Quote:
Originally Posted by h0ughy
so if i get absolutely perfect focus on a star then put the filter over it will be out of focus?
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I can't give a definitive answer to your question.
All I can tell you is what I have done with some success. I have focused carefully using liveview at 8x on the lunar limb ~1min before totality using the Baader film and not felt the need to change focus during totality. During totality I switch off live view and preview of the just taken image so that I don't have a distracting light.
You are asking me about focusing on a star hours before and whether it will be in focus with the filter on. But then when you shoot totality, your filter will be off just like when you focus on the star. So what does it matter about the filter? Forget the partial phases....it's all about the corona. However, now you'll need to think about temperature change, mechanical stability of the focuser as well as depth of field issues since your EFR could be less than mine.
Lets look at the variables
USER Joe
Time from
focusing -> usage 60s
Temp change small
OTA f stop f6 - f8
filter on to focus
filter off shoot totality
USER Houghy
Time from
focusing -> usage hours
Temp change small to large??
OTA f stop f2.8, f4??
filter on shoot partial ecl
filter off to focus
If you are thinking of focusing on a star pre-dawn, you need to be sure that temperature changes and mechanical stability of the focusing mechanism don't trip you up. My eclipse observing partner and I did a star focus on his Nikon lens in Libya back in 2006. I don't know whether it was temp shift or some mechanical focus shift but it just didn't work. All his images were out of focus. We could not pin down the cause, but we never tried it again.
I don't know the answer to your question about the star. You could try this simple experiment. Focus on the star one night then next day put the filter on and check the focus on some sunspots. I can see why you want an alternative, too many cameras to focus in such a short time. An early morning eclipse like this might not see too big a temperature shift. The land doesn't have much time to warm. I observed the last eclipse in this saros series in Bolivia Nov 3, 1994. We were at 4500m altitude. The eclipse was total at 23 deg altitude a bit after 8am local time. Temperature dropped 5C from pre-eclipse to totality. Biggest shift I've measured was in Romania in 1999. I had two sensors, one mounted at 1m measuring air temp the other at ground level.
Before the eclipse:
Air temp 39 Celsius
Ground temp 52 Celsius
During the eclipse:
Air temp 30 Celsius
Ground temp 31 Celsius
I've had more than my fair share of focus problems over numerous eclipses as have many friends of mine. Apart from really stupid errors like forgetting to remove the filter or forgetting to press the shutter or start the computer script, focus is one of the most common problems among eclipse photographers. It sounds silly to be caught by such a simple problem but with so little time, and so many things competing for your attention, it's so easy to make mistakes.
Sorry I can't give you a simple definitive answer.
Joe
