and better according to many tests than the Canon L series equivalent. At f4-f5.6 it utterly rocks, but it is easy to see the effects of diffraction at f11, one full stop below the supposed f16 diffraction limit and in fact it is also noticeable at a bit less than f11 on really close inspection. By f16 images are really starting to soften up. At f4 to 5.6 however its performance is streets ahead of the lenses of a few years ago and this probably makes the diffraction issues appear more starkly obvious in contrast, as it does with most other really sharp lenses.
Macro lenses are designed to be used at smaller apertures and combat diffraction a bit better than other lens types, mainly because manufacturers know you need all the DOF you can get once you go close, but even so, I have yet to see a macro lens of say 50mm focal length perform better at f11 than it does at f 8 and by f16 real sharpness at the actual focussed plane is clearly softer.
The diffraction limit for the compact is normally just a bit under f5.6, say f5 and less at wide angle settings, but this is hardly an issue as there is no need to go beyond f4 in normal circumstances. This by the way makes a mockery of the claims of compacts falling apart image wise at high ISOs. Frankly for most purposes all you need is the lowest ISO and a fairly wide aperture, going to a smaller aperture will just make the pictures soft. Lets put it another way, all things being equal your average compact is about as flexible exposure wise at 100 ISO as your DSLR is at 800 to 1600 ISO!
One thing few amateurs and probably a few pros fail to appreciate about compacts is that once the optimum aperture is passed, going to a smaller aperture even by just a 1/2 stop can have a dramatically dulling effect on resolution, in other words you really need to stick to the optimum apertures. I suspect, (actually I know) that a lot of the complaints folk make regarding compact camera resolution are caused by an unnecessary double whammy.
Here is how it works, they put the camera on 400 ISO or so thinking it’s like their old film camera. Using 400 ISO means that under normal daylight conditions they often run out of shutter speeds as most top out at 1/2000 sec but some even less than this, so the camera is forced to raise the aperture to a smaller setting than optimal. So we get, less resolution due to diffraction plus a couple of other problems tossed in.
Why? Using higher ISO’s increases image noise so the camera increases the noise reduction applied to the image, so what you get is lower resolution, increased noise and image smearing noise reduction, its no wonder they end up with rubbish results.
Very high ISO’s on compacts are of course particularly troubling because the cameras are almost always programmed to add very heavy noise reduction, which kills all fine detail stone dead, rendering photos with a very plastic look, hence the contention that compacts are generally poor performers. The sad thing is that all this heavy noise reduction is actually not needed anyway with modern compacts at ISO’s under 800, as in any reasonable print size and viewing distance the noise would likely not be much noticed anyway, so why all the noise reduction? Simply, I feel to make the image look cleaner in a 100% screen view, which in real world use is utterly meaningless, but I digress...
All of the above makes the constant whinging on web forums about lousy high ISO compact performance seem a little stupid to me, as said most of the time you don’t need high ISO’s with compacts, you just need a steadier hold on the camera and 100 ISO. I do speak from experience here as I almost never use any compact on anything other than its minimum ISO or one step up, with the camera well supported using various methods and frankly it has never proved much of an impediment regardless of the shooting conditions. In other words I am saying folks need to learn how to use their gear better instead of whinging about supposed compact camera shortcomings.
Most modern well designed compacts of moderate focal length range have another ace up the sleeve in that their lenses have far greater resolution than the full frame or even APSC frame size DSLR equivalents, and are not nearly as aberration limited at close to wide open, although 5 years ago this was not the case. It’s a simple fact, the smaller the area the lens has to cover the easier it is to make it really perform. Much of this goes out the window with wide range zooms of course, there are normally some pretty serious compromises with 12 times zooms on compacts, but the fact remains some of the 3 and 4 times zooms are amazingly sharp and well corrected right across the entire image area, in fact if I recall correctly the lens on the Fuji F10/11/20/30/31 actually measured by one testing lab as the sharpest lens they ever tested! Yet the F20/30 is a small cheap compact camera of moderate aspirations, I know from experience with these little gems that even wide open the lens is a stellar performer and there is no reason why other compact lenses could not be as good or even better if the manufactures were serious about upping the optical ante.
