Exposure - Single Colour Photos - What to look for in a histogram

[Brownbear]

When one is taking a photo of a single coloured object, say a red flower (it could also be a green or a blue flower ) what should one look for in the histogram? In the RGB graph and the single channels (blue, green or red) And in the luminosity graph?

For example in the case of a red flower, I would expect that I should look for the histogram to cover the entire base of the graph with no clipping on either end, with the central portion of the histogram at the middle or towards the right... Since red is a primary colour do the blue or green channels matter at all? And if it was a blue flower do the green and red channels matter at all?

Then again I'd have to assume that it was a pure blue or a pure red flower that I was photographing and that is likely not correct.

And what about green? ie yellow and blue make green but our cameras don't have a histogram showing yellow.

Thank you.

[Dave Humphries]

Hi Christina,

In such cases I would only pay attention to the colour channels that are relevant, so red for a red flower, etc.
I would ignore the luminance histogram and the other channels too - unless say, there were brightly lit green leaves also in shot.

A yellow flower would mean both red and green channels are relevant since the yellow light hitting the sensor can be thought of as being composed of red and green - not actually true in real life, but the yellow you see below (from your screen) is.

Purple is comprised of red and blue.
Cyan (a pale blue), is blue and green.

Have a look at the Additive Colour Mixing diagram here;
https://www.cambridgeincolour.com/tu...perception.htm, it may help.



By the way, I wouldn't expect to see the hump spread across the entire width, it is likely to be biassed to the right and all you should do is ensure it isn't 'climbing the wall' on the right.

Here's one:


It does extend to left because it was flash lit and the background colours fall right off in the shadows and distance, you will see that if I had exposed for the luminance histo, I would have over exposed the red channel significantly.

Cheers,

[Brownbear]

Hi Dave,

Thank you so much! This question came about because I bought my Mom some flowers for Mothers' Day and of course they were pretty, so I had to try a few photographs before handing them over... And they included pure green and red flowers and while checking out the histogram I realized that I did not understand what I was looking for when for single colour shots.. (no green leaves just the flower)

A wonderful explanation! Thank you for the link, I missed that tutorial somehow

[Inkanyezi]

It is always helpful to look at the separate channels to understand what might be "clipping" - and remember, clipping occurs only at the right side of the histogram. At the left side, it is just black, and anything that emits less light than the minimum that can be recorded is pure black. The histogram may rise at the left side, you should not expect it to always reach the base line at or before the lower left corner.

The interesting part of the histogram mostly lies within the rightmost quartile and particularly at the very end, at the lower right corner.

If the histogram ends in the lower right corner, there is plenty of data in the brightest parts of the image, and none of it is clipped. Clipping occurs when the rightmost histogram part starts to climb the wall at the right. Only in an image where you would allow clipping should you let the histogram reach any height at all at the right side. Ideally, it should mostly end precisely in the corner, except in a low key image where you don't want any highlight at all to reach saturation, or in images where you allow some highlights to burn out completely.

If there is plenty of empty room to the right, but only the baseline, it indicates that exposure is low. The image is dark. It may be an indication of under-exposure.

The three histograms for each colour channel may be used in the same way as the histogram for all three, but it indicates which channel that would clip. In Dave's image above, it would surely be the red channel that goes to clipping if exposing too much, while the other two have plenty of headroom.

When shooting flowers, mostly the red channel, but sometimes also the blue, has the highest risk of clipping. I have yet to see the green one clip without the red also clipping (saturated yellow). Doesn't usually happen.

Particularly when shooting saturated colours, as red flowers, the individual red histogram is of great value, as the highlight warning "blinkies" mostly will not kick in unless two or all three channels clip. The highlight warning will not tell you that only one colour channel is saturated.

The leftmost side of the histogram indicates a single thing, whether the actual dynamic range covers the entire scene or if there is some part of the image that is black. A histogram climbing the left side shows that there is pure black in the image. A histogram that ends before the left side and leaves a bit of pure baseline to the left, indicates that the sensor's dynamic range was sufficient for the scene, if the right end of the histogram also ends in or before the lower right corner.

[Brownbear]

Thank you for the detailed, informative and easy to understand reply. Extremely helpful.

