Exposure and Depth of Field
Quote:
F/8 and Be There
If you ask a professional for some exposure advice, the typical answer is "f/8 and be there." This is a bit of an in joke. The "f/8" part of it sounds vaguely technical and useful, since f/8 is an actual aperture that you can set on most lenses. But it doesn't mean anything without an accompanying shutter speed or film ISO. The "be there" reminds you that ultimately exposure is pretty easy. The most important thing to have is patience and dedication so that you're around when a great photograph is happening.
There is no correct exposure
As I noted in the chapter on film, the real world generally contains a wider range of tones than you can represent on paper, film, or even with the best digital sensors. You have to make an artistic decision about where you place those tones. Some detail will inevitably be lost as tones that are distinguishable in the real world are mapped to the same number out of a digital sensor or density on film.
This chapter will teach you how to control and predict which details are lost.
The Controls
Single-lens reflex cameras have an intimidating array of buttons. It will please you to know that there are only three controls that affect the imaged: focus, aperture, and shutter speed. The two controls that affect exposure are aperture and shutter speed.
Aperture
If neither the subject nor the camera are moving, the shutter speed is not very important. Aperture, however, affects the depth of field and therefore which portions of the image will be in focus.
What is aperture and why is it useful to change it? Aperture is the degree to which the iris or diaphragm inside the lens is opened. Lenses are designed for maximum light-gathering capability. The diaphragm is just like the iris in your eye; it can be closed or stopped down to block off a portion of the light coming through the lens. A lot of expense and weight went into making your lens fast or good at gathering light. Why would you want to throw away some of that capability away?
The first reason to stop down a lens is that the world might simply be too bright. If you're using high-speed (sensitive) film and have a slow shutter that must expose the film for at least 1/500th of a second, using a smaller aperture is the only way to prevent too much light from striking the film and overexposing it.
A more interesting reason is for aesthetic control of sharpness. Suppose the lens has a maximum aperture of f/2. The f-number is the lens length divided by the diameter of the aperture opening. So for a 100mm lens, this would be a 50mm opening. The depth of field will be shallow. Only the object on which you focussed will be sharp. Things closer or farther from the camera will be out of focus. The range of distances for which objects are acceptably sharp is called the "depth of field". Notice the word "acceptably" in the definition. What is acceptable in an 8x10 print viewed from across the room may not be acceptable in the same print viewed at arm's length. What is acceptable in an 8x10 print viewed at arm's length may not be acceptable in a 30x40 print viewed at arm's length.
If you want more objects in the scene to be acceptably focussed, you have to stop down the lens to a smaller aperture, e.g., f/16 or f/22. This nomenclature is a bit confusing at first for beginners because a smaller aperture means that the lens length divided by the aperture diameter gets larger, yielding a larger f-number. Even more confusing is the fact that lenses are calibrated with a strange succession of apertures: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 45, 64. Each step represents a halving of the amount of light that comes through the lens. Why? The area of the aperture is proportional to half the diameter squared. So multiplying the f-number by the square root of 2 halves the amount of light coming through the lens.
Let's look at some example images.
With a long lens and a wide aperture, the depth of field is very narrow. Only those objects exactly at the focussed distance will be sharp. For example, here are a couple of images taken with a 600mm lens at f/4 or f/5.6:
This is a good article to help you understand what your camera is capable of
Continued at photo.net
http://photo.net/learn/making-photographs/exposure
