Factors to Consider when Choosing a Digital SLR Camera

by Bob Atkins

photography by Bob Atkins and Hannah Thiem

The newcomer to the world of Digital SLR cameras is presented with a bewildering array of options. It's hard to keep track of exactly who is currently making DSLRs and how many models each have, but as of summer 2008 there were at least 9 manufacturers (Canon, Fuji, Leica, Nikon, Olympus, Panasonic, Pentax, Samsung and Sony) and between them there were something like 34 different models.

How do you choose which one to buy, and in what ways are they different? With so many different cameras available and new models being announced every few months it's not really possible to make specific recommendations on which one is "best". The term "best" will depend on many factors unique to the user. However, I will attempt to outline the various factors that you might want to take into account when deciding which one to purchase.

Lens Compatibility

Bob Atkins

If you already own any SLR lenses, that may influence your decision on which brand of DLSR to buy. In the case of Canon, Nikon, Pentax and Sony (Minolta), their DSLRs are fully compatible with lenses used on their autofocus film based SLRs. The Olympus 4/3 system is new and earlier Olympus lenses require an adapter to mount on Olympus Four-Thirds DSLRs. Most Nikon manual focus lenses will mount on most Nikon DSLRs and most Pentax bayonet mount lenses will mount on Pentax DSLRs, so for those systems there is good compatibility with older lenses. Mounting old FD lenses on Canon DLSRs or old Minolta MD lenses on Minolta DLSRs requires the use of an optical adapter, which lowers image quality, so that route is not really recommended. You can also mount many older manual focus lenses on Canon DLSRs with mechanical adapters, including Nikon, Pentax screw mount, Leica R, Contax and Olympus OM lenses.

If you are considering using 3rd party lenses such as those made by Sigma, Tamron and Tokina, make sure that the lenses you are interested in are available for the camera you are considering. Just about all such 3rd party lenses are available in mounts for Nikon and Canon DLSRs, but not all are available in Sony and Pentax mounts and few are available in the Olympus Four-Thirds mount. So, for example, if you really like the Tamron SP AF200-500/5-6.3 Di lens, you should be aware of the fact that it's not currently available in a Pentax (or Olympus) lens mount, but it is available for Canon, Nikon and Sony DSLRs.

System Expandability

Bob Atkins

The question of system expandability and support comes up if you intend to get really serious about photography and need (and can afford) exotic lenses or very high performance camera bodies. In that case you're certainly better off looking at Canon and Nikon cameras. For example, both have 400mm f/2.8, 500mm f/4 and 600mm f/4 lenses (with stabilization), while Sony, Pentax and Olympus do not. If you intended to eventually pursue photography as a profession, both Nikon and Canon have active professional user groups, which are supported by the manufacturers, while Pentax, Sony and Olympus have a much smaller professional support network. Right now Canon, Nikon, and Sony offer a line of full frame DSLRs. In the future it's possible that Pentax will also do so, but Olympus is committed to the Four-Thirds format. When buying a DLSR, you may need to consider what you'll want in the future as well as what you want today. Of course, if you're an enthusiastic amateur who will never buy an $5000 full frame DSLR or an $8000 600/4 lens, then you don't have to worry about system expandability.

For more information on the individual systems, including camera bodies, lenses, flashes and accessories for the different digital camera brands, take a look at the following articles on Photo.net:

• Canon EOS Digital SLR System
• Nikon Digital SLR System
• Pentax Digital SLR System
• Sony Digital SLR System
• Olympus Digital SLR System

Price

Price is obviously a major factor in any camera purchase decision. Currently prices range from a low of around $400: Nikon D40, 18-55mm kit (review), Canon Digital Rebel XT (review), to a high of almost $8000: Canon EOS 1Ds Mark III (review). One thing to bear in mind is that most photographers end up spending a lot more on lenses than they do on a camera body (and indeed that's the way it should be). It would be silly to purchase a $2000 DSLR and then only use a $150 "kit" zoom lens with it. You should balance your budget between the camera and lenses.

