I must have had a bad sample. You could clearly see the loss of sharpness at the
edges of slides. (This was _not_ visible in slides taken with OM Zuikos, so it
couldn't have been the projector.)
The vignetting might be due to Olympus's desire to make the camera as shallow as
possible. The lens sits almost on top of the film (!). If the rear nodal point
has been moved _closer_ to the film than it would be a "normal" 35mm lens, this
would exaggerate cosine^4 effects and increase vignetting. But this is all
hypothesis.
>>>>>
If it's of any interest, close-up lenses are _not_ magnifying lenses. They
actually have the effect of _reducing_ the lens's focal length. The lens can
then focus at a closer distance. This also eliminates the need for an exposure
increase, because the increase in the lens's effective f-stop exactly cancels
out the increase in exposure required.
CU or "plus" lenses (not filters) work fairly well if you don't use too many and
you stop the lens way down. For example, a +1 or +2 on my IS-10 at f/22 gives
really crisp images. But pile up all three for a +7, and you get visible
chromatic aberration, especially towards the side of the image. If you can
afford it, you should buy CU lenses stronger than +2 in achromatic (two-element)
versions.
The nice thing about plus lenses is that they take up almost no space in your
camera bag. You can get a decent closeup when you forgot to bring your macro
lens (or didn't have room for it). It also lets longer lenses focus closer. I
use them on my Polaroid 600 SE's normal and tele lenses, which don't focus very
close.
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