Histograms in post-production

[Abitconfused]

In Photoshop CC Camera RAW the histogram can be altered using the mouse. This is convenient but I notice that the height of the histogram changes as it is altered. We are told that the height of the histogram indicates how many pixels are located in any tonal area of the image. If this is correct, then why doesn't the image's file size change in proportion to alterations made in the histogram?

[Manfred M]

Simple answer is that the distribution of pixels changes, but the total number of pixels that makes up the file does not.

[Dave Humphries]

Because "it doesn't work like that", to over simplify.

File size depends on a lot of things, file type being one of them, any compression (lossy or lossless) applied during saving.

If a specific pixel goes to a different value (e.g. from say 200 to 100); i.e. it's just no longer adding to the particular 'stack' that previously represented the tallest column, that (over-simplifying) just means that it is some where else within the graph, it hasn't 'vaporised' (creating a hole in the image where no pixel exists).

It could be postulated (I won't say 'argued') that the file size MIGHT be better determined by the area under the curve of the graph, which could be thought of as the sum of all pixels present in the image. i.e. if you count how many (the quantity of) pixels have a value of "0" (on extreme left of histogram), then add the quantity of pixels with a value of "1" and repeat 253 more times, ending with the quatity of pixels at peak white "255", then you'll (theoretically) have the total number of pixels in the image and it should match the h x w figure. Except it won't because the whole concept is impractical.

I'll shurrup at this point and see whether that helps.

Dave

[DanK]

In addition to the points already made:

the height of the histogram indicates how many pixels are located in any tonal area of the image.

I don't think this is quite right. Imagine that the scale on the vertical axis ran from 0 to the maximum number of pixels in an image from your camera. That's what would be required for the histogram to fit vertically in all cases. If that were the scale, then an image of uniform brightness would fill only an invisibly small range at the bottom.

For this reason, I have always assumed that the software rescales the number of pixels to fit. The software I use clearly doesn't do this perfectly; sometimes one gets peaks running off the top, and sometimes one gets an entire histogram that has low values. However, on he whole, it appears to be rescaling. If so, the heights of the bars indicate the relative number of pixels at a given tonal value, not the count.

A simple test of this (I am not at a computer with my software) would be to save an image in a lossless format twice, at different sizes, and compare the histograms. If I am right, they should be similar.

[Mike Buckley]

A simple test of this (I am not at a computer with my software) would be to save an image in a lossless format twice, at different sizes, and compare the histograms. If I am right, they should be similar.

A pair of lossy JPEGs of different sizes even confirms that you are correct.

[DanK]

A pair of lossy JPEGs of different sizes even confirms that you are correct.

Mike, thanks for checking.

Dan

[xpatUSA]

I'm finding it hard to follow this thread, sorry.

I've never been able to click on a histogram with the mouse and alter it in any way. Is the assumption that we are adjusting Levels or something and observing the effect on the histogram?

It's not clear to me which type of histogram is being discussed. Luminosity or RGB or what?

Confundido . . .

[Manfred M]

I've never been able to click on a histogram with the mouse and alter it in any way. Is the assumption that we are adjusting Levels or something and observing the effect on the histogram??

Adobe Camera Raw does let one make adjustments this way, by clicking on the luminosity histogram and depending on where on the the x-axis one clicks, the left / right movement corresponds to moving the blacks, shadows, exposure, highlights or whites slider.

The history and the individual sliders are shown in this screen shot.

[xpatUSA]

Adobe Camera Raw does let one make adjustments this way, by clicking on the luminosity histogram and depending on where on the the x-axis one clicks, the left / right movement corresponds to moving the blacks, shadows, exposure, highlights or whites slider. . . .

Thanks Manfred, last version of ACR I ever used was the cut-down version supplied with Elements 6. If it had that function, I either never knew, or never used it.