Histogram/channel limiting help

[tim]

I had a few problems with some images I took yesterday. The histogram looked fine on the camera, but when you look at the images the people look very red in the face, or sometimes white (ie more than just the red channel limited I guess). They're rescuable because I shot RAW, but it'll take a little tweaking. The lighting was theatre lighting, white lighting from overhead and a few mixed colors from below, which combined to give an apparently white light. I used a 550EX for fill flash to remove possible shadows under the eyes, and I shot in Av with no EC or FEC. 1/30th, F5.6, ISO 400.

Have a look at these histograms, the top two from the EOS viewer, the bottom from CS2 RAW. The histogram on the camera looked pretty much like the left EOS viewer histogram, though there was less of a bump on the right.

http://mrwild.co.nz/unprotected/potn/histogram.gif

Now here's what happens when I get CS2 to show me the limiting channels (hold ALT while you click on the exposure slider). I usually adjust things until I get little or no limiting from any channel, and with people it's usually the red channel that limits.

http://mrwild.co.nz/unprotected/potn/histogram2.gif

Does anyone know what's gone on here? By reducing the exposure, increasing the by -1 and changing the brightness to +75 (from +50) the shots look ok. I'd rather not post the images publicly at the moment, but I can if necessary.

[malcolmx]

imho i think you have clipped colours on the histograms which would lead me to think underexposure i dont think the colour detail is recoverable even in cs moy not be any help

[tim]

Underexposure? I never thought of that. I don't know why they'd be underexposed. They definite look overexposed to me, one of them's massively overexposed, the faces are solid white.

The background was a solid black curtain, which will have dragged the average color closer to black (rather than 18% grey) and caused underexposure. I was using partial metering, the focus point was typically the red dress - this seems a likely candidate for the problem to me - anyone else?

Most of the exposures aren't too bad, some are terrible, and most seem to be recoverable. I took plenty of shots so it's not a disaster, more an annoyance. I'd really like to figure this out so it doesn't happen again, so here's a couple of shots. The first is straight from the camera, the second is with exposure down, brightness up, and shadows tweaked.

Original
http://mrwild.co.nz/unprotected/potn/temp/orig.jpg

Tweaked
http://mrwild.co.nz/unprotected/potn/temp/tweak.jpg

[malcolmx]

i genraly thougt that a hard line to the left of the histogram denoted solid blacks and generaly this represented underexposuer but now i seethe pics the matt black curtain must have influenced the whole shot and caused the the bright channell to overexpose which would account for the bright channell being hard to the left these histograms seem to put intensity to the left ,so all the colours will be intensified . i think also although we see theater lights as white the camera is trying to pick up cloured lights,flash,and white light which again may have pushed the brightness off the scale ,ihope this helps and i have not incresed your problem

[tim]

Yeah the bunching on the left will be the black curtains and the black suits. I wouldn't expect many midtones, maybe the red clothes. I'd only expect to see a little at the high end, the white shirts at the very right and the skin further towards the left.

Based on looking at what i've shot, and looking at what falls inside the metering circle, I suspect that the partial metering I chose to use was the major fault.

[PacAce]

Just an educated guess on my part but if you used a flash for fill, that's what the culprit was. Since most of the scene is dark and mostly not reflective, the ETTL tried to set that to mid-gray, resulting in some of the faces and the white shirt to get overexposed. Notice how the shirt is totally blown and without details. Maybe an FEC of -1 to -1 1/3 over what you already had along with a diffuser if you weren't using one might have resulted in less or no overexposure and softened the flash lighting a bit.

[tim]

Thanks for that PacAce. I may never be sure what went wrong, but all the ideas are things i'll make sure i'll do better next time. A diffuser probably wouldn't work, I was using the 70-200 at around 70-100mm, a diffuser would have cut out way too much light i'd have thought. FEC is a good idea.

What I think's more important is to know how to detect these problems at the time they're taken. The histogram looked ok to me, at the time, though I should've picked up the clump on the left and the spike on the right. There weren't any flashing areas on the LCD for the most part. Hmmm.

[robertwgross]

Tim, I saw that histogram at the top of the thread. Doesn't that spike on the right edge concern you? If I had seen that while shooting, I would have cranked the exposure down, for sure. I can deal with it if the blacks are black, but if the whites are blown, that shot is a goner, in my book. However, the final results look good enough.

---Bob Gross---

[tim]

Does it concern me? Yes. Any spike on the right is bad, and I shoud have reduced the EC. I didn't notice it when I took the shots or i'd have taken the EC/FEC down too. I probably checked the first few shots then didn't check after that - i'll not make that mistake again.

I'd still love to know what caused the problem, I suspect partial metering on the wrong part of the scene didn't help, and the flash should have been FEC-1 or so like PacAce suggested.

[robertwgross]

At the time, what was your exposure compensation set for? There is a trap here, as you might have set it positive to make the whites whiter, or you might have set it negative to make the blacks blacker. It's the sort of thing that drives a wedding photographer to drink.

Also, metering mode always gives me a headache. Virtually anytime that I stray off Evaluative, even for good reason, it bites me in the butt. <sigh>

---Bob Gross---

[tim]

I just edited a spelling mistake in my first post - there was no EC or FEC. If anything i'd have set it down because there's so much black in the scene, but since I was on partial metering I didn't bother.

I tend to use partial metering most of the time, especially for theatre stuff, I should get back onto the other modes and just watch the histogram/screen more closely until i'm used to them.

[tzalman]

tim -
Here's a possible explanation, but first a question.
Are you using CF4/1? If so and you set the focus on the red dress with the * button and then recomposed before pressing the shutter release, the partial metering would have been primarily on the black clothing of the gentleman in the center, which would have caused the overexposure.

[tim]

Nope, CF4 is on 0, exposure and focus were set by the standard shutter button. I suspect the centre focus point was pointing at a black suit in this case, and will have covered some of the suit, some red cloth, and a little white. I could post it the circle as displayed by EOS viewer in a couple of hours when i'm home if that'd help.

[robertwgross]

One suggestion would be to spot meter the flesh tones, and then let the lights and darks fall where they will.

Alas, I don't have any spot meter.

---Bob Gross---

[tim]

Yeah, a meter might be nice, but I think i'd rarely use it so i'm not sure if it'd be worthwhile.