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Old 17-10-2010, 08:45 PM
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philiphart (Phil Hart)
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Join Date: Jan 2007
Location: Melbourne
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I'm a little surprised by your experience Martin.. I would have thought that lens would be up to the job (but I've never tried it).

Here's a few thoughts..

With my 24mm f1.4, 50mm f1.4 and 200mm f2.8 lenses, I image at about f3.5. I don't expect to produce sharp stars at f2.2 with any lens (and technically, going from f1.8 to f2.2 is not quite even one stop?). Certainly in my experience, the fixed focal length lenses are better than zoom lenses but I've never used the 85mm.

The only recent lens I've ever had trouble with was a 50mm f1.8 lens which must have got knocked or something and had a tilted lens element so displayed different focus across the frame. I would guess that you would have noticed if the image quality was different across the frame rather than just the same coma in all corners?

I assume you have liveview focus on that camera? Once you've slewed to the scene you want to shoot, you should have a star bright enough to liveview on and focus there. But slewing shouldn't be enough to change focus unless something like a dew heater strap is causing it.

Maybe you can borrow a lens to try something different. A lot of places hire Canon gear.. perhaps you could try that if nobody near you has one you can try?

Phil
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