Dual ISO HDRI 20bit export VFX help

[boogotti84]

Hi i have a afew questions, and the more techy people can help with me
i have been playing with dual iso i think is excellent. I take hdri images for VFX so the output is 32bit image, with a range of ( a mid day image taken) seeing the sun as a disk to seeing the in dark shadows (this is not a toned map image its a full float 32bit exr or .hdr image)

So my question is
- Is the dual iso image a 20bit raw?  i cant seem to export the processed image out from lightroom or adobe raw at 20bit as max is 16bit. Is there away to process the dual iso raw as a 32bit tif or even better exr 32bit full float to retain the full 20bits?

-In theory would i just need to take two dual iso shots raws to cover the dynamic range i need to get 32bit? i take 7- 6 raws bracketed? so is there a setup i can have with Magic lantern to bracket the camera to take  two dual iso shots?with different iso's and shutters speeds. As i take a 360 i take fours shots @ 90 degrees to get the full 360.

Thank you in advance

[dmilligan]

- Is the dual iso image a 20bit raw?

No, it's 16 bit. cr2hdr does it's processing/blending using 20 bits (well, technically I would say it uses 32 bit processing that is self-limited to 20 bits, since you can't really have 20 bit integers), and then converts the image back down to 16 bits for output.

[boogotti84]

Thanks dmilligan, s
Instead of it converting it down to 16bits is it possible for it not to do that? And keep the image as 20bits? Keeping the most amount of data?

[dmilligan]

I doubt there's much reason to, in fact 16 might even be a little overkill (but it is much easier to output because you don't have to do any bit packing). 16 bits is plenty enough to represent what would be at most 14 EV of DR.

[boogotti84]

Ah ok. So for my bracked hdri images i take 6-7 shots so that about 26f-stops of dynamic range in a hdri image combined.
Would be able to take two dual iso shots to get effectively 28f-stops? (14f-stops per image) all i need really is 26f-stops.

[Audionut]

The bottom 2 or 3 stops of an image (including dual_iso), are usually very noisy.  Probably better to work under the assumption of 10 stops of useable dynamic range, and take three images at 0 EV, -9 EV and -18 EV.

[boogotti84]

The bottom 2 or 3 stops of an image (including dual_iso), are usually very noisy.  Probably better to work under the assumption of 10 stops of useable dynamic range, and take three images at 0 EV, -9 EV and -18 EV.

Thanks Audionut for clearing that up.
Is there way to set up with magic lateran to bracket 3 shots for dual iso? Would the HDR bracketing mode work with  dual iso? Or can i do it with intervalometer model?
Thanks in advance

[Audionut]

Use advanced bracketing with dual_iso enabled.  The EV increment maxes out at 8EV, which is probably best for your situation (more midtone overlap), and will net 26 stops with 3 images (assuming 10 stops per image).

You might want to take some test images first to check for aliasing though.  With a single dual_iso image, any aliasing is only in the highlights (which should be white (or thereabouts) anyway).  But in your bracketed usage, you may get some aliasing in other areas of the exposure.  So you might want to use smaller dual_iso EV increments (say ISO 100/800 instead of 100/1600), and/or smaller bracket EV increments.

[boogotti84]

Hi Audionut
i did some tests, with taking 3 shots @ 8EV dual iso 100/6400.
And my control test taking 9 shots at 2EV part

I took both hdri images and then exposed them in photoshop about the same. Now i know that dual iso reduces resolution and will create mosaicing artifacts which im not bothered about. what im looking for how much information the hdri has. As in the exposure information. As you can see from my attached image looks like 3 shots might not be enough i will try with 4shots/ 5shots. You can see the colors going weird and haloing with 3 shots.

I have processed through Photoshop merge to hdri.

[Audionut]

Woah!!!   100/6400 is pretty extreme.  You're gaining a maximum of 0.59 EV (less in reality) in the shadows over 100/1600, and loosing 2 full stops of resolution in the midtones of each bracket (over and above the 4 stops already lost with 100/1600).

So in you're 100/6400 test, the initial exposure @ 0 EV, the top 6 stops have half resolution.  From here, you have about 4 stops of good full resolution detail (this is where ISO 100 is still giving good information).  The next exposure (bracket) @ -8 EV, the top 6 stops of that exposure is half resolution also.  The first exposure got down to -10 EV, but that still leaves 4 stops of half resolution detail, before both ISOs in dual_iso, start overlapping again for full resolution detail.

So from 0 EV through -6 EV is half resolution, -6 EV through -10 EV is full resolution, -10 EV through -14 EV is half resolution, and so on.

You'll net 10 stops of good DR with 100/800, and only suffer 3 stops of resolution loss (compared to 6 stops of resolution loss with 100/6400).  Here, you will have significantly more overlap through your entire 26 stops.  And while you are only interested in exposure information, the loss of resolution (detail) with 100/6400 will affect your perception.  So you should probably try 3 shots @ 100/800.



Exposure information

Contrary to popular belief, and despite what the exposure triangle says, ISO does not affect exposure.  ISO is an post exposure gain.  The light itself that we shoot, actually contains noise, and they call that shot noise.

Using dual_iso in this bracketed manner, the only advantages are reduced shutter count, and less images to process.  From an exposure standpoint, more bracketed images with less EV difference will always produce better results.  So you're looking at a trade off between maximum Signal to Noise ratio (quality), and shutter life/processing.

[boogotti84]

Thanks Audionut, i miss understood i thought more difference in iso e.g 100-6400 iso would give you more dynamic range.  Thank you for clearing that up.
Just to clarify what best setup for 3 shots ?
3 shots
Dual iso 100/800
8 EV part? Or 6EV part?

I'll try doing some tests tomorrow as sun gone down now

Thanks Audionut

[Audionut]

i thought more difference in iso e.g 100-6400 iso would give you more dynamic range.

It does.  It just not a free lunch.  There are drawbacks that I mentioned above.

Just to clarify what best setup for 3 shots ?
3 shots
Dual iso 100/800
8 EV part? Or 6EV part?

Only you can determine that.  I would consider the above @ 8 EV apart as a decent starting point.  Using 100/800 should not have any issues with resolution loss, and the only remaining issue may be noise.  The noise will be due to the nature of light itself, and can only be dealt with by using smaller EV increments for your brackets.

If you find that you are needing to use smaller then 6 EV part brackets, you should probably just stick to regular ISO (not dual_iso) @ 4 EV apart, and call it a day.