Format Size

There are currently four different "35mm" DSLR formats (there are medium format DSLRs too, but I'm not going to deal with them here):

1. Full Frame-DSLRs with a sensor which is the same size as 35mm film, i.e. 36mm x 24mm. These tend to be higher end cameras, costing somewhere between $2000: Canon EOS 5D (review) and $8000: Canon EOS 1Ds Mark III (review). It's pretty unlikely any of these cameras will be the choice of a first time buyer. Nikon calls their full frame sensor, such as the one used in the Nikon D3, the "FX" format. Sony doesn't have a specific name for their full-frame model's sensor, the A900.
2. APS-H-There aren't many cameras with this format, which corresponds approximately to the H- format of APS film. The Canon EOS 1D MkIII is one, and the sensor is 18.7×28.7mm. The Leica digital module R (for R8 and R9 bodies) had a similarly sized 26.4×17.6 mm sensor.
3. APS-C-DSLRs with a sensor roughly the frame size of the old "APS C-format" film, i.e around 15mm x 22.5mm for Canon and around 15.6mm x 23.7mm for Nikon and the others. Most of the consumer and prosumer DSLRs from manufacturers other than Olympus use an APS-C size sensor. Nikon refers to the APS-C sensor size as their "DX" format.
4. Four-Thirds-Olympus (and Panasonic) chose to design their DLSR systems around the "Four-Thirds" format, which uses a 13.5mm x 18mm sensor. Note that not only is this sensor smaller than the others, but it also has a different aspect ratio (3:4 vs. 2:3).

Hannah Thiem

Which format is best? That's a question which is almost impossible to answer. All else being equal, a larger format will yield a higher quality image, but all is rarely equal. In truth, for most amateur applications, all these formats are capable of yielding excellent images up to at least 11×14", and that's about as large a print as most amateur photographers will ever make.

Since most first-time buyers will be looking at cameras priced under $2000 (and probably under $1000), the choice is really between APS-C and Four-Thirds. Strictly from the point of view of format size, there's not a huge amount of difference here. APS-C is a little larger, which all else being equal might yield a higher image quality, but as I said earlier, all else is rarely equal. From an average user's point of view, there's really not a huge difference between the image quality of these two formats, and the decision on which to buy should be made on other factors than just format size.

Note also that some lenses are designed only to cover the APS-C format, and so if mounted on a full frame camera will result in images with dark corners and low edge quality. Of course the image can be cropped, and indeed the full frame Nikon cameras, Nikon D3 (review) and Nikon D700 (review), have a "DX crop" mode for use when using lenses designed for APS-C format cameras in which only the center part of the sensor corresponding to an APS-C size frame is used. The Canon APS-C format lenses are designated "EF-S" (rather then "EF" which is the designation for full frame lenses). In the Canon system EF-S lenses cannot physically be mounted on any full frame body, which gets around the problem of dark corners in a different (and frankly less useful) way. EF-S lenses would interfere with the mirror mechanism of full frame cameras since they protrude deeper into the camera (by design). Thus EF-S lenses are not usable on Canon full frame (or APS-H) format bodies. Third party APS-C coverage lenses can be used on Canon full frame bodies, and the images manually cropped to to the APS-C format to eliminate vignetting, though you have to "guesstimate" composition since the viewfinder does not have markings to show what will be within an APS-C sized frame.

For more information on this topic, Bob Atkins' article on Size Matters discusses pixel count and sensor size.

Image Stabilization

Bob Atkins

One factor, which may be the biggest difference between basic DSLRs is the image stabilization system used. Both Nikon and Canon decided that they would achieve optical image stabilization by building lenses which have internal gyros and moving optical groups. The gyros sense movement and move the optical group in such a way as to keep it stationary on the sensor. Pentax, Sony and Olympus all decided to go a different route and put the stabilization system in the body. Again, gyros are used to sense camera movement, but in this case image stabilization is achieved by moving the sensor around to compensate for any movement of the image. This is sometimes called "sensor shift" stabilization".

