Author Topic: Advanced HDR bracketing questions  (Read 12307 times)

engardeknave

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Advanced HDR bracketing questions
« on: October 25, 2013, 04:12:02 AM »
So my current protocol for HDR starts by exposing for the brightest thing in the frame, usually what's outside the windows (shooting indoors). I do this by looking at the RAW histogram and setting it to E4.0-E3.0 (depending on the dynamic range of the scene). I don't even know exactly what this number represents, but it seems like a good way achieving consistent results. Then I start the bracket, allowing autodetect to choose the number of frames.

Is there some way of automatically choosing a good starting exposure? If not, this might be a feature request: a way of automatically selecting a Tv such that the RAW histogram reads as some predetermined value. Now I set this manually this each time, and for 35 photos it gets redundant.


The last frame is often way, way too overexposed. Sometimes so overexposed that I end up just deleting it and using the previous frame as the final exposure for the shadows. Sometimes I reduce the exposure of the final frame to sane level for the shadows before Enfuse. This is probably due to some small black object in the frame. I don't think there's any way to fix it though because there's no way for the camera to differentiate between underexposed areas and black objects.

Perhaps exposing each photo in the reverse order, from the shadows to the highlights would help? Anyone have experience with this?


Somehow I had overlooked the ISO-shifting feature until just now. I am very interested in any way of making shoots go faster. Also of potentially avoiding blub shots (which take forever and then switch all the camera settings from those of the C mode I'm in to whatever random settings M mode has at the moment). My question is: does ISO-shifting at "half" mean that my MKII is going to be shooting some photos at ISO3200 (with ISO expansion off)?

edit:
Ok, I didn't think I'd be able to test these things now, but I was able to do so even though it's night.

1) It's not going to be possible to do the brackets in reverse order because with my settings I'd have to start in blub mode to expose the shadows properly. If the settings are such that the shadows can be exposed in less than 32 seconds, then it can be done from shadows to highlights. Not sure if there is any advantage in this still.

2) ISO-shifting set to "half" on my MKII with ISO expansion off took a photos at a maximum ISO of 1600. This should really be something we can set.

3) There is basically no perceivable difference between finished HDR photos shot at ISO 100 and 400. Probably could take that to 800 without noticing noise on an MKII.

Greg

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Re: Advanced HDR bracketing questions
« Reply #1 on: October 25, 2013, 04:45:05 AM »
I use the bracketing 0 + ++
Exposure to not be over-exposed, and bracketing the 6x2EV produces shadows.

engardeknave

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Re: Advanced HDR bracketing questions
« Reply #2 on: October 25, 2013, 04:55:45 AM »
Just tested a few things and edited the OP.

Six is too many frames for a lot of rooms in houses though. Some of them don't even need to be HDR, but I shoot them HDR anyway just so they all look consistent.

a1ex

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Re: Advanced HDR bracketing questions
« Reply #3 on: October 25, 2013, 09:24:51 AM »
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I do this by looking at the RAW histogram and setting it to E4.0-E3.0 (depending on the dynamic range of the scene). I don't even know exactly what this number represents

This means the image is underexposed by 3-4 stops according to ETTR (but if you use SNR limits, that's relative to noise levels, not to the brightest point). To expose for highlights only, disable SNR limits in ETTR.

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This is probably due to some small black object in the frame. I don't think there's any way to fix it though because there's no way for the camera to differentiate between underexposed areas and black objects.

After implementing ETTR, I've noticed that for highlights it's best to ignore a very small percentage of them (1% is too much), but for shadows it's usually better to ignore around 5% of them. Maybe I should use similar percentiles as in ETTR defaults (0.2% and 5%) for HDR bracketing too (now it has around 0.005% for both, don't ask me why).

Experiment with these values in hdr_check_for_under_or_over_exposure (e.g. start with under_numpix = pu > 500 and over_numpix = po > 20 and let me know what works best).

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Is there some way of automatically choosing a good starting exposure?
Canon meter, ETTR... whatever is in inside the bracketing range, or at one of the ends, should be just as good.

2) ISO-shifting set to "half" on my MKII with ISO expansion off took a photos at a maximum ISO of 1600. This should really be something we can set.

