Framed and Exposed: Buying a Digital Camera, Part 3
As with any purchasing decision, buying a digital camera involves balancing lots of competing features and parameters. Your goal is to find a camera that provides the shooting controls and image quality you need for your intended results, in a package that's comfortable to use and easy to work with. And, of course, your final decision has to be something that you can actually afford.
Over the last two installments of this column, you've been working your way through a series of questions aimed at helping you cull the huge field of digital cameras down to a few models that offer the features and performance that you need. If you've been following along, then you've made most of the core decisions that you need to consider, including price, resolution, size, and photographic controls.
At this point, you're more than halfway there. Now, you're ready to look at light meters and lenses, the two remaining essential factors you need to consider.
Whether you prefer to let the camera make lighting decisions for you in its fully automatic mode or prefer to exercise your own judgment by using some form of manual mode, a good light meter is essential to getting a properly exposed image. Though it's possible to get into geeky, technical arguments over the merits of one light metering system or another, the fact is that these days, just about any camera from a reputable manufacturer is going to pack a high-quality, sophisticated light meter. Sure, there are times when it's possible to confuse a meter, and get a poorly exposed image, but in general the light meters on most cameras these days are very good.
Unfortunately, evaluating a light meter's performance can be a little tricky when checking out a camera in a camera store. In general, you're not going to find the types of mixed lighting situations that can hamstring a good light meter. So, to learn about light meter performance, you'll usually have to turn to camera reviews and user reports, all of which are readily available on the Web.
What you can assess when examining a camera is what metering modes are provided. Most cameras include one or two modes, and though you can usually get by just fine with a good matrix meter (sometimes called multi-segment meter, or referred to as segment meter), a couple of extra modes can be handy for dealing with trickier lighting situations.
A matrix meter works by dividing your image into a grid, metering each cell of the grid, and then calculating an average metering that works for the whole scene. Matrix meters are good for just about any situation and the metering algorithms that most vendors employ can handle even difficult mixed-lighting situations.
Nevertheless, there are times when a matrix meter will underexpose the foreground -- for example, if you have a bright window or landscape in the background. For these instances, a centerweight meter -- which works just like a matrix meter, but favors the center cells in your scene -- can help you get a correct exposure that doesn't leave your foreground elements in shadow.
A spot meter is an essential tool for any photographer who wants to ensure that specific parts of the image are exposed in a particular way. For zone-system photography, or any instance where you want to capture as much dynamic range as possible, you'll want to be sure your camera provides a spot meter.
As with many digital camera decisions, you don't have to look for a model that provides all of these options. But, when selecting a camera, it is important to consider which metering options you need for the type of shooting you want to do.
As with all features, you'll want to consider the camera's light meter interface. How complex is it to change metering modes? Meter-mode selection is not something you'll need to do often, and usually not something you'll need to do especially quickly, so this feature can be a few button presses away without compromising your picture-taking capabilities.
Most cameras use a through-the-lens (TTL) metering mechanism. This means that the light meter actually looks through the camera's lens when reading your scene. Though less common these days, some cameras use a light meter that's external to the lens. In rare circumstances, external meters can be confused and deliver inaccurate meter readings. More importantly, though, a TTL meter has the advantage of being able to correctly meter, even if you've attached filters, or special lens attachments to your camera. Though not a deal-breaker, you might want to look into the type of metering interface your camera provides.
Through a Lens, Sharply
When choosing a camera, it's very easy to get hung up on resolution and image processing controls as the final arbiter of image quality. But, if your camera has a lousy lens, all that extra resolution and image-processing power ain't gonna count for much.
To evaluate a lens you need to be able to see some output from the camera. Unfortunately, the LCD on the back of a camera is far too small to accurately judge lens quality (or most potential image-quality problems, actually). To get a more accurate assessment of lens quality you're going to want to view some images on a computer monitor. If your camera store of choice doesn't provide a facility for such evaluation, then consider buying a media card that's compatible with the cameras you want to test. You can take it to your local camera vendor, shoot some images onto it, then spend as much time as you need assessing the results at home.
If you're considering a camera with a point-and-shoot design that has a built-in lens, your lens evaluation task is fairly simple; you get what you pay for. If you've opted for a digital SLR, though, then you're going to need to consider what lens (or lenses) you'll want to initially purchase with your camera. You'll need to make some decisions about what focal length ranges are appropriate for the type of shooting you tend to do -- telephoto for sports and nature photography, wide angle for landscape photography, somewhere in between for general shooting.
Whether you're choosing a camera with a built-in lens, or a removable lens for an SLR, the issues you need to consider when evaluating a lens are the same. In particular, you should consider the following issues:
- How's the general focus and sharpness? Does the lens do a good job of rendering fine detail? Are the corners as sharp as the middle? Check for all of these concerns throughout the camera's zoom range. Often, cameras will have trouble maintaining corner sharpness as you move to a wider angle. Also, shoot with a variety of apertures. Most cameras have a "sweet spot" in their aperture range that yields the best focus. In particular, see how the lens fares with wider (lower-numbered) apertures.
