Digital Camera Buying Guide: Part 2
Part 2: Exposure controls (program modes, shutter speed, aperture, white balance, and ISO)
In the first installment of this series, I embarked on a discussion of the process of buying a digital camera. I didn't evaluate specific cameras -- the market changes too fast for that to be useful for long. Instead, I gave you a set of questions that help winnow the field to a few models that are right for the way you shoot.
If you followed along, you settled on a budget and made the big decision about the type of camera you need: SLR or point-and-shoot. After those two cuts, you addressed resolution and hhow much you need for the type of output you hope to create.
With those three questions out of the way, you should be zeroed in on a very specific class of camera and might have already identified some prospective models. Now it's time to evaluate features that will further refine your search.
Digital cameras include lots of features not available on film cameras, from automatic panorama modes to bracketable color parameters. There are times when special features like these can mean the difference between getting or missing the shot. But for your bread-and-butter shots, what really counts are the basics: control of exposure shooting modes, shutter speed, aperture, white balance, and ISO.
It used to be that every camera offered manual controls, and you had to pay a lot of money to get a fully automated model. Nowadays, every camera offers automatic mode, and you have to pay more for manual controls.
Manual controls let you handle difficult lighting situations, such as harsh backlighting, and ensure that image elements are rendered with the proper tone and color. Even if you don't consider yourself an avid photographer, I recommend at least a little manual exposure control so you're equipped to handle a greater range of situations.
Most digital cameras offer several shooting modes with different levels of exposure control. Program Mode is a fully automatic mode that makes the camera perform all exposure decisions. Depending on the quality of your camera, this mode might suffice for 80 percent of your shooting.
Even fully automated cameras usually include Preset Exposure Modes. These special modes force the camera into exposure parameters tailored to specific situations. For example, a Landscape exposure mode will lock focus on infinity and use a smaller aperture for maximum depth of field. A Portrait exposure mode, on the other hand, will try to use as wide an aperture as possible to blur the background and bring more focus to your subject.
These modes don't allow any fine-tuning, but for many tricky situations, the ability to bias the camera's automatic modes can be all you need to get a good shot. (Higher end cameras usually don't have these types of modes, since most high-end users prefer to craft their exposures by hand when shooting in difficult situations.)
For true power, select a camera that offers more than simple preset exposure modes. Priority Modes let you wrest some exposure control back from your camera's automatic mechanism:Aperture Priority Mode lets you select an aperture, leaving the camera to choose the appropriate shutter speed. To evaluate an aperture priority mode, pay particular attention to the range of f-stops available. Not all digital cameras let you pick any of the camera's possible apertures. Shutter Priority Mode works just like aperture priority, but lets you select the shutter speed. In most cases, Priority Modes give you sufficient manual control. But again, for maximum control, you might want to opt for a camera that also provides a full manual control mode. Then you can manually select any combination of shutter speed and aperture. Though you'll probably be able to spend the bulk of your time in the fully automatic Program Mode, you'll still want speedy access to whatever manual controls your camera provides. Ideally, you should be able to change shooting modes through an external control on the camera's body. Some smaller cameras lack external controls, so you must use an internal menu to change modes (see Figure 1). If you tend to shoot in situations that call for quick decisions and changes, your camera's mode controls should be quickly accessible. Figure 1: For changing modes, most cameras offer either a dial or switch on the outside of the camera, or a selection screen within the camera's menuing system. Shutter Speed and Apertures By opening or closing your camera's aperture and changing the speed of its shutter, you can control how much light strikes the image sensor. Your primary goal in selecting a shutter speed/aperture combination is to produce a final image that's neither too bright nor too dark, and that reveals good detail from the shadows through the highlights. By changing one parameter or the other, you can make artistic decisions about an image's content. Altering shutter speed can give you some control over the motion in your image -- letting you blur or stop a fast-moving image, for example -- while aperture selection gives you control over which areas of your image are in focus. Shutter speed and aperture have a reciprocal relationship. In other words, if you change one parameter in one direction -- increasing your shutter speed to freeze a fast moving object, say -- then you need to change the other parameter in the opposite direction and open the aperture to allow more light. Different combinations of shutter speeds and apertures can yield the same exposure; that is, the same amount of light ends up striking the focal plane. Many digital cameras include an automatic reciprocity control. This mechanism lets you automatically cycle through all equivalent (reciprocal) settings for a given exposure. If your camera meters a scene at 1/100th of a second at f16 