P3D Re: Realist image size distortion

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Ken Luker brings an interesting idea in the Realist table.
What appears to be yet another Realist "feature":  A crooked
film plane.  The difference in sharpness is not a problem at
f11 or f16 (says Ken) but the difference in image size is.

I did a rough calculation.  Do you know how much the film plane
moves when the focus changes from 10 ft to infinity?  Answer:
0.4 mm.  Do you know what difference in size this causes?
Answer: 0.3 mm (in the vertical direction)

So, assume that the film plane in one side is focused for 10 feet
and in the other side it is focused for infinity.  This will result
in one image being 0.3 mm larger than the other (in the vertical
direction).  This is easily noticeable with an RBT mount which is
graduated for differences in height in increments of 0.1 mm.

But still, 0.3 mm is only 1.2% change in size.  We have discussed
how the eyes can fuse images of much different sizes, without
much problem.  Somehow I doubt that Ken is bothered by this
difference in size and cannot see a difference in sharpness.

I am not convinced that this is happening and don't recall seeing
anything similar with my Realists (but I have noticed that there
is some varying vertical registration - shift, not change in size)
in some Realists.

A call to Ron Zakowski is in order, IMO.

-- George Themelis


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