ISO speed

[thing]

A simpleton who is a friend asks........:If I'm imaging at ISO 800 and decide to go to 400 for less noise (I'm using a DSLR) does it follow that I'll need twice the exposure to get the same image? If so doesn't that give twice the noise? So finishing up with the same amount of noise?

[Psychobilly]

Hmmm thats interesting

50/50 phone a friend or ask the audience... :scratch:

[blinky]

You know I never thought of that :scratch:

[rawhead]

cranking it up...

on my D40

a 3200 @ 30 secs has more noise than a 1600 @ 60 secs

So I would say yes and no........

[moondog]

Dave

In this stuation, I've found that halving the ISO speed and doubling the exposure time does give a bit less noise.

There is much more difference though in going from 1600 to 800 ISO than from 800 to 400.

MD

[Psychobilly]

logically

The sensor would have captured less photons in the shorter exposure and upped the gain to produce the "same" image ...

Billy...

[Kaptain Klevtsov]

Imaging as in what we do with the sky, or more sane daylit subjects? If you can drop the ISO and still get what you want with daylight / flash etc. then drop it. With dark stuff you get very slightly less read noise as there are 50% fewer reads. Then you add amp. glow and stuff, but it adds up to - most DSLRs have a sweet spot at around 800ISO but it does depend on the camera model, or so I've read. Do the darks / flats / bias stuff and you get better data from longer exposures, so lower ISO means less noise. But, more tracking errors unless you have good polar alignment and good guiding.

In short, it depends on how far you take the process.

Kaptain Klevtsov

[thing]

My Dad who has been a photographer (not astro) for some 60 years reckons that once you get above 400 the noise really starts to crank up and he suggested that I try imaging at 400. He says the difference between 400 and below is barely noticeable but 400 to 800 is huge. Just wondered what the consensus was, and what the answer was to my original question!

[thing]

Ah, I see Jon has neatly filled in the gaps, thanks KK.

[ngc2403]

i just loaded up this to another thread but i might well you here.

this was taken with my DSLR and a lens at the as focal length pointing at a wall. the black lines are the best fit to the curves as the camera only has so many different exposure times.

(click to enlarge)

as for the noise that will increase with the iso and i would say that if you can us a lower one if you find that the noise is to high in your images.

i find ISO 1600 bring up loads of LP in my images so i tend to avoid it