shooting the moon: focus issue
 

Hey everyone. I was out taking some shots of the lunar eclipse the other night with my A1. Zoomed in all the way I was pretty happy with the image. I don't know what the temps were but I'll guess around 15-20 degF.

All camera functions were set on manual. Every once in a while the focus would let go. It would act as if the AF was suddenly activated and 'searching' (while recording). Very annoying. It would do this sporadically (no cause or average time intervals that I noticed). I would correct the focus to infinity, sometimes once, sometimes 2-3-4 times repeatedly and then the focus would hold for a while.

Any ideas on what was going on there?

Thanks
Paul

Hi Paul.............
 

I'd say that if the temp really was 15 - 20 degF, the lens/ camera were simply adjusting to the new reality of everything contracting as they cooled.

The amount of change in the lens barrel going from 70 degF to 20 degF would be considerable.

Reckon if you'd left it outside for an hour before shooting it would pretty well have "done it's thing" and held focus.

Just a guess but best I can do.


CS

thanks
 

Thanks, Chris. It hadn't occurred to me that the lens barrell changes due to temperature. I'll have to read up on that (and a whole lot more). It was very cold that night, maybe even colder than I said above.

Guess I also have to leave the possibility open that I could have accidentally had the camera set on AF. 238,855 miles is pretty far for the autofocus to work!