iso comparison

[Lester]

Hi all, this is 2 images of ngc3132, the first is iso 800, 2 minute exposures and the second is iso 400 and 5 minute exposures. The second, unfortunately is slightly out of focus,and that is why the fainter stars are less prominent in the second image.

The interesting thing I found is the colour difference. Could iso 400 be more red sensitive with the Astro 40D?

[gbeal]

Wow Lester, big difference, same night/time/scope/processing?
Be good to try this again, and getting the focus/tracking spot on as well.
Great comparison though,
Gary

[h0ughy]

how could you have that much colour difference? wow. might have to try that with mine

[rogerg]

Regarding the colour:

- were they taken on the same night?
- was there much time between them for the object to move far across the sky?
- were they both shot at the same white balance?

Roger

[jjjnettie]

Would the difference in colour be due to the different wavelengths? Would red, having the longer wavelength, take more exposure time to become apparent??

[Octane]

I have always been an advocate for ISO-400. :grin:

H

[jjjnettie]

:) There's a lot to be said for the short and sweet subs you can get at iso800. If you're strapped for time, or clouds are moving in it's the only way to go.
But I know where you're coming from Humayan, if you're in for a long run, iso400 is the only way to go.

[troypiggo]

You're not comparing equivalent exposures. 2mins at ISO 800 should be compared with 4mins at ISO 400. Perhaps that extra minute of exposure is what's making it appear brighter? ;)

And as Gary and Roger have asked - were the conditions and processing identical?

Another question: was your camera set to AWB?

[Lester]

Hi all, the two comparisons were not taken on the same night, about a week apart, although I cannot see how that would influence colour, there was not any smog, haze, or fog on those nights. The processing was the same. Here are the images after being stacked in DSS.

[troypiggo]

What WB was the camera set to in each?

[Lester]

Hi Troy, WB=AWB, I have never changed the setting.

[Lester]

Here are cropped RAW images straight out of the camera, and the colour is similar. So it seems that DSS is altering the colour somehow, with its setting unchanged.

The first is iso 800.


The second is iso 400

[Bassnut]

The original pics you posted are the best IMO. The 800ISO pics shows better blue in the core, I like it. Iso 1600 wouldve been interesting, perhaps wouldve shown more neb extention.

[Vanda]

The stars change colour as well - seems to be an artifact of some kind. Serves to remind us what a "tough" test stars are of any optical system.

[Lester]

Thanks Fred and Ian, perhaps I need to trial more iso settings on this object. Although faster isos's will burn out bright areas very quickly, but may record fainter areas better.

[DavidTrap]

Eric has e-mailed me tonight to say he has received my 350D for modification this weekend.

I'm certainly planning to do some ISO 400 v 800 trials upon its return.

DT

[rogerg]

I've often found that colour varies dramatically between nights, hence I asked if they were on different nights :) I find that different amounts of dust, haze, etc in the sky which I don't always notice visually make quite a difference to the colour of my DSLR shots (and film shots back in those days). Especially out at my country sites I get huge variations between nice rich blues/grays to very yellow dusty colours. For wiefield stuff I like that, I like keepig the uniqueness of the shots to show what it was like :)

Using AWB will give you unreliable variation too, best to use a fixed setting for astro stuff, I use daylight WB, but it's not the end of the world eitherway, especially if you're shooting RAW :)

[Phil Hart]

i applaud any attempt at objective testing rather than gut feel approaches to learning what matters - it's a great way to improve your skills. in this case there are a few too many variables to say what the results mean from an iso point of view.

i've done a similar test with a 40D, and there is next to no difference what iso setting you use, except that ISO200 is just starting to look worse than higher isos:

http://www.philhart.com/canon_test

for an equivalent test, the sub exposures should be the same length for each iso, otherwise you have different amounts of read noise in each and are not comparing the effect of only ISO anymore. (having said that, with an uncooled camera, thermal noise dominates and read noise becomes insignificant even with 1 minute subs).

i'm happy for people to believe whatever they want, but my testing convinces me that there is very little difference between ISO settings on a 40D but that the lowest ISO settings are not the best.

my results have no bearing on any other camera (except a 450D with same sensor). with a 20D (or 350D) previous tests were more strongly in favour of ISO 800/1600.

Phil

[RB]

I only use ISO 400, I find it ideal for astro work.

Lester if you shoot and process in Raw then the WB setting shouldn't matter.

If you're using the jpgs then AWB will vary between sessions and is not a reliable setting to use.

I always shoot in Raw + jpg and leave the WB on daylight on the 20Da and only process the RAWs.

:)

[Lester]

Thanks for your comments and input everyone; I agree that the test has too many variables to be a good iso test. I was really trying to capture more of the fainter exterea of the planetary nebula, and thought iso400 at 5 minutes may do it.

All the best.