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"There is no issue with the small diameter field lens of the 4x Powermate since the high magnification factor only requires a smaller field stop."
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I have to admit, I'm foggy on field stops and entrance pupils. It seems criminal to throw away a large part of the light cone. I'd always presumed it was because the optics design couldn't correct for aberrations any wider than that, but thinking about it, maybe it's simply that, being closer to the focal plane the outer part of the image is simply not required at higher focal lengths.
Makes me wonder if using the 2.5x PM will be a good idea, since it's only 1 1/4". The entrance pupil on that is a similar size to the 4x, which doesn't seem right. I wonder if I'm discarding precious photons?
Markus
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