Both systems are quite effective and can stabilize an image to the extent that you can photograph hand held at shutter speeds 2-3 stops slower than you could without stabilization. Some (notably Nikon and Canon) claim that lens-based stabilization is more effective, especially for long lenses. Others really don't see a lot of difference between the two systems, at least not for lenses of 300mm and less. My experience has been that the difference between the two systems for typical zoom lenses isn't very large. The lens-based stabilization is nicer to use because it stabilizes the viewfinder image, which is somewhat reassuring and if you're good you may be able to time your shots based on when the image is moving least. On the other hand, the obvious advantage of a stabilization system in the camera body is that you only have to pay for it once, and it stabilizes every lens you attach to the camera. With the lens-based system you have to pay for it in each lens, and if the lens doesn't have stabilization, then you're out of luck. For example, no fast prime lenses under 200mm from either Nikon or Canon has stabilization. In contrast, any fast prime under (or over) 200mm becomes part of a stabilized system when mounted on a Pentax, Sony or Olympus body with built-in stabilization. If you want a system with a stabilized fast 24, 35, 50 or 85mm prime lens, you'll need to look at body-based stabilization since Nikon and Canon have no such stabilized lenses.

Size and Weight

Size, weight and price tend to go hand-in-hand, with the smallest and lightest cameras also being among the least expensive. If weight and size are both important, then the Olympus four-Thirds DSLRs would lead the pack. For example, the 10MP Olympus Evolt E-420, is 130mm x 91mm x 53mm  and weighs just 440g. However, 10MP APS-C models from Nikon and Canon aren't that much larger or heavier. For example, the Nikon D40x (review), measures 124mm x 94mm x 64mm and weights 522g, while the Canon Digital Rebel XSi (review), measures 126mm  x 98mm  x 65 mm   and weighs 502g.

Whether size and weight are important depends on your application. No DSLR (with lens) is small enough to fit in a jacket pocket, so small differences in size and weight may not be a big issue if you're carrying around a camera bag anyway.

Pixel Count

Hannah Thiem

The pixel count of currently available DSLRs ranges from about 6MP: Nikon D40, 18-55mm kit, $410 (review) to 22MP: Canon EOS 1Ds Mark III, $6550 (review). How many pixels do you need? One way to look at this is to compare the size of the  largest print you can make from each camera. For this calculation I'll assume that the print is made using 240 ppi (pixels per inch), which is generally regarded as a good number to use for a high-quality print. If the image is viewed from up close you might need 300 ppi, or if it's viewed from a slightly greater distance you might not see any real difference at 180 dpi, but 240 ppi is a good number for a high quality print viewed from a normal viewing distance.

As you can see, even a 6MP camera can make an excellent quality 8×10 image, which is actually as large as most people ever print and 10MP is enough give you a very high quality 11×14 print.

It's important to note that these are pretty conservative numbers. Many users report excellent 16×20 prints from 8MP cameras like the Canon EOS 30D, $650 (review), especially if the images are optimized for large printing using software like Adobe Photoshop CS4 [Mac], $597, Genuine Fractals 5, $130, or Nik Sharpener. The bottom line here is that unless you are trying to make a living by selling high quality 24×36 art prints, any of the current DSLRs should be capable of yielding prints, which are large enough and of high enough quality to satisfy the needs of the average photographer. While pixel count is a factor you might certainly want to consider when choosing a DSLR, it's not the only-or even the most important-factor.

ISO Settings

DSLRs currently span the range from ISO 50 to ISO 25,600. If you are interested in low light work you may want to look for a camera which allows a high ISO setting. Most entry level DSLRs offer a range from around ISO 100 to ISO 1600. Mid level DSLRs, such as the Canon EOS 50D (review), now offer a range of ISO 100 to 6400 and some higher end DSLRs, such as the Nikon D3 (review), and Canon EOS 5D Mark II (review), have ISO settings up to 25,600, as well as a low setting of ISO 50, or the Sony Alpha A900 (review), another higher end DSLR has ISO settings from 200-6400 with boost.

