Depth of Field, Part 1
This week's tip is the first in a three part series on depth of field, one of the key concepts necessary to gaining better control of the images you take in the field.
Depth of field, or DOF, refers to the distance range within the image that is in relative focus. Although the lens is actually only in focus on a particular plane perpendicular to the lens axis, there exists a zone of acceptable sharpness extending in front of and behind that plane. I say "acceptable sharpness" because this is indeed a somewhat subjective thing.
If a lens is focused on a sharp point some given distance away, points at varying other distances will appear somewhat fuzzy. There exists a concept known as the "circle of confusion" that defines the size of fuzzy point below which sharp points still appear to be dots. For certain critical work, the acceptable circle of confusion would be smaller than for other, less critical work. In other words, some people and some situations force greater demands on the sharpness of an image.
With any one lens, your choice of aperture controls the depth of field for a given subject from a fixed vantage point; a larger aperture results in less depth of field, and a smaller aperture results in greater depth of field. Similarly, with any one lens set to a constant aperture, your depth of field decreases as you move closer to your subject (refocusing of course) and increases as you move further away. From any given spot, magnification also affects depth of field. If you use a longer focal length without changing your position, you will see a smaller section of your subject, and depth of field will also be correspondingly smaller for any given aperture.
The only constant when discussing depth of field is that for any given aperture, and any given image size, depth of field will not change, regardless of the lens being used, its focal length or your position relative to the subject. For example, if you frame a subject at a given aperture with a 50mm lens and then back away and focus on the same subject with a 200mm lens so it appears the same size in the frame as before, your apparent depth of field will be identical, so long as you use the same aperture.
Learning what controls depth of field is the first step towards using it creatively.