ISO Settings again

[JohnnyD333]

Hi SGL,

I know the question about ISO settings has been asked quite a bit. I have read numerous posts and a few photography books about the subject, but I am always second guessing myself when I choose a setting. Could anyone possibly link me to a good thread, post, or tutorial on the proper ISO settings for Astro. I know it varies from DSO to DSO, but now that I am autoguiding I question the use of such a high ISO (1600) and if I should go for longer exposures at lower settings (400 or 800). Does it even make a difference? Thanks SGL as always:icon_salut:

[Earl]

From what I have read it really depends on how your camera works at the different ISO, try 800 and try 1600 and see what works best.

[JohnnyD333]

Thanks Earl, I was having issues the past week with my photos. I thought I found the sweet spot for my camera's ISO settings. It worked well for a while, all of a sudden my photos are washed out and overexposed. I couldn't figure out why. I then realized how much the moon light was polluting my photos. I was shocked, but glad to know it was a simple beginner mistake:)

[26Left]

I agree that it depends on your camera. The important thing is to work out the exposure you want by taking a test shot. You'd be looking for a peak in the histogram a fifth or perhaps a quarter of the way along.

My method is to take that test shot at ISO 6400. Let's say that a 30 second exposure works best at that ISO. If you double to exposure time, you can halve the ISO and the exposure should stay the same. So, it follows that the following combinations would work:

• 60 seconds @ ISO 3200
  
• 120 seconds @ ISO 1600
  
• 240 seconds @ ISO 800
  

At this point, your experience with the camera comes into play. With my camera, I know that exposures over four minutes will be pretty noisy and that will outweigh the benefit of using a lower ISO (lower ISO = less noise). So, I'd probably go for ISO 800 or ISO 1000.

But, each camera's different.

[Earl]

Thanks Earl, I was having issues the past week with my photos. I thought I found the sweet spot for my camera's ISO settings. It worked well for a while, all of a sudden my photos are washed out and overexposed. I couldn't figure out why. I then realized how much the moon light was polluting my photos. I was shocked, but glad to know it was a simple beginner mistake:)

Ah the moon is none to friendly, to be honest if its out in full force, either switch to narrowband, or image the moon.

[JoLo]

For my Canon 450D i basically use two settings - 400 and 800. At home, with light/moderate light pollution, i use the 400 setting and get three to four minutes before sky fog takes over...the time frame is dependent upon haze / transparency / humidity for that particular night. At my dark site i use 800 and get five to seven minutes before brown out.

I occasionally use 1600 on short tripod shots of the Milky Way, but I get a lot of noise at that ISO so i avoid. I also occasionally use 200, when the moon is out and i still have the AP itch I can't scratch. Otherwise, 400 and 800 serve me well for this camera under those conditions.

[Grinde]

Hi!

An astrophotographer named Phil Hart has made an excellent study in the subject, shooting the same target with various iso's & exposurelengths, but all with the same sum of exposuretime (12 minutes), read it here, the result might suprise you

Astrophotography Test: Canon 20D/40D - ISO Setting - Sub-Exposure Length

Full 100% zoom crops in full size can be found here:

EtaCarTest.jpg

I've always shot my pictures using ISO1600 & 5min subs with my f/5 system from pretty dark locations, never burning any galaxy-cores... Going for lower iso & longer subs just adds the risk of having to throw away alot of data in the event of satellites & so on, since the total exposure-time when stacking the subs. The total exposure time is the biggest player in the noisebusiness, not individual sublenght or iso (but make sure you don't saturate bright parts of the dso's!)

Just my opinion, as I haven't done so many tests myself, hope this helps!

[dph1nm]

Excellent. 15sec subs are virtually indistinguishable from 3min ones. Which is good news for those who don't (can't) autoguide, or have alt-az mounts.

NigelM

[blackparticle]

From what I know changing ISO doesn't affect the sensitivity of the sensor. All it does is apply a mathematical multiplier inside the camera.. usually so you can view it on the screen.

Obviously there is an optimal setting depending on exposure time / aperture and background level of darkness to make best use of the 16-bit data register but from the results of those tests, there isn't much difference regardless of what settings you use and the equation trades off on how long you can keep the shutter open compared to how much processing power your computer has.

This would also suggest there isn't much point in using anything but the lowest ISO setting on the DSLR unless you can't see the image to check if it is in focus or not and even then, you could focus at ISO 1600 and drop the camera down to start the run.

Longer subs do have the advantage of preserving the life of the shutter mechanism though. Avid imaging at 10 - 20 seconds exposure times and you will go through one DSLR per year.

[Grinde]

From what I know changing ISO doesn't affect the sensitivity of the sensor. All it does is apply a mathematical multiplier inside the camera.. usually so you can view it on the screen.

Obviously there is an optimal setting depending on exposure time / aperture and background level of darkness to make best use of the 16-bit data register but from the results of those tests, there isn't much difference regardless of what settings you use and the equation trades off on how long you can keep the shutter open compared to how much processing power your computer has.

This would also suggest there isn't much point in using anything but the lowest ISO setting on the DSLR unless you can't see the image to check if it is in focus or not and even then, you could focus at ISO 1600 and drop the camera down to start the run.

Longer subs do have the advantage of preserving the life of the shutter mechanism though. Avid imaging at 10 - 20 seconds exposure times and you will go through one DSLR per year.

Many good points, changing ISO doesn't change the sensors sensitivity. & setting a lower won't give you a better signal to noise ratio. I wonder if the tests above goes for imaging targets with very low surface brightness, any clues?

Although, I've read that cameras like the Canon 550D for example is rated for 100.000 exposures, which means 555 hours of data using 20s exposures, before reaching 100.000 exposures. Which I belive is plenty more than a years imaging

[blackparticle]

Heh.. I did say "avid" imaging.. I went through over 10,000 DSLR subs in my first month with a telescope although a lot of that was trying to photograph Saturn.