This is it: The final installment of Ben Long's guide to selecting a digital camera model. We're down to the pivotal issue of image quality. Here's how to evaluate cameras for color, noise, and other image essentials.
Over the last five installments of this column, you have been presented with a barrage of questions and options related to choosing a digital camera. If you've been following along, you've asked yourself what type of shooting you do, what features you think you want, what you need in terms of size and weight. After answering all of these questions, you should have narrowed down the field of possible candidates to just a handful of cameras. You're now at the very last stage: the final choice.
The good news is that making the final choice may not be too hard, because, in the end, one single question should outweigh all others when selecting a camera: How good are the images it produces? The bad news is that, if you're already favoring a particular camera for its features and design, you may have to nix it and go with a secondary camera that yields better image quality.
Image quality is, of course, a very subjective topic -- one person's hideous noise might be another's beautiful texture. Nevertheless, with a little bit of attention, you can identify problems up front that might vex you later.
In this final "Choosing a camera" installment, we're going to quickly take a look at the most common image-quality issues that you should consider when choosing a camera. In addition to aiding your camera choice, learning to identify these problems now can help in your image-editing efforts later.
Print or Monitor?
Before you can begin comparing image quality, you've gotta have some images to evaluate. A camera LCD is not good enough to evaluate image quality, because color and contrast are rarely accurate on a camera LCD. Camera screens are not intended for accurate output, and they're designed to be visible in very bright sunlight, which means that vendors often have to increase brightness and contrast to improve image quality.
So, if you want to evaluate image quality from a particular camera, you need to get some images out of the camera and back to your computer at home. The easiest way to do this is to buy a memory card that you can take to the camera store. You can then fill it up with images that you can look at back at home. If you're considering cameras that use different formats, then you might need to invest in several cards, but media is cheap right now and you don't need high-capacity cards for these kinds of tests.
Laptop owners have the option of taking their computer to the store and loading images in directly from the camera.
Finally, there are a number of camera review sites that regularly post full-resolution output from their review camera units. These are perfectly suited to evaluation, and usually cover a much greater range of lighting and shooting conditions than you could ever find in a typical camera store.
Obviously, your first evaluations will be performed on your computer monitor, and there's much that you can learn from these assessments. However, when viewing images onscreen, it's also possible to get hung up on issues that may not be a problem in your final output. Simply put: When you can zoom in to look at the individual pixels of an 8-megapixel image, you're likely to find some troublesome artifacts. Whether these artifacts will be visible in an 8-x-10-inch glossy is another question.
So, when evaluating an image, lend a little thought to what your final output will be. If you mostly shoot for the Web, or on-screen viewing, then consider the typical size that you use, and do a round of evaluations at that size.
If you predominantly take images for print, then you'll need a round of evaluation using your typical printing technology, media, and print sizes. Many troubles and artifacts that are visible onscreen may vanish when resized and output using a particular printing technology.
Noise Levels
Though there are a lot of things that can go wrong with an image, the issue you're most likely to notice first when comparing cameras is noise. Noise is roughly analogous to grain in a piece of film and, like grain, it's not necessarily a bad thing, as it can add texture and atmosphere to an image.
However, sometimes digital-camera noise has a more intrusive character than film grain, manifesting as speckly colored patterns rather than an underlying texture. What's more, there will be times when you don't want to have any noise in your images. Because noise can easily be added using an image-editing program, it's better to get a camera with very low noise, so that you'll have the choice of producing clean or noisy images.
The good news is that, these days, most cameras have their noise issues under control. What's more, on a higher-res camera, noise is typically much finer, meaning it will often vanish if you downsize the image.
Noise is almost always more prominent in the shadow parts of an image, and this is the first place to go looking when evaluating an image.
There are two types of noise: chrominance noise and luminance noise. Luminance noise is the preferable of the two, as it looks more like film grain. Where luminance noise is simply a variation in the brightness of the pixels in the image, chrominance noise is a variation in the color -- sometimes an extreme variation. If you see brightly colored blue, red, or even purple pixels, then your camera is exhibiting chrominance noise (see Figure 1).
Again, noise is not a problem that has to be completely avoided, but it's good to know the noise characteristics of a particular camera.
Noise gets worse as you increase the camera's ISO settings, so you'll want to do noise tests across the camera's ISO range. Because ISO is a valuable exposure parameter, you'll want to carefully consider noise levels at higher ISOs. If possible, you might also want to try evaluating some long-exposure images, as noise also increases with exposure time.
