My Exposure+ISO Test

[fatboy1271]

Working with M27 I did four nights, each with 2 sets of different exposures + ISO. Below is a screenshot of the Preprocessed versions.

First two sets
RAW: 600sec x 400ISO
Lights: 22 = 3.667 hours total exposure
Darks: 9 from previous night's session
Bias: 100 from previous night's session
Flats: 100 from each night

Second two sets
RAW: 180sec x 800ISO
Lights: 74 = 3.7 hours total exposure
Darks: 20 from each night
Bias: 100 from each night
Flats: 100 from each night

From both the RAW and the Preprocessed I find that the 600sec x 400ISO has less noise; however, the 180sec x 800ISO has just the slightest bit more detail. I used the Preprocessed versions because they are pretty equal as far as the processing goes. If I used fully processed versions I couldn't guarantee that I would process them exactly the same... Unfortunately, due to Los Angeles light pollution, 180sec is the longest exposure I can go at 800ISO.

Unless I can't find a new target to shoot I think I'm done collecting M27 data for now; therefore, I'll use all four nights data and do a full process! Any thoughts on my "findings" are appreciated

Preprocessed: Left = 600sec x 400ISO | Right = 180sec x 800ISO

[madperk]

I think the iso 400 image looks better. The colors are more defined and there is slightly less noise. That being said, you gathered twice as much light in the same amount of time with the iso 800 exposure. I think your starfield will be more pronounced with the second shot.

Use them both for your final combined image. Overlay the 400 for color and the 800 for the starfield. Not sure how you could do it with PI but it would be a Snap with Photoshop. I often shoot a single target with different settings and use two stacks for this purpose. Use the brighter image as a luminosity layer to bring out some of the fainter starfields.

[KathyNS]

That being said, you gathered twice as much light in the same amount of time with the iso 800 exposure.

I have to take exception to that. The ISO setting has no effect at all on the amount of light collected. The two series have almost the same total exposure time, so they collected almost the same number of photons.

ISO is simply a gain control. You get the same number of photons, but the camera records bigger numbers.

If the two, I prefer the ISO 400 version. It does seem to have lower noise. The difference in detail may be due to seeing. Marginal seeing would blur the longer exposure more.

[JayTee]

Hi Patrick,

Check out the Clarkvision site: https://clarkvision.com/articles/digita ... e.summary/

It is useful info for your Canon 6D since it is a more established (read: older design) camera and the info in the articles directly applies to your camera. These articles will give you a better understanding of why you're doing what you're doing ie, the correct ISO versus exposure duration.

To me, you can process out noise but you can't add detail that is NOT there!

Cheers,
JT

[fatboy1271]

Thanks All!!!

JT, I'll check out that article

[madperk]

I stand corrected Kathy. I still want to think of ISO in film terms of sensitivity to light. Sometimes it hard to shake 25 years of shooting film.