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rainstr
26th of February 2009 (Thu), 15:36
Last night I tried for the first time my 100-400L for shooting start and comet Lulin, but I encountered problems with focusing. While 17-85 (at 85mm) lens seems to be very forgiving to the focusing error, the 100-400 (especially at 400 mm) is not - the stars do not seem to be very sharp. The infinity mark is wide and the night was cold so that the actual infinity was constantly moving & it was too cold for a trial-and-error focusing. Then, the focusing ring does not appear to be very precise - a tiny rotation results in a large focus difference. (OK, I use 350D attached to a guiding telescope on an equatorial mount). Can you share your experiences and techniques?

The second portion of my question is here. Has anybody used stabilizer mode panning mode when shooting with 100-400 (or the like) with a not very stable equatorial mount? Most of the vibration is in the up-down direction...
chris.bailey
27th of February 2009 (Fri), 03:03
I have tried the 100-400 and experienced all the issues you have found! I would not use IS even on a very stable mount.
rainstr
27th of February 2009 (Fri), 14:49
So how do you deal with the 100-400 manual focusing? Are you able to focus it manually at 400 to get sharp stars? Any photos?

I've just noticed ad on Bahtinov mask, has anybody used masks for focusing 100-400 or other telephoto lens? Focusing a telescope is fine because the stars are so bright, but the stars through 100-400 and 350D look very dim.
chris.bailey
28th of February 2009 (Sat), 02:08
I gave up with the 100-400 for anything other than a couple of moon shots. A Baht mask might work but to be honest I think the infinity focus adjustment is just too coarse to be useful.
rainstr
28th of February 2009 (Sat), 07:29
I gave up with the 100-400 for anything other than a couple of moon shots. A Baht mask might work but to be honest I think the infinity focus adjustment is just too coarse to be useful.

Sounds too bad! Does anybody else in this forum use 100-400 for shooting stars? :confused:
rainstr
16th of March 2009 (Mon), 17:09
Ok, I've experimented with my lens, take a look at my experiences here (http://sites.google.com/site/boriskrassi/photography/astro/focusing-canon-100-400l)