The bits that control exposure

You need to understand the controls that adjust the exposure. What's exposure, you ask? A picture's exposure is nothing more than the amount of photons available for capture by the sensor. A good exposure requires exactly the right number of photons captured. Too few, and the image doesn't register at all. Too many photons, and the photo is overexposed.

BE# As you might guess, if a lot of light is bouncing around a scene, a large number of photons can illuminate the sensor in a very brief time. If the light is dimmer, it might take longer for enough photons to reach the sensor. The important detail to remember is that for any given scene, an ideal exposure exists — one that provides just the right number of photos to capture the image.

Your digital SLR's exposure system is designed to improve your chances of getting that ideal exposure, either by adjusting the length of time the sensor is allowed to suck up photons or by modifying the number of photons that reach the sensor in any particular instant. You make these adjustments by using the camera's shutter speed and lens opening/aperture controls, which I describe in the following sections.

Your exposure time machine

Of course, in a digital SLR, the sensor isn't exposed to incoming light all the time. Instead, it sees photons for a brief interval, called the exposure time, usually measured in fractions of a second. The exposure time can extend for many seconds in the case of a time exposure.

The gatekeeper that controls these time slices, a sort of exposure-time machine, is called the shutter. The shutter can be a mechanical device (usually a curtain in front of the sensor that opens and closes very quickly) or an electronic mechanism that activates the sensor for a specific instant of time. Digital SLRs might have both, using a mechanical shutter for exposures measured in seconds from about Mm to 34o of a second and an electronic shutter for exposures in the 34o to >8,000 of a second range.

Longer shutter speeds let in more light but can produce blurring if the subject or camera move during the exposure. Shorter shutter speeds cut down the amount of light admitted, but they also reduce the chance that movement will cause blurriness.

An aperture is a lens opening is an f-stop

You can control photons by using the lens aperture (also called the f-stop). The aperture is the size of the opening through which the photons pass. You can think of an f-stop as a pipe: Larger pipes let more light flow in a given period of time, and smaller pipes restrict the amount of light that can pass. The aperture is a clever little adjustable mechanism that uses a sliding set of overlapping metal leaves to create an opening of the desired size, as shown in Figure 2-8.

To get the right amount of light for an exposure, you need to choose the right f-stop. And to choose the right f-stop, it helps to understand three confusing facts about f-stops:

Figure 2-8: Overlapping metal leaves create various sized lens openings.

il F-stops seem to be named wrong. That is, f2 is larger than f4, which is larger than f8. As the numbers get larger, the amount of light an aperture can admit gets smaller.

I F-stops don't seem to be properly proportioned. An f2 opening lets in four times as much light as f4, and f4 admits four times as much as f8. You'd think numbers like 2, 4, and 8 would represent double (or half) as much — not four times.

I F-stops use all these weird intermediate numbers that do represent halving and doubling the amount of light passed by the aperture. For example, between f2 and f4 is f2.8, which is exactly twice as large as f4, and half the size of f2. The actual sequence of f-stops, each half the size of the previous aperture, is this:

f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32 What's going on here?

BE# Everything becomes clear when you realize that f-numbers are actually denominators of fractions that represent the size of the aperture opening, just as K, >4, >8, or Xe represent ever-smaller quantities. So f11 allows your camera's sensor to collect more light than f16 because »1 is a bigger number than »6.

Two of one, half a dozen of the other

As far as the camera is concerned, f-stops and shutter speeds are equivalent. Cutting the shutter speed in half produces the same effect on exposure as using an f-stop that cuts the size of the lens opening in half. An exposure that is twice as long is the same as one exposed for the same length of time but with an f-stop that is twice as large.

So, if your camera's exposure system suggests an exposure of M™ of a second at f8, you could reduce the exposure to »,000 of a second (half as long) at f8, or get the exact same exposure at 340 of a second at f11 (with the aperture half as wide).

Similarly, various combinations of f-stops and shutter speeds can produce the same exposure value. An exposure of X00 of a second at f8 is the same as »000 of a second at f5.6 (halving the shutter speed, but doubling the size of the lens opening), while K50 of a second at f11 (twice the shutter speed, but half the size of the lens opening) is the same, too. Although these reciprocal relationships might be confusing at first, they always seem natural after a few weeks using a camera.

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Reasonable care has been taken to ensure that the information presented in this book is  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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