The main wide aperture limitation with compact lenses tends to be chromatic aberration rather than resolution. Chromatic aberration puts nasty purple colour fringes around high contrast details, stopping down one aperture stop usually reduces it somewhat, but my experience with a number of compacts is that the problem is more likely to be sensor bloom than true CA and thankfully CA in newer compacts is far better controlled than compacts of even a couple of years ago.
Underexposing a little bit often reduces the bloom and CA problem, and whilst you may baulk at this, the fact is most compacts tend to overexpose a bit more than ideal so as to make the images look punchy straight out of the camera and to keep shadow noise low. Slightly underexposed images at the lowest ISO and appropriately post edited can actually end up looking a lot better and the underexposure will also mean your shutter speed is a bit higher which might render the shot a bit sharper....food for thought. Image noise however will also be fractionally increased, but it’s a welcome trade-off in many cases if it kills the bloom and burnt out highlights.
In the film days, film size was the great leveller, there was no way a pocket 110 camera, regardless of how good the lens was, could provide images anywhere near as clear as the 35 mm camera, which in turn was shamed by the medium format gear. The film was pretty much the same in all the camera formats; you just got less of it in the smaller formats, with the silver halides remaining the same size regardless of format. But these days compact digital cameras and DSLR’s have pretty much the same number of pixels so they should resolve much the same amount of actual detail all things being equal. Sure the image from the compact is a bit noisier and maybe the colour fractionally less pure, but it’s certainly not a major or even minor issue at the lowest ISO, and as you may have gathered the real need for high ISO’s in compacts is pretty debatable. In my opinion I would say high ISOs are more likely a marketing tool than an effective imaging solution in much the same way digital zoom was touted as a key feature of older compacts for extra marketing clout.
Many folks, I realise will point out that increasing the number of pixels on a small sensor will increase resolution but make the image very noisy, but once again while the theory sounds well, sound, the actual performance we are seeing in print from the latest crop of 14/15 megapixel compacts does not appear to support those concerns at all.
The mistake that these “computer chair critics” make is that you cannot equate a 100% “on screen” view with the final “on paper” result. Yes the 15 megapixel image probably looks a little noisier on screen at 100% (I am not convinced it always does) but it requires far less magnification for printing so the noise is much less intrusive, and usually not visible at all in regular prints, and helpfully what noise there is often serves to make the images look a little less digital anyway.
Computer chair critics fail to give the sensor developers’ and software engineers credit for all the hard work they have put in and the improvements they are making with image processing technology and also lenses as the pixel counts escalate. If you doubt the veracity of my claim just grab a 6 year old 3 megapixel digicam and a new 12 megapixel one, take the same photos have them properly printed, and have a good look, actually you won’t need to look too hard, the new digicams’ total superiority in all image aspects will likely be utterly obvious even on the computer screen.
One often quoted “fact” of compact resolution is that due to pixel pitch issues once you go beyond about 8 megapixels there should be no increase in real resolution due to theoretical diffraction limits. The theory seems sound and is definitely well supported by the maths, but somehow the scientists must have failed to tell Canon, Sony and all the other compact digicam makers, as their new 15 megapixel compacts definitely resolve more detail than the lower megapixel models of present and past. The tests prove it, so there must be hole a in the theory or application somewhere, don’t ask me why though; I am clearly no physics expert or camera designer whiz.
Even disregarding the issue of DOF many compacts produce images straight out of the camera that are more detailed and sharper than DSLR’s on a “per pixel” basis simply because the anti-alaising filter in the compact is far less intrusive to image quality. The AA filter is an unfortunate trade off with “larger pixel” sensors, the bigger the pixels the stronger the AA filter needed and the more detail that is sacrificed as a result. Some compacts apparently have no anti alaising filter at all!
In theory if you have enough pixels which are really small you would not need the anti alaising filter and then suddenly you would get a big increase in clarity, I predict that some manufactures of DSLRs are working on that very concept at present and within a couple of years the anti alaising filter will be gone and the pixel count somewhere around 30 megs but of course the compact sensors will also get more advanced technology along the way.
