How to use R/G/B Histograms in the Field?

Brendan

A little while ago, I upgraded to a D80 and gave my D70s to my girlfriend. One of the features on the D80 I want to learn how to use better is the R/G/B histograms (the individual channel histograms).

I understand, to some extent, what it means if there is clipping on one channel (red seems to do this a lot), and understand to some extent how to correct this later in Photoshop.

I guess what I'm confused about is how to really make good use of this when I'm out shooting.

If you chould share some thoughts, examples, etc., that'd be awesome.

Thanks!

—Brendan

pathfinder

My cameras are set to display the three channel RGB histogram at all times. I can quickly check for loss of highlight detail in a channel with just a glance. I find this much more useful than guessing with a white single channel histogram.

If one channel is over exposed, I then, have to decrease exposure, or accept some loss of highlight detail in one channel. It is true, that by shooting RAW, one can regain some of this data, but it is better not lost in the first place.

One good use of this is when shooting gray cards for custom white balance. You should end up with a histogram with three spikes ( the red, green, and blue channels ) lined up vertically in the middle of your histogram window.

Blackwood

pathfinder wrote:

One good use of this is when shooting gray cards for custom white balance. You should end up with a histogram with three spikes ( the red, green, and blue channels ) lined up vertically in the middle of your histogram window.

Great tip. Thanks.

Brendan

Eek... I haven't followed up on this in a very timely fashion. Life is busy. :X

pathfinder wrote:

My cameras are set to display the three channel RGB histogram at all times. I can quickly check for loss of highlight detail in a channel with just a glance. I find this much more useful than guessing with a white single channel histogram.

I get this part, I think.

If one channel is over exposed, I then, have to decrease exposure, or accept some loss of highlight detail in one channel. It is true, that by shooting RAW, one can regain some of this data, but it is better not lost in the first place.

This is the part I'm a bit confused about. Are the only solutions to either accept the clipping or decrease exposure? I guess this makes sense on a technical level, but part of me wants to think that because the histogram shows three channels, there's a way to get a proper exposure for all channels in the field. Perhaps not..?

Wouldn't this involve a trade-off as well: either accept clipping, or decrease exposure, which would result in higher noise if you go raise the exposure again in post-processing? Which is preferable? I would tend to think that I'd rather have a little bit of clipping on one channel than increased noise from processing, but if anyone has some examples/etc. I'd be interested to hear it.

One good use of this is when shooting gray cards for custom white balance. You should end up with a histogram with three spikes ( the red, green, and blue channels ) lined up vertically in the middle of your histogram window.

This is useful information -- thanks!

—Brendan

jfriend

Biarien wrote:

This is the part I'm a bit confused about. Are the only solutions to either accept the clipping or decrease exposure? I guess this makes sense on a technical level, but part of me wants to think that because the histogram shows three channels, there's a way to get a proper exposure for all channels in the field. Perhaps not..?

Wouldn't this involve a trade-off as well: either accept clipping, or decrease exposure, which would result in higher noise if you go raise the exposure again in post-processing? Which is preferable? I would tend to think that I'd rather have a little bit of clipping on one channel than increased noise from processing, but if anyone has some examples/etc. I'd be interested to hear it.

It is possible to adjust the channels relative to one another. That's what happens when you change the white balance. It moves one channel relative to another (increases red, or increases blue, etc...).

But, what you typically want to do in the field is set the white balance to what you think is an accurate setting for the white balance and then turn up/down the exposure until you don't have clipping, but don't have a big empty space on the right side of the histogram.

There is no goal to fully expose every color in the histogram. An image with lots of red and little blue or bright reds and only really dark blues will have a red histogram all the way to the right and have an empty space on the right side of the blue histogram. That's the way that image is supposed to be exposed.

Clipping is generally the worst choice because when a color is clipped, detail in the brighter parts of the image is irreversibly lost and can never be recovered. The only time I accept that clipping is if I want to blow all detail in that part of the image. This happens sometimes when shooting a subject in the shade with a sunlit background. The only way to get a semi-decent foreground exposure is to blow the detail in the background. Of course, it's better to compose so this isn't an issue, but if you have to, that's what I do.

For a normal shot, turning down the exposure by 1/2 stop to stop the clipping will not introduce enough noise that it makes a worse picture. You will generally rather have detail in your highlights and then restore shadows in post processing.

pathfinder

Biarien wrote:

This is the part I'm a bit confused about. Are the only solutions to either accept the clipping or decrease exposure? I guess this makes sense on a technical level, but part of me wants to think that because the histogram shows three channels, there's a way to get a proper exposure for all channels in the field. Perhaps not..?

Wouldn't this involve a trade-off as well: either accept clipping, or decrease exposure, which would result in higher noise if you go raise the exposure again in post-processing? Which is preferable? I would tend to think that I'd rather have a little bit of clipping on one channel than increased noise from processing, but if anyone has some examples/etc. I'd be interested to hear it.



This is useful information -- thanks!

—Brendan

I have been thinking about this question, and i think there may be a third alternative. If all three channels are blown, you will lose all detail, so you want to avoid that. If only one channel is blown ( an if it is only ONE CHANNEL) You should be able to dial back the Saturation slider for the hot color in the RAW converter, and thereby diminish the exposure slightly I suspect. This could be tried with a deeply saturated red rose.

I have not actually tried this, as I am inclined to dial back my exposure if one individual channel is at risk of being blown. But I am pretty sure it will help. For out of the camera jpgs, this is of no help at all.

My 40D has the ability to display the individual RGB channels, as well as a white light histogram. If you use this display, it becomes apparent very quickly that the white light histogram will still look "safe", while the red channel is showing a blown spike on the right side of its histogram. This tells me the the three color, R,G,B histogram is more trustworthy than the white light version - at least on my 40D!!

Elaine

Something I'm confused about related to histograms...aren't they based on an in-camera jpeg, even if you're only shooting raw? If this is so, it seems that some of your camera settings (such as picture styles) may play a role in you deciphering your histogram/LCD. Can anyone sheld some light on this?

Scott_Quier

Elaine wrote:

Something I'm confused about related to histograms...aren't they based on an in-camera jpeg, even if you're only shooting raw? If this is so, it seems that some of your camera settings (such as picture styles) may play a role in you deciphering your histogram/LCD. Can anyone sheld some light on this?

I was wondering the same thing as I also understand that the histogram is an interpretation of the converted JPG. This is why, when you shoot RAW you have the ability to recover some of the "blown" highlights - because they are not not actually blown in the RAW data, just in the camera converted JPG.