Why? Above 1600 it's pointless on 5D2 and below you can just use a fixed ISO.

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Sometimes I reduce the exposure of the final frame to sane level for the shadows before Enfuse.
This is good in theory, because you get much lower noise levels. Though it may cause problems with clipped highlights => something like ZeroNoise should handle it better than enfuse.

engardeknave

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Re: Advanced HDR bracketing questions
« Reply #4 on: October 25, 2013, 11:25:19 AM »
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To expose for highlights only, disable SNR limits in ETTR.

I thought of trying this earlier but ETTR is disabled with bracketing (even when triggered with SET) and constantly toggling it didn't seem like much of a time-saver over just dropping the exposure manually. Again, it's already simple to do manually, but I'm doing it over and over and over again. I just wish it were automated.

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Experiment with these values in hdr_check_for_under_or_over_exposure (e.g. start with under_numpix = pu > 500 and over_numpix = po > 20 and let me know what works best).

I think it's great how you work with the assumption that anyone can do this. Positive leadership. But I haven't managed to get that far yet. (If you just change it I doubt anyone is going to miss that nearly all white frame.)

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Canon meter, ETTR... whatever is in inside the bracketing range, or at one of the ends, should be just as good.

Canon meter? Spot metering? Doesn't exist in LV, which I shoot in for a number of reasons. (And metering a spot outside LV, then reframing/readjusting a tripod would be waaaay more work).

Like I said, ETTR doesn't work with bracketing. I just tried it now and it looks promising, but I'd need daylight to know for sure. And for it to make sense to use, it would have to be enabled when bracketing is enabled.

Shooting 0 - -- sequence won't work in many (if not most) cases because I'd have to start in bulb mode somehow when a room isn't lit well.

I think when I tried 0 - + -- ++ it didn't underexpose the room enough to get the stuff outside windows exposed correctly. I'd have to test it again.

Come to think of it, I'm not sure sequences other than 0 + ++ are useful at all.

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Why? Above 1600 it's pointless on 5D2 and below you can just use a fixed ISO.

I brighten everything a lot in post. I don't think 1600 would suffice for real estate, but I haven't tried it in a while.

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This is good in theory, because you get much lower noise levels. Though it may cause problems with clipped highlights => something like ZeroNoise should handle it better than enfuse.

The highlights in the last frame are always waaay blown out and I've never lowered them so much that they are lower than the previous frame. This is only a slight adjustment from light gray shadows to dark gray. I'm not worried about noise so much as situations where I have a shadow that's as bright as a midtone. You end up with the back of a sofa or something that is unnaturally illuminated, and you can't fix it with any type of contrast adjustment.

a1ex

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Re: Advanced HDR bracketing questions
« Reply #5 on: October 25, 2013, 11:43:48 AM »
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I brighten everything a lot in post. I don't think 1600 would suffice for real estate, but I haven't tried it in a while.

On 5D2, brightening in post from 1600 is better in all aspects (except maybe the number of mouse clicks). 5D3 is a little better, but not much.

Do your research; I've expected a bit more common sense from you ;)

engardeknave

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Re: Advanced HDR bracketing questions
« Reply #6 on: October 25, 2013, 11:53:54 AM »
I have a weird mental block about shooting at higher ISOs. I have made the decision to shoot at ISO 400 tomorrow instead of my usual 100. I guess I'll try testing 800 and 1600 too.

engardeknave

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Re: Advanced HDR bracketing questions
« Reply #7 on: October 26, 2013, 07:13:19 AM »
ISO 400 was fine. Didn't test any higher because my camera kept freezing and I had to retake a bunch of photos until I finally disabled RAW histograms.

I have a couple of (over-)overexposed final frames if you want to see them.

Marsu42

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Re: Advanced HDR bracketing questions
« Reply #8 on: October 26, 2013, 01:38:48 PM »
I have a weird mental block about shooting at higher ISOs.

That's indeed a bit weird because with current Canon ff sensors, shooting at a bit higher iso won't make much of a difference because at base iso read noise kills most of the advantage you might have (unlike Nikon exmor sensors). See here: http://www.sensorgen.info/CanonEOS_5D_MkII.html