dual_iso: Add min. ev gain (& min. delta ev gain)

[Marsu42]

I've written something like this before (sorry for the dupe) In the mega-thread, but here it's goes again as a dedicated feature request:

Problem: With the current "dumb" +/- ev options, you're quickly running into high iso values that don't make any sense, and specifiying absolute isos makes (to me) even less sense when shooting in changing light.

Suggestions (a few more advanced options for dual_iso won't matter :-p):

1. Add a "Min. EV gain" option ... if the ev gained drops below this, do a shot w/o dual_iso

2. Additionally add a "Min. EV delta" option ... if each of the higher iso values don't add more than this ev gain, use a lower iso value instead ... this will prevent dual_iso from choosing too high values that don't add any useful dr gain

Btw there was a declined pull request about somehting like this, and I agree hardcoding iso values is not the way to do it: https://bitbucket.org/hudson/magic-lantern/pull-request/229/dual-iso-fix-comments-limit-isos-on-7d-and/diff

Edit: deleted off-topic item about high iso differences help.

[Audionut]

Edit: also a help text what +/- ev values still give ok quality after cr2hdr would make sense, no need for everybody to find out on her/himself that +6ev looks kinda funny.

All +/- EV values give ok quality depending on scene, print size, personal requirements.

[Marsu42]

All +/- EV values give ok quality depending on scene, print size, personal requirements.

That's as fuzzy as it gets, isn't it? Imho the the "default" dual_iso should be expected to be indistinguishable from single iso @100% crop, and that excludes iso100/26800 shots. There are questions and comments cr2hdr performance on high iso differences, so why not share this knowledge in a help item... not that it matters much to me now that I know, so no point in a lenghy discussion, it's just a suggestion for alex and not the main item of this feature request.

[Audionut]

Imho the the "default" dual_iso should be expected to be indistinguishable from single iso @100% crop,

Which by definition is not possible.

The level of artifacts is dependent not only on the EV spacing, but also on the subjects in the scene.

https://bitbucket.org/hudson/magic-lantern/src/136004f68e89d28cefa61f73b003309d1b7abbea/modules/dual_iso/README.rst?at=unified

[Marsu42]

https://bitbucket.org/hudson/magic-lantern/src/136004f68e89d28cefa61f73b003309d1b7abbea/modules/dual_iso/README.rst?at=unified

Ah, thanks, I didn't see the updated README - that should be sufficient. Now back to the min ev gain/delta options...