High ISO settings come with some problems, such as noise and occasionally some patterning (lines) in the image. Maintaining image quality at very high ISO settings is difficult. While at lower ISO settings (100-800) most cameras yield low noise images, differences start to get greater at higher ISO settings. Noise can be reduced via digital noise reduction techniques, but those techniques can also reduce image detail along with noise. If high ISO operation is important to you, try to look at some sample images from the cameras you are considering and see how they compare. Not all cameras are equal.

Noise

Hannah Thiem

All digital images have noise. Though an imperfect analogy, you can think of it as being like film grain. The lower the ISO setting, the lower the noise, just as lower ISO films have finer grain.  Noise comes from a number of sources including the digital sensor itself and the electronics associated with collecting the (analog) signal from the sensor and converting it into digital form. All else being equal (which it rarely is), larger pixels generate less noise. This can quite clearly be seen when comparing images from DLSRs with those from P&S digicams. Typically the pixels of the DSLR sensor have 10x to 20x the area of the pixels of a small digicam sensor and the DSLR images show much lower noise.

Among DSLRs there isn't a huge difference in pixel size. It ranges from about 5×5 microns (1 micron is 1/1000mm) for cameras like the Olympus Evolt E-520 (review), to around 8×8 microns for cameras like the Canon EOS 5D (review), so the pixel area varies by a factor of about 2.5x or less.

All DSLRs apply some sort of noise filtering and all will yield low noise images at ISO settings up to 800. It's really only at ISO settings above 800 where you may find differences between models. The differences are due both to the intrinsic noise level of the sensor and electronics and the amount of noise reduction applied to the images. The more noise reduction the lower the noise, but the softer the images. If you intend to use high ISO settings, it's worth finding reviews of the cameras you are interested in and looking closely at any comments they make about image quality at settings over ISO 800. If you don't do much high ISO low light photography, you'd be unlikely to notice much difference between DSLRs in normal prints.

Metering

All DSLRs have several metering modes. These normally include some form of muti-segment metering where the image is split up into multiple segments, each one is metered seperately, and the camera uses an internal "smart" algorithm to analyze the data and decide on the optimum exposure. Canon calls this "evaluative" metering, while Nikon calls it "matrix" metering. When it comes to multi segment metering, most systems do a good job whether they have 16 zones like the Pentax K20D (review), or 63 zones like the Nikon D3 (review). While more zones may be theoretically better, the extra zones may only make a difference for a small percentage of shots and/or captures under difficult lighting conditions. In addition, there may be center-weighted and averaging meter modes, as well as partial and spot metering modes.

Center-weighted and averaging modes use a much simpler algorithm than multi-segment modes and most people won't use them very often. They are somewhat more predictable than multi-segment metering and so they may be useful if you are an "old school" photographer who likes to use exposure compensation for scenes where you know a center-weighted or average reading will be wrong. Compensating multi-segment metering can be more difficult since you can never be quite sure what "smart" corrections the camera has already made!

Partial and spot metering take a reading from a small area of the image. The only real difference is that the spot mode takes its reading from a smaller area. Spot metering can be useful when it's critical that one particular part of the image gets the correct exposure, even if other areas have to suffer. A typical partial meter might use the center 8% to 10% of the frame, while a typical spot meter might only use 1% or 2% of the total frame area.

Autofocus

Bob Atkins

Autofocus is a very complex subject and it's not really possible to predict how well an AF system will work by just looking at the specifications. Just about all current DSLRs can achieve good focus on a high contrast static subject in bright light, but they may differ in performance when trying to get a focus lock or tracking focus on a moving low contrast subject in dim light.

