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Forum: Lens Clubs
06-17-2015, 02:39 PM
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A plano-parallel "lens", such as the rear filter will shift the focus slightly backwards. The difference between the focal plane with and without filter in place is small. That is easily compensated with longer lenses by re-adjusting the focus and will get unnoticed. There are a few lenses, where the filter serves other purposes as well, mainly reducing colour aberrations, but that is more often the case with front filters. Lenses that require a front lens (quite common with older 300mm f/2.8 lenses), will indeed be sharper when the filter is in place, than without it. There was a very nicely illustrated and well-informed thread in an Olympus discussion forum about that, where the images published clearly showed the improved sharpness with the filter in place.
Ben
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Forum: Lens Clubs
10-26-2011, 12:08 PM
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The Rubinars are indeed good. Many suffer from slight astigmatism, but that be remedied easily, by making the front lens element slightly looser, the retaining ring usually is screwed down too forcefully. The real problem with the Rubinars, ist that they are soooo slow.
Ben
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Forum: Lens Clubs
07-12-2010, 01:54 PM
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I think, you got it down the wrong throat :)
Birds in flight are not exactly my favourite passion, but if I do chase them, I use refractive lenses, as I find mirrors too slow and too cumbersome to focus. I use mirror lenses for more or less static (or slowly moving) objects, like this:
This was taken with my 3000mm fl f/12.5 mirror scope.
Ben
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Forum: Lens Clubs
07-12-2010, 12:32 PM
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I have no experience with the Pentax reflex lenses, but used the Tokina 500(8 and the MTO 1000/10 in the past (apart from my 3000/12.5 scope)
. The Tokina is one of the better catadioptrics, especially given its very tiny size.
But your example images are not only soft, but simply not sharp to my eyes. Is this a compression artefact (from the upload here) or is it real?
Ben
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