At 19:04 08.10.01, Roger Wesson wrote:
Hmm. This has got me thinking. 8mm at f/2.8 = 2.9mm absolute aperture.
For astrophotography purposes this is small. Now, I've got a 0.42x
adaptor thing that screws onto a 49mm filter thread, giving a circle
frame fisheye when used with a 28mm lens. It seems to me that, as it's
a 28/2.8 lens, with adaptor it must effectively be a 12/1.2 lens,
because the absolute aperture hasn't been changed, and indeed the
viewfinder image looks brighter to me. Therefore it seems to me an
adaptor is a better solution than a proper fisheye lens. Is this right
or have I missed some salient point?
You've got a point if the adaptor uses the entire opening of the 28/2.8. If
it does and it's about f/1.2 it would also explain why the image quality is
so incredibly poor! I had one of these 0.42x adaptors years ago but I had
to stop it down to f/11 (effectively f/5.6 or there abouts) on the main
lens to have decent quality. I can't remember if I tried it for astro, but
it would have been a disaster. The 0.12x Spiratone and Accura units from
decades ago have a pretty good reputation, though. The nicest thing about
the adaptors is they work for medium and large format too, where the cost
of a real fisheye would be prohibitive for most of us (if available at all).
Regards,
Thomas Bryhn
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