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From Ollie's Web Site

Camera Theory

Picture-taking is a complex blend of technical knowledge, chiefly related to how to get a good exposure that fully exploits the tonal and color range of the media, while at the same time achieving an artistic composition. This, of course, is no different than other artistic endeavors, where knowledge of paints, brushes, carving techniques, modeling clay, etc. are all necessary companions to artistic vision. One without the other leads to sterile products. While I will spend some time on composition and other areas of the "art" side of camera work, I will chiefly address the technical side of picture-taking.

What are some of the areas to be covered? The main ones are:

• Exposure
• Focus
• Depth of Field
• StopAction
• Color Balance
• Lenses, including focal length and zoom capability.

Note that Exposure is treated on this page, while the other areas occupy their own pages. Click on any one of them to go to its page.

Exposure

At its simplest, esposure is about getting the just enough light onto the film or digital sensor to lead to a full range of tones on the media. The earliest photographers worked with terribly insensitive coated glass plates, and so worked with very long exposures of either fixed landscapes or rigidly bound human subjects. Exposure durations were measured in _minutes_ as opposed to today's fractions of seconds. That day's photographer merely uncapped his lens for the appropriate duration and recapped it when done. He didn't have to worry about fancy shutters.

Nevertheless, these early photographers developed the basic principles of exposure, principles that are as important to today's digital cameras as they were for their early film ancestors: increasing the amount of light striking the sensor brightens the ultimate image, while decreasing it darkens it.

The range of light from the lowest amount of light that creates a barely discernable image in the shadow to the greatest amount that still generates an image viewable in the highlights is called the film or sensor latitude. The effort of correctly exposing a picure revolves around taking the actual subject brightness and adapting the camera so the amount of light hitting the sensor fits into the latitude.

Your camera today gives two primary exposure controls:

• Exposure duration (shutter speed); and
• Lens opening or aperture.

For a given brightness of subject, a whole variety of combination of shutter speeds and apertures can lead to equivalent exposures. Thus, a very brief shutter speed and a wide aperture are the equivalent of a somewhat longer shutter speed and a narrower aperture, and so on.

Shutter speeds today are most frequently fractions of seconds, e.g., 1/125th of a second, 1/60th, 1/250, 1/500, etc., although tripod-supported exposures may get down to 1/4 sec. or 1/2 or even multiple second exposures.

The aperture is set in a lens by an iris that, like the pupil in you eye, varies in diameter to allow more or less light through. Apertures are described via a rather strange series of numbers: 1, 1.4, 2, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32, etc.

These numbers are more precisely stated as f/2 or f/4, where "f" stands for the focal length of the lens. If you are dealing with an f/2 opening on a 50 mm focal length lens, this means that the opening is half of the focal length, or 25 mm in diameter. f/8 on that same lens would mean an opening of 1/8th of the focal length, or 50/8 or a little over 6 mm.

The interesting part of all this is that this rather arbitrary-looking series is a ratio scale where each step doubles or halves the amount of light admitted through the lens. Going, for example, from f/2.8 to f/4 halves the amount of light passing through the lens, while going one step in the opposite direction, e.g., from f/8 to f/5.6doubles the amount of light. The numbers appear to be backwards, with f/1.4 being a much larger opening than f/16. This is because these are fractions, so the larger the denominator, the smaller the fraction.

Since overall exposure is a combination of shutter speed and aperture, it is easy to see that if 1/125 sec and f/8 lead to correct exposure, then doubling the time to 1/60 and halving the aperture to f/11 is the same thing as far as exposure is concerned as is 1/250 (half the shutter time) and f/5.6 (double the aperture). You can keep up this doubling or halving to create other equivalent exposures. All the following are equivalent exposures:

As you will learn below, while these are equivalent exposures, they may lead to substantial differences in the image insofar as either stop-action (i.e., sharpness of moving objects in the photograph) or depth of field (the distance range of objects in focus in the image). Both of these will be treated in some detail below.

All these matters are discussed in greater depth on A Tedious Explanation of the f/stop.

Canon 10d

Once you have absorbed all this, you should pick up the camera and look at the mode dial. You should always keep it set for one of the "Creative" settings (P, Av, T). These do the following:

• P - fully program mode. The camera's metering system sets the exposure, and picks the shutter speed / aperture combination.

• Av - Aperture Priority mode. The camera's metering system sets the exposure, but you pick the aperture with the wheel closest to the shutter release button. The camera adjusts the shutter speed to maintain the exposure.

• T - Shutter Priority mode. As above, the camera's metering system sets the exposure. Here, though, you pick the shutter speed with the wheel closest to the shutter release button. The camera adjusts the aperture to maintain the exposure.

Why would you use T or Av? Because they give you more creative control over the image: if depth of field is important, then you can control that by adjusting the aperture. If, on the other hand, stop action or blur motion is important, then you can control that with a faster (i.e., shorter) or slower (i.e., longer) shutter speed.

As if this wasn't complicated enough, Canon recognizes that sometimes the meter will get it wrong and so allows you to adjust the meter through Exposure Compensation. To do this, depress the shutter button half way and then move the back dial while looking at the top LCD. You will see the index move up and down the Exposure Compensation scale. The whole numbers are f/stops, so moving to +1 means you are effectively doubling the exposure (either opening the lens one stop or doubling the shutter duration). Remember to reset this after you go back to normal metering situations.

Play with these settings for a while until these controls feel familiar. Then go to the Focus page.