longer subs over higher iso?

[Daniel-K]

sorry if this has come up before. im currently in the obsy taking 400 sec subs iso400 am i going to get more SNR compared to my normal 180secs subs iso800?

[lensman57]

sorry if this has come up before. im currently in the obsy taking 400 sec subs iso400 am i going to get more SNR compared to my normal 180secs subs iso800?

Hi Daniel,

Yes you would but up to a limit, from there on all you get is saturated sky and no more build up of signal.

Have a read of this, though may not apply to your DSLR but it shows that there is an optimum exposure time and it can be calculated depending on the sky background ADU for a 3 minute exposure, the read noise and the dark signal.

http://www.skyhound.com/sub_exposure_guide.html

Regards,

A.G

[dph1nm]

ISO isn't relevant (doesn't change signal-to-noise). The sub length might make a difference, depending if the 180s is affect by read-noise or not. This assumes the same total exposure after stacking of course.

NigelM