Depth-of-field in Photography

Depth-of-Field in Photography

by Robert Meeks

One subject which can definitely engender any number of opinions in trying to present an appropriate definition is certainly depth-of-field. Whether defined as a zone where objects are in focus or a zone before objects fall out of focus, other than for the sake of argument, these variances in opinion do nothing to change the phenomenon itself. The objective of this article is to offer an understanding of depth-of-field, but certainly not the definitive, and to offer ways to take advantage of it to effect creativity within our photos.

When we focus a lens on an object, any point of light (any part of the object which is visible), which is at that point of focus is brought into focus as a point of light on the film plane (or digital sensor). Any object which is not within that point of focus forms as a point of light before or after (would theoretically form) the film plane. When the latter happens, a corresponding but larger point of light is formed on the film plane. These points of light are called circles of confusion (and what an appropriate name).

The circles of confusion are conversely out-of-focus and larger than the points of light which are in-focus. If, however, we reduce the size of the circles of confusion, the resultant image appears to be sharper. In order to reduce the size, we need to reduce the diameter of the light reaching the film. This is done by reducing the aperture (the diameter of the lens opening) which causes the circles of confusion to effectively shrink in size, thus making them sharper. As we reduce our aperture, we can improve the sharpness of these circles further from our point of focus.

Although it would appear as though the more we close down the aperture, the sharper the pictures will be, this is not the case as we reach a point when we close down the aperture that light begins to deflect off the blades themselves. When this happens, the light which deflects is formed as more circles of confusion on the film plane reducing sharpness at all ranges. Even though we still effectively have our depth of field, sharpness has been noticeably reduced throughout the image.

We have an interesting effect at the other end of the aperture range. As we open the aperture, a larger diameter, the circles of confusion become larger and lose even more sharpness. This has the effect of isolating the image which is in-focus by throwing any objects outside of the focus range further out-of-focus. This also causes an interesting play on contrast as the contrast in the in-focus area is maintained but in the out-of-focus area contrast is reduced as well color saturation. This adds to the effect of isolating an image and using it to its advantage is called selective focus.

The opposite of selective focus is the hyperfocal distance. This is the setting on a lens when we want most everything in a scene, from close to far, to be acceptably sharp. If we set the lens to focus at infinity the objects closer to the lens will be out of focus and vice versa if we focus on objects close to the lens. We can accomplish getting everything (most everything) near and far acceptably sharp by using the hyperfocal distance.

If our lens has DOF marks on the barrel, it is relatively easy to determine hyperfocal distance. Set infinity at the aperture at which we are shooting. If we are shooting at an f/11, set infinity at the f/11 on the DOF scale of our lens. The opposite side of the DOF scale will now show the closest focusing distance at the f/11 mark. The actual hyperfocal distance will be at the focus mark on our lens.

If there is no such scale on the lens, then we can resort to a standard rule of thumb (which is a lot better than the algebra required to calculate this). Since our depth of field will fall 1/3 before our point of focus and 2/3 behind it, we want to set our focus to twice the range of our closest subject while using a small aperture like f/16. If our closest subject is 15ft, then we want to set the focus at 30ft which will put everything within the depth of field from 15ft to infinity. This is working with a lens which is 50mm or shorter (for 35mm film) and a subject no closer than 7ft with which this rule of thumb mostly works.

This has been a short introduction to some of the basics of depth-of-field. There is a lot more which can be studied about depth-of-field and which I could have written. Being, however, that I do not care for algebraic equations when I am using my camera, rules of thumb work for me just fine.

-Bob

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