P3D Re: 2519 - focus

[Duncan Waldron <J.Waldron@xxxxxxxxx>]

Jim Norman wrote:

> 	On a related topic, I want to note that I'm sometimes irritated by what I
> think is a certain imprecision in the terms that we all use.
> ... [deleted] ...
> Also, we use "depth of field" to describe the limits of a lens under specific
> circumstances to register the full range of a scene with acuity.  But we're
> not really talking about the "field" here; rather we're talking about the
> depth of "focus," an appropriate term to use in photography.  "Field," IMO,
> should be reserved to describe the width of a scene, as I think it is in
> binoculars.

Sorry Jim (and others who replied), but the terms "depth of field" and
"depth of focus" are very clearly defined in photography, and cannot be
interchanged. 

Depth of field refers to the distance range in front of and behind the
subject - the point of focus - which will be rendered acceptably sharp;
for example, when focusing on an object 5 metres away, the depth of field
may, with the appropriate aperture, extend from (say) 2 metres to 10
metres in front of the camera (strictly speaking, in front of the focal
plane).

Depth of focus refers to the distance in front of and behind the focal
plane within which an acceptably sharp image of the subject exists.

When focusing near infinity, depth of field is great, and depth of focus
is small; as the point of focus gets closer to the camera, depth of field
decreases, and depth of focus increases. When you get into the realm of
photomacrography (not macrophotography, which is literally making big
photographs), you need to stop down to very small apertures in order to
get a decent depth of field; on the other hand, you can move the focal
plane a long way (relatively speaking), before seeing a significant shift
in focus. 

For the width of a scene being photographed, or viewed though a binocular,
you would refer to the field of view.

Duncan W



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End of PHOTO-3D Digest 2527
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