Tuesday, August 21, 2012
Look Into My Iris
Understanding Apertures • An Eagle-Eyed Reader • Calibrating The Color Of An iPad • Speed Up Your Laptop
Q I will just never quite get apertures; in the May 2011 issue of OP, your colleague Ralph Hopkins talks about shooting from ships using varying apertures at a distance depending on lenses, etc. I just cannot interface apertures used for light vs. focus.
A The aperture setting is critical to three different aspects of image capture. The first is to control the amount of light that's allowed through the lens. The smaller the ƒ-stop number, the larger the lens opening; ƒ/1.4 is wide open and ƒ/16 is very small. Ideally, the aperture will allow enough light so that the image will be rendered with the proper tonal properties; that is, there's detail in the highlights, as well as information in the shadows. Too much light, and the image is "blown out"—too bright. Too little light, and the image holds deep, indiscernible shadows—too dark. High-contrast scenes (those with both very bright and very dark areas) may be outside the normal possibilities of film or sensors.
Second, the size of the aperture also dictates the size of the area of the image that can be rendered in sharp focus by a particular lens; that is, the depth of field. Large apertures (e.g., ƒ/1.4, ƒ/2.8, ƒ/3.5) render a smaller depth of field than small apertures (e.g., ƒ/11 and ƒ/16). So as we stop a lens down (that is, decrease the size of the opening), we increase the area of relative sharpness (depth of field). The depth of field is doubled by every two-stop decrease in aperture.
The problem, of course, is that by increasing the depth of field, we're also decreasing the amount of light that passes through the lens, and this is the "light vs. focus" concept that you're trying to grasp. Serious photographers are always weighing the need for depth of field vs. attaining more light (for faster shutter speeds); each image is a compromise of sorts. The alternative is to leave your camera set on "P for Perfect," in which case the camera can make every decision for itself without considering what you're trying to accomplish.
So say you're using a telephoto lens to photograph a running deer. If you wish to stop the action, you'll choose a large lens opening to let in plenty of light so that a fast shutter speed can be employed. Stopping the animal in mid-stride is more critical in this case than the depth of field; as long as the animal is sharp, you don't care if the background goes out of focus.
But if you're photographing a landscape with interesting rocks or flowers in the foreground and mountains in the background, you would use a small aperture to gain maximum depth of field (from close in front of you into the distance) and compensate for the loss of light with a longer shutter speed.
The third aspect of aperture is its impact on the sharpness that a lens can render. Obviously, lens sharpness varies depending on the quality and construction of the optic. But generally, any lens isn't as sharp at its widest aperture as it will be when stopped down two to three stops. At the other end of the range, a lens stopped down too far (ƒ/22, ƒ/32) will lose sharpness due to diffraction.
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