Quote Originally Posted by MarkChap View Post
Mr Felix, it is simple exposure calculation, if you halve your ISO, then you need to double your shutter time.

Unless you can achieve ISO 50 on your camera, which i doubt, if I remember correctly you have a rebel ?? then you won't have 5 stops of downward adjustment in your ISO any way.
SO ISO 1600 = 30sec -> 800 = 60 sec -> 400 = 120 sec -> 200 = 240 sec -> 100 = 480 sec.
So as you see, you could only achieve 4 stops adjustment
Hi Mark.

I have the 550D - or that is what I used to take the image.

Yes, I sort of remember what you explained about ISO/shutter speed. But have forgotten the "extra" numbers.

So where I went unstuck is when I pushed the ISO to a high level, and got the shutter speed to 30 seconds, then wound the ISO back to a less noisy level, but then I didn't know the shutter speed time.

But thanks, now reading your reply, it is "double" if I read it correct.

So if I have ISO 400 and 30 seconds, and take it back to ISO 200, it becomes 60 seconds - right?