If you're entering the digital SLR world from the realms of film cameras or non-SLR digital photography, you'll note some significant differences that can only be called quirks. They're idiosyncrasies of the dSLR that you must compensate for or grudgingly put up with. Some might even drive you crazy. The following sections offer some advice for contending with these quirks.
Out, out damned spot: Cleaning the sensor
Every time you remove your dSLR's lens to replace it with another, you could be admitting tiny specks of dust that might, if you're unlucky, find their way past the shutter when it opens for an exposure, and thence onto the sensor. It might take a few weeks or a few months, but eventually, you'll end up with artifacts on your sensor.
You might not even notice this dust because it's most apparent when using the small f-stops that produce the largest range of sharp focus. (I explain why in the section, "The bits that control exposure," earlier in this chapter.) If you take most of your pictures at f/8, f/5.6, or larger, any dust on your sensor might be blurry and almost invisible. In addition, if the dark dust spots happen to fall into dark areas of your image, they're masked.
So, if you own a digital SLR, you can plan on the need to clean your sensor from time to time. It's not particularly difficult, and cleaning kits are available at camera stores and online. Some tips to remember include f>
i If possible, change lenses only in relatively clean environments.
Do not attempt to clean your sensor with canned air, compressed air blowers, lens cleaning liquids, or other methods that seem to make sense but which can damage your sensor. Use only bulb blowers and swabs intended expressly for sensor cleaning.
1 If you have any doubts about your ability to clean your sensor yourself, let your local camera shop or the manufacturer to it.
Multiplication fables: Working around the crop factor
As I mention earlier, many digital SLR cameras use a sensor that's smaller than the 35mm film frame even though they typically use lenses that were originally designed for full-frame cameras. The smaller sensor, in effect, crops out part of the image, so you're using only, say, 75 percent of the area produced, as shown in Figure 2-12. When you use a 100mm lens, say, only the center area is imaged by a smaller sensor, capturing an image with the same field of view as a 150mm lens.
This effect is sometimes called, inaccurately, a multiplication factor or lens multiplier because you can represent the equivalent field of view by multiplying the focal length of the lens by the factor. In truth, no multiplication is involved. A 100mm f/2.8 lens used on a camera with a 1.5 multiplier is still a 100mm f/2.8 lens and provides the exact same image. It's just being cropped to a smaller rectangle by the camera. The correct terminology is crop factor. Common crop factors with today's dSLRs are 1.3X, 1.5X, 1.6X, and 2X. A camera that provides a full-frame image, with no cropping at all, is sometimes said to have a 1X crop factor.
The crop factor affects your picture-taking in two ways:
^ Your telephoto lenses seem to be magically converted to much longer focal lengths. A 200mm telephoto lens becomes a 300mm telephoto lens; a 400mm-long lens is transformed into a 600mm super telephoto lens. Of course, your results would be identical to those you'd get taking the same picture with a full-frame camera and cropping the image, but it's a convenient fable even so.
^ The view of wide-angle lenses is cropped so that they no longer take in as much of your surroundings as they would on a full-frame camera. A nice, super-wide 20mm lens on a camera with a 1.6X crop factor has the same field of view as an ordinary 32mm wide-angle lens. A useful 35mm wide-angle lens view, like the one shown in the upper left in Figure 2-12, becomes the mundane 56mm normal lens perspective, as shown at lower right. The crop factor means that, to get a true wide-angle view, you have to purchase expensive, ridiculously wide lenses, such as the popular 12mm-24mm lenses offered by Nikon, Sigma, and Tokina.
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