and you want shallower depth of field, you can simply use your automatic reciprocity control to select a combination that has a wider aperture. Automatic reciprocity can be a reasonable substitute for Priority Modes or full manual override. Even if you want manual modes, keep an eye out for a reciprocity control also. You'll love the flexibility it gives you. Your camera's light meter calculates a shutter speed and aperture combination that will yield an evenly exposed, well-lit image. However, there are times when evenly exposed is not the best choice for an image -- you may opt for over- or under-exposure to properly render certain elements. If you have manual controls, you can use your base metering as a starting point and calculate an over-exposure by hand, adjusting your shutter speed or aperture appropriately. A much easier method is to use your camera's Exposure Compensation controls to tell the camera to over- or under-expose. Even the tiniest point-and-shoots now include exposure compensation controls. These simple mechanisms let you elect to change the exposure by up to two stops, usually in half or one-third stop increments. The advantage of exposure compensation controls is that you don't have to think in terms of specific shutter speeds and apertures. Instead, you can simply think about relative exposure changes -- overexposing by a stop, for example. Because it's such a powerful exposure tool, look for easy access to exposure compensation controls. I prefer an exposure compensation control that's on the camera's body, with a readout on the LCD screen or camera's viewfinder. If a camera provides exposure compensation through a menu, be sure it's only one or two button presses away. Some cameras have external exposure compensation controls that you access by pressing a combination of buttons. If this is the case on a camera you're considering, get yourself to a brick-and-mortar store and test that you can hit the combination comfortably, without losing your grip on the camera. Exposure compensation and automatic reciprocity can often take care of your exposure control needs. With them, you can specify over- or under-exposure, or configure a particular exposure parameter to your liking. Ideally, you want these controls accessible while looking through (or at) your camera's viewfinder. ISO In a film camera, you have to make certain decisions when selecting a type of film. First, you need to figure out how sensitive, or "fast," your film should be. Faster film is more sensitive to light and therefore needn't be exposed for as long. Faster films let you shoot in lower light and afford different exposure options when shooting in brighter light. The downside is that as film speed increases, so does graininess. Film speed is measured using the ISO scale -- higher numbers means faster film, which translates into greater sensitivity, and usually more grain. One of the limitations to shooting with film is that once you select a particular speed, you're stuck with that speed for the entire roll. Digital cameras also have a sensitivity to light that's rated using the ISO scale. Most cameras default to an ISO rating around 100, but you can change the ISO of a digital camera on a shot-by-shot basis, letting you crank up the ISO if you suddenly find yourself in a darker environment, or if you need to shoot with a smaller aperture or faster shutter speed. Because of this ability to adjust ISO on the fly, it effectively works as a third exposure parameter. Pay attention to how accessible the ISO control is. You'll also want to assess how well the camera performs as you increase its ISO. I'll cover this topic more when I get to the installment on evaluating image quality. White Balancing One of the amazing traits of the human eye is that it correctly renders color no matter what type of light you're in. Unfortunately, neither film nor digital cameras can pull off the same feat. When shooting with a digital camera, you have to calibrate the camera for your current lighting situation, a process called white balancing. (Note that if you plan on shooting predominantly in the Raw format, white balance controls aren't a significant concern for you, as you can easily adjust the white balance of your image in your raw conversion software.) These days, most cameras provide very good automatic white balancing. However, these mechanisms can be fooled by unusual subjects or lighting situations -- for example, a band wearing green costumes marching on a green field. Many cameras include preset white balance options, such as Sun, Tungsten (indoor), Cloudy, or Fluorescent. These can handle some tricky situations. Many digital cameras also offer white balance fine-tuning so you can make minor adjustments to the camera's preset white balance modes. For the ultimate in control, I recommend a camera with manual white balance, which lets you create a white balance setting that's tailored for your particular situation. Because white balance is based on your current lighting situation -- something that's usually stable throughout a shoot -- you don't need the same quick, fingertip access to white balance that you want for exposure compensation or other exposure parameters. Just make sure the white balance settings aren't buried too deep in the camera's interface. As with ISO, you'll want to evaluate how a camera's white balance affects your final image, something I'll discuss in a later installment of this buying guide. Congratulations! You're Well On Your Way At this point, having made decisions about price, type of camera, resolution, and exposure controls, you've made it past the most critical camera buying decisions. You're now ready to evaluate the cameras' secondary features, a process I'll cover next week.
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