For a long time we've been told that both depth of field and the spatial relationships between objects in a scene are influenced by lens focal length.

We've always believed that because (1) it sounded good, (2) textbooks said so, and (3) our experience seemed to confirm it.

But when you investigate the facts you find that neither depth of field or perspective is dependent upon lens focal length.

Focal Length and Depth of Field

Although focal length appears to affect depth of field (the area in focus along the lens axis), this appearance is actually based only on differences in camera-to-subject distance and target image size.

This has been confirmed by tests including the one illustrated in Popular Photography ("Depth of Field 101," July, 1994.)

For a specific lens-to-subject distance and comparable image size on the target, all lenses of comparable optical design will (regardless of focal length) have the same depth of field when used at the same f-stop.

Sure, it's true that a zoom lens used at 10mm appears to have a much greater depth of field than when the same lens used at 100mm. But the 10mm view simply is able to hide the existing lack of sharpness through reduced image size.

This explanation is more than just academic. It explains among other things why a subject which seems perfectly sharp can suddenly go completely out of focus when you zoom in.

By zooming in you end up magnifying the existing out of focus area until it becomes noticeable, and objectionable.

Focal Length and Perspective

It also seems that focal length alters spatial relationships between objects in a scene-makes them appear closer together with a telephoto length lens and farther apart with a wide-angle lens. This too is based on an unequal comparison of image areas (sizes).

Try this experiment: Take a wide-angle still photo of a scene. Without moving your camera position switch to a telephoto lens of the same basic optical design and take another photo. Then enlarge a section out of the wide-angle shot equal to what you got with the telephoto shot.

When you compare the two you will find (1) depth of field in the selected area is the same, and (2) the perspective is the same.

In other words, the blown-up section of the wide-angle scene will be identical to the telephoto version of the same scene (with allowances for some grain and lack of sharpness associated with the enlarging process).

It is only when we make a comparison based on different image sizes that the wide-angle lens ends up having a different perspective and depth of field than a telephoto lens. 

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