DSLRs also vary in the number of AF zones as well as their performance. For example, the Nikon D3 (review), has 51 AF zones, while the Olympus Evolt E-520 (review), has only 3 (when not in LiveView mode). In general, more is better, though for many applications even those cameras with only a few AF and metering zones do a perfectly good job. I'd say that well over 90% of the time I only use the center AF zone myself, since it tends to be the most accurate/fastest on many DSLRs. If I want the subject off-center I focus and recompose. Of course, that's just my personal shooting style. For those photographers who like to select an off-center composition without recomposition, a camera with more AF zones distributed as widely as possible across the image maybe more useful. Note however that most cameras, even those with multiple AF zones, concentrate them around the center of the frame.

AF zones come in at least two types, cross and linear. The cross type are better since they can focus on both horizontal and vertical detail. Linear AF zones can only focus on detail in one direction. Some work on horizontal detail, some work on vertical detail. Most DSLRs use a cross type AF zone in the center of the frame, but some only use linear sensors for the other zones.

On some cameras (e.g. certain Canon EOS models) some of the AF zones may be high precision, which means that with fast lenses (usually f/2.8 or faster), they are capable of higher focus accuracy than with slower lenses.

Continuous Capture Rate and Buffer Size

If you photograph a lot of sports or other action subjects, the continuous shooting rate and buffer size might be important to you. The buffer size determines the number of consecutive shots you can take at the maximum rate before the camera has to slow down. Current DSLRs range between 2.5 frames/sec (fps)-Sony Alpha A350 (review), Nikon D40, 18-55mm kit (review),-to 10 fps-Canon EOS 1Ds Mark III (review). Unless you are shooting action sequences, 2.5 fps is probably fast enough for the needs of most amateurs. Some mid-level cameras offer significantly faster rates without breaking the bank, for example the Canon EOS 40D has 6.5 fps and can shoot up to 75 JPEGs before slowing down.

A large buffer is only really required by those shooting action sequences. Many of the slower frame rate cameras, such as the Sony Alpha A350 (review), and Nikon D40, 18-55mm kit (review), can shoot JPEGs at their maximum frame rate (2.5 fps) until the memory card fills up. However, if you shoot RAW images you may only get a few images before the frame rate slows down. For example, with the Nikon 40D you can shoot around 6 RAW images before the buffer fills, though even after that happens you can keep shooting at a slower rate (just under 2 fps in the case of the D40).

Live View

"Live View" is the name given to the capability of some DSLRs to display a live image on the rear LCD. While all digicams have this feature, not all DSLRs do. The utility of Live View is somewhat questionable in my opinion. It can be useful in some circumstances, and on the few DSLRs with a fold out LCD display it can be useful for high and low level shooting. In general, the optical viewfinder is more useful on DSLRs. There are two focusing methods in the Live View mode. One, used for example on the Canon EOS 40D (review), requires the reflex mirror to drop causing the LCD display to blank out. The normal DSLR AF system is then used to achieve focus, the mirror goes up and the Live View display reappears. Other cameras use a contrast detection scheme based on an analysis of the live image. This may be less accurate than the normal AF system, but doesn't require the reflex mirror to be raised and lowered. Both systems usually result in somewhat slower AF than is possible with normal reflex viewing. On some cameras there's also a significant delay after pressing the shutter before the exposure is taken when in Live View mode. Live View mode also uses significantly more battery power than the normal SLR mode, so battery life is shortened. If Live View is important to you, be sure to check out exactly what capabilities it has or doesn't have on the particular DSLR you are considering.

Video

For the first time, two new DSLRs are also capable of capturing HD video. The Nikon D90 (review), has limited video capability and the Canon EOS 5D Mark II (review), has somewhat more extensive video features. I would expect that in 2009 and beyond, video will become a standard feature on DSLRs, just as Live View is now something that most new DSLRs feature.

