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Old 22-09-2009, 06:24 AM
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troypiggo (Troy)
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troypiggo is offline
 
Join Date: Nov 2008
Location: Brisbane, Australia
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DSLR different exposures - do you vary shutter speed or ISO?

For a target that requires varying exposures, such as M42's bright inner core and fainter outer areas, without even thinking about it I would keep my ISO setting the same (typically ISO800) and vary shutter speeds from 8m or 4m longer exposures down to the 15s or 30s for the brighter parts.

I guess I see the added benefit that the extra time for the other exposures, and the associated darks, is at least much shorter time to do so.

At a talk at Astrofest, and also some discussions with some other guys here, I have realised that another popular option is to keep shutter speeds the same and drop the ISO down instead.

I'd like to know thoughts on the benefits of this.

Is it lower noise in the lower ISO range? Would have thought that the calibration steps we take in astrophotography would pretty much negate the difference in noise values between ISO800 and ISO100 that you would see for terrestrial photography.

I see the downside being much longer exposure times for lights and darks. Not only time I guess, but increased exposure time may also increase risk of tracking errors etc, and therefore more chances of ruining some shots.

What are your thoughts and what method do you use?
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