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Need our opinion on a telescope Optics Planet has on sale, go ahead and ask! Post some hints and tips on various telescope brands and configuration.


Postby iogearadam on Mon Sep 22, 2008 10:24 pm

I have a quick question regarding the difference of f numbers on telescopes and camera lenses.
I have a Nikon 70-300mm f/4-f/5.6 zoom lens which is so poor at night.
If you compare the lens to WilliamOptics 66mm f/5.9 refractor, is this refractor slower than the Nikon lens ?
I don't think this is true, but I'm not sure about it.
Hope you answer for me.
Thanks a lot.
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Postby Jne_K on Tue Sep 23, 2008 5:37 am


Your lens at 70mm is f/4 and at 300mm, a f/5.6 You compare f numbers, so yes, the Williams at f/5.9 is slower, even compared to your 300mm setting.

If you like the zoom range on your lens, time to go shopping for a faster lens. Won't be cheap, because the faster the lens (lower the f/number), the bigger the lens and that's what drives the price up. I'd look for at least an f/4 at 300mm to make any significant difference.
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