Re: [Leica] Depth of field summary with regard to the argument at hand

[John Collier <jbcollier@home.com>]

The reason I use the word perceived is to imply that it is not fixed. If I
move closer or farther away from the final output, the depth of field that I
"perceive" will change. If I may refer back to the two quotations from the
posted article:
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It should be realised that "seeing" a thing is not the same as "resolving"
it.

The fact that the eye cannot distinguish between a point and a very small
disc of light leads to the concept of "depth of field".
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The problem of being unable to see small characters on the eye chart is not
a matter of focus, assuming your vision is good, but of the eye being unable
to resolve the small characters. If the eye was not focused on the chart,
then you would have trouble seeing the large characters as well.

I am not sure I can correctly follow your example of the tree. It seems that
you are photographing the tree from different angles. One straight on the
other looking up. Parts of the tree will be at significantly different
distances from the plane of focus in the low and close photograph as opposed
to the straight on and distant one. If we switched the low and close
photograph so that it was taken along the same sight line as the distant
one, we would then be able to compare the depths of field. Please elaborate
if I have misunderstood your example.

One other quote from the article also has to considered as well:
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These calculations assume that the lenses on the camera and enlarger or
projector are perfect and free from aberrations.
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John Collier

> From: "Austin Franklin" <austin@darkroom.com>
> 
> 
>> Perceived depth of field varies with the magnification of the subject
>> (combination of output size and viewing distance), not with the actual
>> physical size of the subject.
> 
> But, it is the word 'perceived' that I have trouble with.  If you are going
> to throw perception into the equation, then perception will vary with
> conditions, and one of those conditions IS the size of the object, separate
> from the magnification it is printed at.
> 
> A picture of a tree, up close...and looking up...it occupies almost the
> entire width of the bottom of the image, and obviously tapers off to the
> top.  What 'appears' to be in focus will 'appear' to be in focus from a
> greater distance, than say a picture looking down the street.  The size of
> the object does relate to what 'appears' to be in focus.
> 
> Eye charts are viewed from one distance, yet, the smaller characters on the
> eye chart are 'out of focus', but the large objects are in focus, as far as
> our eye is concerned.
>