At 50 feet the 750/6 has DOF of about 6 inches. At the same distance
the 800/11 has DOF of about 10 inches. At 100 feet the 750/6 has 33
inches and the 800/11 has 41 inches.
My guess is that better results with the 750 are probably due to easier
focusing from a much brighter image and perhaps an inherent superiority
in resolution due to the size of the Celestron. Theoretical resolution
of a lens is based on its diameter. The aperture for the 800/11 is only
73mm whereas the Celestron is 125mm.
Chuck Norcutt
Jeff Keller wrote:
> The 750mm f6 seems to have plenty of DOF for birds. It seems the opposite of
> what I would expect - bird photos are almost always better focused and thus
> sharper with the Celestron 750mm f6 than with the 800 mm f11 Vivitar S1.
> I've mostly used it when the birds are too far away for my Tamron 400 w 1.4x
> teleconverter. If I had taken more pictures focused closer I may have seen
> more problems.
>
> The 800mm f11 can take good pictures but the reject percentage is very high.
> Most of the 750mm f6 have been at least technically acceptable. The 750 is
> an imposing lens. I can't imagine very many people being comfortable with it
> pointed at them even 50 feet away.
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