(meteorobs) Meteor/astro photography and ISO

[Leo S]

Hi all,

I've been a bit of homework, and I came across some advice that 
contradicts the advice to use lower-ISO, given here earlier, at least in 
the case of some DSLRs.

Here is a response to the question "Why would you use a higher ISO over 
a lower one?" posted by Daniel Browning on the Cloudy Nights forum:

"Because the high gain (high ISO) sometimes have less read noise. The 
reason is that there are many electronic components which contribute to 
the read noise. When they are fed with high voltages, their own noise 
becomes less significant. It's like a audio speaker that emits a 
constant low-volume hiss (noise). For low-gain signals (soft music), you 
can hear the noise. Feed the speaker with a high-gain signal (loud 
music), and the hiss is drowned out. Ideally, of course, there would be 
no hiss, and we would have the same (low) read noise (in electrons) at 
all ISO settings, but that's not the case with many current DSLR cameras."

The thread can be found here:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3337943/page/20/view/collapsed/sb/5/o/all/fpart/1/vc/1

Here is the methord Daniel Browning recommends for determining which ISO 
to use:

"So the technique for selecting ISO is very simple: ETTR then ITTR.

1. Expose to the right (ETTR).

That means to increase exposure (photons) as much as possible without 
clipping highlights that you care about or causing other problems. For 
example, if your mount can only handle 30 seconds, don't shoot 10 minute 
subs. If light pollution limits you to 1 minute, that's where you have 
to stop.

2. ISO to the right (ITTR).

This means to increase ISO as high as possible without clipping 
highlights that you care about or going over the highest useful gain of 
the camera. You only perform this step *after* ETTR and if you still 
have room on the right of your histogram. The "highest useful gain" 
varies by camera. For most it is 1600. The 5D2 allows selection of up to 
ISO 25600, but the highest useful gain is only ISO 1600. On the Canon 
10D it was ISO 800.

Here are some examples:

* Your mount can handle 4-minute subs, but the light pollution is so bad 
that you can only do two minutes at ISO 100, any more would clip 
important highlights. The optimal ISO is 100.

* Your mount can handle 2-minute subs, but at ISO 100 the histogram is 
not to the right yet. You increase ISO as high as possible (ISO 1600) 
before clipping important highlights.

The ETTR-then-ITTR technique will always give you the highest possible 
SNR and the best image data. "


Leo