Potentially Whopper III

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

This one was really confusing and so I'm going to expand on it.

>> And one could argue that
>> all lenses have same depth of field at same f/stop (if blown up to
>> equal magnification!!).

> One could argue that, but I don't think you would go for the same
> final magnification in a photo taken with a short lens unless you
> crop heavily because the print would get too big.  If you crop
> heavily, you should/could have used a longer lens in the first place
> because you've just done the equivalent.

First let me say there are a couple of possibilities as to what's
meant by our statements.  1) If you take photos of a scene from the 
same point in space, then regardless of the focal length of the lens 
you used, the depth of field will be the same if the linear aperture 
is the same and you blow up the photo so the magnification is the same.  
As an example, if you use a 50 mm lens and f/8, you blow up the photo 
2X and you get the same result, the same depth of field, as if you had
used a 100 mm lens at f/16 and made a contact print.  And then of course 
you view the two photos from the same distance regardless of their size.  
2) If you take photos so that the magnification on film of the in-focus 
object is the same, then the depth of field will be the same regardless 
of focal length and only dependent on the relative aperture.  Now in 
this case, the perspective changes because you are moving the camera to 
get the same magnification whereas in the previous case you just changed 
the final magnification through enlargement but kept the camera in the 
same spot for both exposures.  So you can see Bob was probably talking 
about case 2).  I was probably 8-) talking about case 1) as you can see 
from an earlier statement I made in that same post:

> It was there to say you could get a lot of depth of field because 
> the linear (not relative) aperture is small.

I will be so bold as to say that case 1) is closer to what we should use
in stereo because we always try to view from the center of perspective to 
prevent stretch/squash.  In case 1) as described, we might not be viewing
from the center of perspective but we are at least viewing from the same
percentage of the center of perspective so could view from the correct 
center of perspective of both photos if we wished without changing the 
conclusion.  This is no reflection on Bob's statement because he wasn't 
talking about stereo at the time he made it.

Physical optics has been completely ignored in this discussion.  With 
any luck, we aren't operating so stopped down that diffraction is a 
problem compared to the lens' inherent aberration problems.

John B


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End of TECH-3D Digest 226
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