ISO Bracketing

[popeye10]

I'm trying to do iso bracketing by setting to manual mode, fixing shutter and f stop, and using auto iso. I'm getting very erratic results. Sometimes it works, sometimes it only takes one or two photos when 5 were selected. I've tried varying number of shots and step size but I can't figure out what the internal camera "rules" are for what it does so I can't make it work reliably or predictably. Has anyone ever done this? Any insight into what's happening?

[XKAES]

The options for this technique vary from camera to camera.  For example, I can't do this at all on my cameras. 

First, tell us what camera you are using, and second, tell us if you have a user manual.

[Pieter]

Out of pure curiosity: why would you want to shoot a bracket using ISO as your means to control exposure? Sony cameras are virtually ISO invariant (save for the second gain step), so raising ISO in camera or in post-production yields nearly identical results. Most people shoot bracketed exposures to gain detail in over- and underexposed areas, but when you use ISO as your exposure variable, you might as well just create the bracket in post by raising/dropping exposure. You don't gain any dynamic range that way.

Edited June 23 by Pieter

[popeye10]

Thank you XKAES  and Pieter for responding to my query. Since my post, I figured out what I was doing wrong, and now I am getting more predictable results. To answer a few questions, I am using a Sony A7R5, and am aware that it is ISO invariant, however at least for what I've tried, there are limits to how much you can "push" exposure in post. I am doing some night/astrophotography and when I set iso to 400, and increase exposure in post, I get more noise in dark foreground areas than if I use a higher iso in camera when I increase exposure 4 stops or more. I wanted to do iso bracketing so that I could further compare the results and choose the settings that gave me the best results. I had hoped that I could set the minimum iso to 320 and take 3 or more images at 2 stop increments, but I now doubt that's possible. Since the camera is choosing iso based on the light it detects, it may or may not start at 320. Any thoughts or suggestions would be appreciated. 

[Richard A]

I do a bit of astro photography and do a lot of research etc. on best settings to use. I have never heard of anyone using ISO bracketing! Typically, higher ISO settings such as 1600, 3200 or 6400 are used and multiple light and dark frames are stacked to reduce noise. If you are worried about noise in dark foreground areas then just use a seperate exposure for them then blend the exposures in post processing.

By the way, I also have an A7R5.

[popeye10]

Thanks for the information. Since my original post, I did some "experiments" at different iso settings. Best results were obtained at iso 320, and then increasing the exposure by four stops in Lightroom. The biggest difference compared to using a higher iso was that there was detail in dark foreground areas, while at high iso the dark areas were blocked. This is consistent with articles I've read about ISO invariance.

[Richard A]

ISO 320 is the A7R5's second base ISO setting. You will find at ISO 320 you will get better results than even ISO 200.

I normally take a set of shots at 1600, 3200 and 6400 ISO. I am taking 60 light frames and 10 dark frames. I don't do any adjustments to the files before stacking. I really need to get organised and do some bias frames now.

Here is pretty much my first successful Milky Way shot from a few months ago. I was combatting a bit of ambient light and quite a lot of cloud but I'm pretty happy with this. It was shot using my Sigma 16-28 f2.8 which is better at astro than I had anticipated, at ISO 1600.

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