Depth of Field and Focal Length - Event Photography

From any given distance and at any given aperture, a wide angle lens gives greater depth of field than a longer focal length lens. This leads many photographers to rely on the wide angle lens for increased depth sharpness in their pictures. They feel that accurate focus will be less critical and that they can use a larger aperture and still retain greater depth of field than they would with a longer lens.

This works out nicely in practical picture taking provided you use the same aperture and shooting distance for each picture. Then, the shot taken with the wide angle lens always shows greater depth of field, even at greater degrees of enlargement.

But suppose you use the wide angle lens to move in close and get the principle subject the same size on the film as you would with the longer lens and a more distant viewpoint. What happens to your depth of field then? Well, in this case, the wide angle lens does not give greater depth of field. Even this does not always apply but you will find in most picture taking situations that when lenses of different focal lengths are used to produce the same size image on the film, the depth of field is the same when the same aperture is used, regardless of focal length.

All this may seem to be of purely theoretical interest, but it does have practical applications and can help you to avoid some pitfalls into which many a photographer has fallen. For instance if you purchase a wide angle lens to achieve a greater viewing angle from any given distance, well and good. However, if you purchase a wide angle lens with the idea that it will provide greater sharpness in depth than your normal or tele lens, remember that this will not be true if you use the wide angle lens to give the same size image on the film as the longer lens gives. Only by closing down the aperture of your lens, wide angle, normal, or tele, can you be sure of getting an actual gain in sharpness with images of the same size.

The impossibility of maintaining image size on the film while altering depth of field by changing focal lengths obviates some photographic old wives' tales. One tale suggests you use a wide angle lens for greater depth in close-ups. This rarely works. If you photograph the same picture area with a long focal length lens, your actual depth of sharpness between objects in the picture is the same at the same aperture.

The conclusion that can be drawn is: choose your focal length lens primarily for image size needed and apparent perspective desired.