Color Troubles
Color reproduction is probably the most subjective of all image quality considerations. What you think of as beautiful, saturated color, someone else might perceive to be garish, velvet-Elvis-painting color. Neither is wrong, of course, so in the end you need to base your consideration on the color you like. As with noise, though, it's still a good idea to know if a particular camera has certain color tendencies. Some color issues to be aware of include:
* Color casts. Some cameras have a tendency to produce images with a particular cast -- an overall color tone that makes the image appear as if it were shot through a colored filter of some kind. Sometimes, the cast will only affect a particular part of the image -- perhaps shadows are too blue, for example.
* Bad white balance. Just as you must select a film that's appropriate for the light you're shooting in, your camera must calibrate its idea of color according to the current light. This process, called white balancing, can sometimes go awry, resulting in images with out-of-whack color. A bad white balance is similar to a color cast, except that white balance problems usually affect the entire color range, not just the shadows or highlights. White balance prowess is something you want to be very conscious of, as removing a bad white balance can be difficult, if not impossible. Most cameras offer a choice of automatic and manual white balances. Because you'll most often use automatic white balance, you'll want to closely evaluate images shot in auto white balance mode.
* Chromatic aberration. All cameras, including film cameras are subject to this weird type of artifact that's sometimes referred to as "purple fringing" (see Figure 2). Sometimes, chromatic aberration is caused by poor lens optics; at other times it's the result of pixels on the camera's image sensor essentially overflowing with light and corrupting neighboring pixels. Chromatic aberrations usually only occur when shooting high-contrast subjects in bright light -- you'll often see it appear along the edges of leaves against a bright sky -- and even then usually only at extreme focal lengths. Chromatic aberration that's readily apparent on-screen is often invisible in print, and it's usually not too difficult to remove, if need be.
* Inaccurate color. This one's a tricky one, because a camera can sometimes yield colors that are dead wrong, but that still look great -- maybe even better than the original. What's more, they might do this consistently. Some cameras, for example, pump up the saturation in an image to produce a "glossy" result. This process can boost the colors into the realm of "wrong" yet yield a more pleasing image. If you're a stickler for accuracy, then you're going to want to evaluate color with this in mind. If you simply want consistently attractive images that require little editing, then accuracy may not be as important.
In the Details
The amount of sharpness and detail in an image is the result of several camera factors: How much resolution your camera has, how good your lens is, and how good your camera's built-in sharpening routines are.
The tricky thing about assessing detail is that you may not know that there's detail missing. In other words, you may think the image looks fine simply because you're not aware of what's there.
The figure below shows the same scene shot with the same camera - a Canon EOS 10D - but using two different lenses (see figure 3). Though neither image looks bad, there's a marked difference in detail.
The easiest way to assess detail when comparing cameras is to examine identical images side-by-side. For this kind of testing, you'll want to go to your camera store and shoot the same scene with all of your candidate cameras. Even if all you can manage is a shot of the cash register, this should still be enough to give you an idea of the differences in detail and sharpness between the different models.
Because in-camera sharpening can greatly affect the quality of an image -- sometimes for the better, sometimes for the worse -- you'll want to take some images with varying degrees of sharpening applied.
Although you may think that more sharpness is inherently better, it is possible to oversharpen an image. To evaluate a camera's sharpening mechanism, look for bright and dark halos around high-contrast areas in an image.
Bad Exposure
You'll use your camera's automatic metering for almost all of your images, so you'll want to get a sense of how well it exposes in difficult lighting situations. Again, sample pictures downloadable from camera review Web sites should ease this problem. Pay particular attention to how well the camera meters in low light, in complex mixed lighting situations (lots of bright and dark in the scene), when shooting with flash, as well as overall consistency of metering.
Most cameras these days have very good light meters, and a skilled photographer can compensate for most difficult metering situations.
Some cameras exhibit lens distortions when shooting at the extreme wide or telephoto ends of their focal ranges. Though these problems are rarely deal-breakers, and can be corrected in post-production, it's worth keeping an eye out for them when evaluating image quality.
Finally, since JPEG compression can seriously degrade the quality of an image, look for JPEG artifacts -- regular, square blocky patterns throughout an image -- when shooting at the camera's highest quality level.
Conclusion
As discussed in the first installment of "Framed and Exposed," most modern digital cameras yield very good images. There are differences, and some might be more to your taste than others, but the good news is that your image evaluations should not be too trying, because you're probably going to see nice results from most cameras.
When considering the image quality issues discussed here, remember to factor in price. Yes, you may not be seeing images that are as good as what your friend gets from his $1,500 digital SLR, but if you're only aiming to spend $500, then you should expect to take a hit in image quality. For this reason, when comparing image quality, be sure to compare comparable cameras. Don't get hung up on how your candidate cameras compare to models that you can't afford.
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