- Does the lens vignette at wide angles? Vignetting is a darkening of the corners and sometimes the edges.
- As you zoom the lens to either extreme, you might see barrel or pincushion distortion, a bowing of vertical lines either in or out. This is usually only a problem on extreme wide-angle lenses. Even if a lens has some slight distortions, they might not be a problem for everyday shooting.
- Does the lens have a trouble with flaring -- colored circles and reflections that usually only occur at wide angles. Flares occur when shooting toward a light source, and are almost impossible to remove later.
There are also some practical lens issues that you'll want to consider:
- Does the lens offer attachments? Many point-and-shoot cameras with built-in lenses support the addition of wide-angle or telephoto attachments that allow you to effectively extend their focal length range. If you think you want this option, then look into how easily these attachments are to add to a lens (a bayonet style mount is much easier to use than a threaded mount), as well as how expensive the attachments are.
- Does the camera have an electronic or manual zoom control? A manual zoom control is always preferable, both for the speed and flexibility that it affords. On smaller cameras, though, there simply isn't room in the unit for a manual-zoom control.
As you learned in an earlier installment of "Framed and Exposed", digital camera image sensors are much smaller than a piece of film. As you change the size of a camera's focal plane, the field of view of any given focal length changes. So, while a 50mm lens on a 35mm camera is roughly equivalent to the field of view of the naked eye, a 50mm lens on a typical digital camera is extremely telephoto.
Most point-and-shoot and mid-size digital cameras have very tiny focal lengths, usually between 8 and 20mm. However, because sensor sizes can vary, there's no simple way of predicting just how wide or telephoto 8 mm might be -- the actual field of view of a given focal length can vary greatly depending on the size of the image sensor. Fortunately, most vendors are very diligent about publishing 35mm equivalencies for a camera's focal length range. These numbers tell you what the equivalent range would be on a 35mm camera.
(For those of you who aren't already familiar with 35mm focal lengths: 50mm is about the same field of view and magnification as the naked eye; lenses longer than 50mm yield more magnification, and therefore a narrower field of view, while lenses shorter than 50mm yield a wider field of view.)
Unfortunately, digital camera vendors have adopted a convention from the video camera market, and taken to demarcating their zoom lenses in terms of a multiplication factor -- a 3x zoom or 4x zoom, for example. This isn't a particularly useful piece of information, because it doesn't actually tell you anything about the particular range of the lens. Does it start very wide and go 3x from there? Or does it start somewhat mid-range, and go 3x from there, into a telephoto range? Some vendors are now placing 35mm equivalencies on the front of the lens barrel, but most are still only including this information in the camera's manual, so you may have to do a little digging to discover the camera's range.
When evaluating a lens you'll need to think about what type of shooting you tend to do. If you're a sports or nature photographer, you're going to want a lens with a lot of telephoto power. For architectural, or landscape work, you might want to favor a wider angle. Most cameras provide a good range for everyday shooting.
Take note of the camera's widest aperture (remember, wider apertures have a lower f-stop number). A lens with that has a really wide maximum aperture affords shooting with shallower depth of field, and enables you to use faster shutter speeds for more motion stopping power.
Autofocus and Digital Zoom
All digital cameras these days come with autofocus mechanisms, and some mechanisms are definitely better than others, so it's worth evaluating a candidate camera's autofocus facility.
Your first question regarding focus is how quickly the camera can achieve and lock focus. If you find the camera spends a lot of time focusing in and out before deciding where the correct focus point is, then there's a good chance that the camera won't be fast enough to capture fleeting moments.
Most cameras focus by measuring the contrast in a scene. Where there's more contrast there's greater sharpness and therefore better focus. While such contrast-detecting autofocus mechanisms have a lot of advantages -- they're not limited by range, and they can work through windows -- they can be tripped up in low-light scenes where there is little contrast. Ideally, you want a camera with a focus-assist lamp -- an extra light that the camera can shine into a scene to create more contrast. If the camera seems to lack such a lamp, then find out if it can use its built-in flash as a focus-assist mechanism.
Many cameras offer multiple autofocus points -- that is, they can calculate focus from different parts of the image, not just the center. These mechanisms make it possible for the camera to differentiate between foreground and background elements and focus accordingly. Try to assess how well the camera evaluates tricky compositions. Also, because no mechanism is perfect, make sure the camera offers a manual override that lets you control which autofocus point will be used.
Evaluating a digital zoom feature is a simple, one-step process: Don't bother. Digital zoom mechanisms work by interpolating an image up to a larger size. However, your image editing software is probably capable of doing a much higher-quality job of this type of interpolation, so it's better to do this sort of cropping and enlarging yourself later.
At this point, you should have greatly reduced the field of candidates to just a handful of cameras. All that's left now is to consider some of the secondary features that might separate one camera from another, and to weigh image quality. We'll be discussing the remaining features in the next installment of "Framed and Exposed."
Read more by Ben Long.
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