> I often use a relatively cheap very high magnification
> loup. This loup shows distortion and it is not very
> sharp at the edges. But then I usually only
> concentrate on the middle and move the loup the
> different regions of interest. It works quite well for
> me. Actually, I also have a 4x loup but hardly use it.
> Most of the time I judge exposure and composition with
> my bare eyes and then check focus with the high
> magnification loup before I scanning.
>
This is the approach I took when I bought a Schneider 4x and a Peak 15x
magnifiers. Being an optics geek, I proceeded to test the magnifiers. The
following is the result of testing I did several years ago.
One thing to note about the results is that I am very near sighted (-8
diopters). In addition, my eyes do not accommodate very well - I use
separate glasses for reading and viewing distant objects. The results that
I report while wearing my reading glasses probably compare most closely to
what persons with normal eye sight can resolve.
I have available to me two different positive, high contrast,
chrome-on-glass, test targets: the NBS 1963A five bar and the USAF 1951
three bar. I shall only report the results from the USAF test target
because the NBS target only goes up to 18 lp/mm. In the tests that I
performed where I could not resolve 18 lp/mm, the resolutions obtained from
the NBS target were 25% higher than from the USAF target. All targets were
placed on a light table for examination.
Test Results:
lp/mm Description
2.4 Wearing glasses for distance.
4.7 Wearing reading glasses.
9.5 No glasses, nose to target.
22.6 Schneider 4x loupe + reading glasses.
32.0 Schneider 4x loupe, no glasses (loupe skirt removed to obtain
focus).
45.3 Projected, using 20 year old Kodak projector with standard lens.
51.1 Peak 15x Loupe (#1962) w/ reading glasses.
64.4 Peak 15x Loupe no glasses, before focus adjustment.
71.8 Peak 15x Loupe no glasses, after focus adjustment.
144. Edmund microscope, unknown power or model number.
Some general comments about the magnifiers:
Schneider 4x - excellent optics, covers full 35 mm slide with no apparent
distortion, sharp center to edge (I obtained the same edge center and
sharpness), and no color fringes. There is a reason why this magnifier is
the defacto standard against which all others seem to be compared.
Excellent for checking sharpness for print sizes up to 5 by 7 inches.
When I removed the plastic skirt so that I could focus when not wearing
reading glasses, the image was just barely soft. This is not surprising as
the magnifier was no longer being used at its intended conjugate distance.
I have not decided yet if the increased resolution is worth the lack of
visual sharpness.
Kodak projector - Disappointing results. Sharpness equivalent of only a 10x
enlargement. Visually, the images seen with the Schneider 4x were much
better. I think its time for me to look for a better projector lens.
Peak 15x - Approximately 1/2 inch FOV with strong color fringes at edge.
Center slightly soft. No built in focus adjustment. The focus adjustment
consisted of sanding the plastic skirt until I obtained best focus with no
glasses on. I would use this magnifier for checking sharpness of slides or
negatives prior to making 16 by 20 prints. If the slide appears sharp at 72
lp/mm under this magnifier, it should resolve 4.5 lp/mm in the print. This
is basically my resolution when wearing reading glasses.
Edmund microscope - This is the type of cheapie microscope that gets sold
for use by kids and hobbiest. Very high resolution but almost impossible to
use. It was so hard to focus and to hold steady that I could not determine
the FOV or edge sharpness. If this microscope had a stand to hold it
steady, it would be great for critical use. Without the stand, I tossed it
back into the kids drawer for them to play with.
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