[Ken MT]

This is an example of a peony, underexposed by 1 stop. However there still appears an area where the colour is partially washed out, but I note that even in the field the colour to the eye is similiar

[rambler4466]

Well, I took a close-up image with the entire frame filled with a green leaf. In the color channel box when I disabled the green, the entire leaf turned red! If you look at the colored sliders you will see yellow and blue on on. So, to lessen the blues push the slider toward the yellow, etc. Using Photoshop Curves, one can also adjust the RGB colors individually.

Another way to see the colors is to use the Temperature slider in Adobe Camera Raw. Move it to the extreme left and you will see a lot of blue in that red flower. For example, I have an image of a bright red flowering tree with only a few green leaves showing. With my eye I see no blue. Lower the "temperature" and a lot of blue hue appears.

I believe the histogram also reflects the intensity or brightness of the color. A washed out red with little detail will be spiked to the right, an underexposed or dark red will be spiked to the left. Notice that the histogram is showing the the green is dark in Ken's image above. Notice that the red is also touching the right edge just a touch which relates to some of the red on the bloom being washed out, you can see the overexposed spots on the petals.

[revi]

One more thing to note:
if there are saturated dark colour in the image, it might not be possible to avoid "under-exposing" one channel:
a predominant colour means that light of one colour is missing (the colour opposite or complementary to the predominant one).
The channel in the histogram corresponding to that colour will appear to be underexposed (it isn't, really)...
E.g. a bright red flower on dark green leaves: the red will be far to the right (from the bright flower) and climbing the wall on the left
(the green leaves, dark and little or no red).

Something else to note when comparing the histograms:
the luminosity channel is very heavily influenced by the green channel. (Iirc the weighting of the R:B:G channels is something like 20:10:70,
so green has more than 3 times the influence of red. That means that for any scene with saturated colours other than green, you're better off
checking the colour channels and keep a close eye on your exposition, which is based on the luminosity values afaik...

What makes this kind of clipping in saturated blues or reds especially unpleasant is that there is no information in the other channels to restore part
of the detail in post production (I know, poor technique to rely on post, but it happens sometimes, and then it is nice to have at least something to
work with; won't work for those nice flowers though, as there is nothing).

@Frank: I think the washed out bits you refer to are actually highlights. Over exposing in one channel only does not lead to washed out colours
(for that you need to have a contribution of the other colours). What happens is that you lose detail in the overexposed areas
and that they look 'flat', very easy to do with flowers, as they tend to have bright colours that contrast well with green...

[Inkanyezi]

I think there is a lot confusion of the terms "over-exposure" and "under-exposure" and how those are related to the histogram.

The histogram can give a hint of both those conditions, but none of them is in any way related to the leftmost part of the histogram. Many people think of lots of data piled up to the left as if it would be under-exposed, but it is not so. It is to the right we can see signs of either over- or under-exposure.

And when we talk about "washed out", generally all three colours are clipping, while the condition of only one of them clipping will cause the "flat" appearance that you can often see particularly in the petals of red flowers, and two channels may also clip when a flower is a violet tint, which can often be observed in rhododendron flowers.

When any colour channel is clipping, also the luminosity histogram will show it, but you might not be alarmed, as the rise to the right may be small, if the clipped area is not very large. And as I pointed out before, the highlight blinkies will not kick in with most cameras until all three channels clip.

I think we should not get fooled into thinking that we under-expose by data piled up to the left. Under-exposure is not indicated in the histogram in any other way than in the rightmost quartile, where the histogram of an under-exposed image is flat at the bottom with no data at all; an empty bit of only base-line with no ripples. The left part of a histogram piling up to the left only indicates that many of the pixels in the image are dark.

[Brownbear]

Thank you Frank, Remco and Urban... Somehow I missed all this additional wonderful extra information until today...

Truly appreciated.

[xpatUSA]

I've been converting flower shots into ProPhoto TIFFs lately and have noticed colors that are certainly out of the sRGB gamut and probably Adobe RGB (not that I use it). It's quite a game to get such colors nicely into those smaller color spaces, especially with my older editing software. For yellow flowers in sunlight, although there are blues in ProPhoto, many methods will render blues as bottomed out at zero, I've found. For a purple flower recently shot, it's the greens than got zapped. Still trying out all kinds of ways but, in answer to the OP, I would recommend checking the individual R, G and B histograms for an image or using a histogram that shows all three colors at the same time. Luminance and RGB histogram can be mis-leading, I reckon, as revi has pointed out.