Updated: 11/18/2011
Focal Length
and F-Stop Myths
We've been told that depth of field and the spatial relationships between objects in a scene
are directly related to lens focal length.
We've always believed that because many textbooks said so and 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.
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...all things being equal, focal length does NOT affect depth of field.
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 because of the reduced image size.
This explanation is more than just academic. Among other things, it explains why a subject which seems perfectly sharp can suddenly seem to 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.
If you have correctly focused on and main subject to start with, you will generally see the background "appear" to go out of focus as you zoom in.
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. (See the "wandering road signs" in Module 11.)
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).
Be aware, however, that the above refers to prime lenses. Some zoom lenses shift lens elements when zoomed, which, in effect, alters the "design" of the lens.
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