The only conventional 35 mm sensor DSLR produced to date without an AA filter was to my knowledge the Kodak 14N, it had a 14 megapixel sensor and for its time was a very high-resolution device indeed. Unfortunately it had some serious shortcomings, the main ones being that diagonal lines exhibited jagged edges and some colours were a bit off, both of which are standard issues if the AA filter is junked, but maybe it was just too far ahead of its time.
Leica have also introduced a rangefinder camera with no AA filter and it definitely produces images of amazing clarity, but there are still some issues with the reproduction of diagonal lines and repeating patterns.
Another more subtle issue at play in images needing extensive DOF is the way in which the image becomes unsharp with distance. The shorter the focal length the more gradual the fade into unsharpness beyond the actual focus point. This effect would be desirable for landscape shots and since the compact camera uses a far shorter focal length lens it may just give a superior result for many of these types of images where you need close and distant objects in sharp focus.
Larger format cameras work very well if there is no need to render close up and distant details at the same time as you can set the focus close to infinity, then you are typically operating in the lenses optimum focus range and are also able to employ the optimum aperture without running into DOF issues. I should also add at this juncture that compacts normally blitz DSLR’s at macro settings unless the DSLR is fitted with a true macro lens, but even then the compact may deliver the better results due to increased DOF, which in the macro world is always at a premium. Most macro shots with DSLR’s have such limited DOF that focus becomes utterly critical making it very hard for the casual user to obtain consistently good images.
One potentially neat aspect with compacts is that the actual focus point does not need to be anywhere near so accurately placed, in most instances with standard or wide focal lengths it will make little difference, so long as the focus is in the ball park. Much of the sprouted superiority of the DSLR in terms of focus accuracy and speed is rather moot all things considered. DSLR focus absolutely has to be fast and accurate if reasonably sharp images are to be consistently obtained, whereas the compact camera has a lot of “sneeze room” making it much easier for the casual shooter.
The compacts lack of need for super accurate focus makes it highly suitable for shooting from the hip so to speak, where for example the focus is fixed and the camera aimed in the general direction of the subject. Usually so long as you are at least able to aim the camera you can get good, if not great results. Some compacts such as many A series Canons can be manually focused and fixed at a certain point (say 2 metres) and will render pretty much anything in focus regardless of distance unless shooting really close up. This makes them ideal for candids at weddings and parties.
A little understood factor in overall camera performance is the need to match the lens performance to the sensors capability, this is an area where DSLR’s are normally outgunned by compacts, compacts are normally very well matched to their lenses, and especially so these days with some even having adaptive processing built into the camera to take into account lens performance issues at differing focal lengths. DSLR’s being designed to work with lenses of all sorts of focal length and optical standards are nowhere near as well matched. Currently the new Panasonic LX3 is probably the best example of a compact with adaptive lens performance processing. Adaptive processing is also available on some DSLR’s but only works with specific lenses.
In the end the compact may well be the better tool where you need to capture images with close and distant details under good light and if you really examined your own images you will find that you probably take many shots that meet this criteria. The superiority in these instances is quite real and as explained is a result of many factors, which in the scheme of things often well outweigh the small compromises on image noise and perhaps lower dynamic range.
Camera manufacturers are well aware of the potential benefits and are now bringing out some compacts that really do capitalise on the compacts strength and promise some amazing potential. The stellar examples are currently the Sony W300, Canon G11 and the Panasonic LX3. These will soon be followed by cameras of probably even better performance from Leica and more particularly Fuji in a few months time.
When a lens is referred to as “diffraction limited” it is often taken to mean that it is diffraction rather than other optical issues that is ultimately limiting the clarity of the image. Such lenses are exceptional performers for in most cases other factors in a lenses performance have a far greater limiting effect on final results, but ultimately if a lens is really good then the final limit on performance will be diffraction. Clearly some lenses are so poor that they won’t produce sharp images at any aperture and the diffraction issues will likely be lost in amongst all the other problems the lens has.