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Ingo a Canon 300mm F2.8L is as good at f/2.8 as it is at f/8! The only difference is a very slight astigmatism at the corners on full frame cameras at F/2.8. and is only noticeable on very bright stars.
We are comparing a 300mm focal length F/2.8 lens to a 1800mm FL f/9 mirror optic. I can assure you focus was spot on in both cases. The TAL 200k has an aperture of 200mm diameter compared to the lens of 105mm diameter.
Thanks for even thinking about the differences, it has been a comparison I have been meaning to do for a while.
Bert
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