Viewfinder Size

Bob Atkins

A larger viewfinder usually gives you a clearer view and is more desirable, but how can you tell how large the viewfinder image is? Most manufacturers give viewfinder size specified by two numbers, coverage and magnification. Coverage is how much of the image the viewfinder shows. Typical consumer DSLRs have an optical viewfinder with around 95% coverage. This is linear coverage not area coverage despite what you might read on some other web sites. It would show 95% of the frame width and 95% of the frame height. Magnification is the magnification you would get with a 50mm lens. If you held the camera up to one eye and kept the other eye open, with a 1x magnification both eyes would see an object at the same size. However, there's another factor involved in determining how large the viewfinder image looks-the format size. Let's look at three cameras:

• Canon EOS 5D (review):   96% coverage, 0.71x magnification
• Canon EOS 40D (review):   95% coverage, 0.95x magnification
• Olympus Evolt E-410:   95% coverage, 0.92x magnification

Which one do you think would have the largest viewfinder image? Would you expect much difference between the 40D (95/.95x) and the E-410 (95/.02x)? It may not be at all obvious from the manufacturer's specifications.

To find the relative viewfinder size you have to multiply the coverage by the magnification and then divide by the digital multiplier factor for the format size. If we do this we get:

• Canon EOS 5D:  (0.96×0.71)/ 1 = 0.682
• Canon EOS 40D:   (0.95×0.95)/1.6 = 0.564
• Olympus E-410:  (0.95×0.92)/2 = 0.437 (approx)

The EOS 5D viewfinder is 21% larger than the EOS 40D viewfinder, which is in turn 29% larger than the Olympus E-410 viewfinder (though note that the Four-Thirds format has a different aspect ration from the APS-C and full frame formats, which means the numbers are approximate). Again these are linear differences (i.e differences in height and width), not differences in area. In fact, the Olympus E-410 viewfinder area is about 41% of that of the EOS 5D. The EOS 40D viewfinder area is 68% of that of the EOS 5D.

Flash Systems

Hannah Thiem

If you intend to do a lot of flash photography you should look at the available flash accessories and multiple flash systems. Some manufacturers, like Nikon and Sony, make DLSRs with a built-in flash, which can act as a wireless controller for an off-camera flash. In the case of Canon, you need a shoe mount flash or flash controller to do this, which means added expense. If you intend to do a lot of macro flash work, both Nikon and Canon offer special flash systems designed for just that purpose.

Other features

Most DSLRs are crammed with features that you may or may not ever use or notice. For example, some use 12-bit A/D converters while others use 14-bit A/D converters. In principle 14-bits are better than 12-bits, but in practice I haven't noticed a significant difference (and I do shoot with both 12-bit and 14-bit A/D cameras).

Most DSLRs have user selectable custom functions, which allow you to do things like select exposure settings or ISO settings in 1 stop, 1/2 stop or 1/3 stop steps, choose the order in which bracketed shots are taken, change the function of the control buttons and dials on the camera, etc. There are far too many such custom functions to go into any detail about them here. Just be sure to check the DSLR you're interested in if there's something important you want to do. Don't assume all cameras have all functions. For example, on the Canon EOS 40D you can set ISO in 1 stop or 1/3 stop steps. On the EOS 5D you can only set the ISO in 1/3 stop steps. Not really a big deal for me, but it could be for you!

The Bottom Line

Bob Atkins

DSLRs are a maturing technology. Just about any DSLR you chose will be capable of yielding excellent images over an ISO 100/200 to 800 range up to a size of 11" x 14". That's all most people will ask of them too, so it's difficult to make a wrong choice if your needs are simple. Bigger differences can be found when you want to move outside the "normal" range and shoot at high ISO settings, or at fast frame rates with AF tracking, or if you want to make prints of 24" x 36" and larger. In that case the decision on which DSLR to chose is more difficult and more important. Specific recommendations are impossible to make and the technology is still changing, so what's "best" today may well be "second best" next week or next month (and certainly next year!). All you can do is carefully read the camera specifications to make sure it has the features you need, and then look for reviews on the web to see what actual users of the camera have to say.

Original text ©2008 Bob Atkins. Photos ©2008 Bob Atkins or Hannah Thiem