Post-production black point & white point question

[Abitconfused]

In many post-production programs the photographer can establish a black point and a white point to correct a deficiency in an image. I might use "Black" and "White" sliders or a Levels tool to move sliders to a point on the black side and white side to suit my taste (or lack thereof). Is there any substantive difference between these two methods?

[Manfred M]

Each software supplier can (and does) implement functionality somewhat differently, so without know which specific tool you are referring to, it is impossible to give a definitive answer.

What is important to understand, all of these functions are effectively simplifications of the "curves" tools.

In general, I would say the answer is maybe. In some software the black and white sliders allow tweaks in the shadows and highlights part of the image; but the adjustments in the rest of the areas (mostly the quarter tones through three-quarter tones could be affected differently). They may or may not affect saturation as well as brightness levels. Unless you have some way of reading what the adjustment you have made, it's hard to tell.

A black point and white point adjustment with either a curves or levels tool have a different function. Assuming that pure black is on the left side of the slider and pure white on the right, when you move the values, everything to the left of the black is assigned a value of 0 (pure black) and conversely everything on the right hand side of the white point is assigned a value of 255 (pure white).

[Abitconfused]

I use Photoshop CC and ON1. I also use PhotoLab 3 and Topaz but it all gets a tad ridiculous at some point. Each has something I like a lot! ON1 has curves and tonal sliders as do they all. Interestingly, ON1 has Curves as a "Filter" attachment and this is as good as any of the others.

[xpatUSA]

In many post-production programs the photographer can establish a black point and a white point to correct a deficiency in an image. I might use "Black" and "White" sliders or a Levels tool to move sliders to a point on the black side and white side to suit my taste (or lack thereof). Is there any substantive difference between these two methods?

Most 'Levels' tools allow more than just the two sliders mentioned above. For example, after the input black and white levels have been set, the mid-point can be adjusted. Furthermore, the output levels can be adjusted such that what was black becomes for example a dark gray and similarly white can become a light gray. For sure, simple "Black" and "White" sliders can not do all that. So they are indeed substantially different, IMHO.

Manfred mentioned Curves tools. They all vary, but in one of my apps, the type of curve can be selected e.g. Linear provides no curve at all: - others are Standard, Flexible, Parametric and Control Cage. Way beyond just "Black" and "White" sliders and beyond Levels, too.

You pays yer money and takes yer choice ...

[Manfred M]

I use Photoshop CC and ON1. I also use PhotoLab 3 and Topaz but it all gets a tad ridiculous at some point. Each has something I like a lot! ON1 has curves and tonal sliders as do they all. Interestingly, ON1 has Curves as a "Filter" attachment and this is as good as any of the others.

I do virtually all of my adjustments using curves; a curves adjustment layer with layer masks (using selection tools) in Photoshop. The only thing I can't handle with curves are some saturation issues and there I use the Hue / Saturation adjustments (almost always reducing the values, but in individual colour channels; never globally), again as an adjustment layer. I will use different blending modes, as required.

If you master those basic tools, you really don't need too much else.

[Abitconfused]

Manfred, Do you use Curves channels other than RGB?

[Manfred M]

Manfred, Do you use Curves channels other than RGB?

Yes I do, but usually only globally to fix specific colour issues. It works very well when restoring old colour prints where the dyes have deteriorated. No more than two colour channels are used.

[DanK]

I use Photoshop CC and ON1. I also use PhotoLab 3 and Topaz but it all gets a tad ridiculous at some point. Each has something I like a lot! ON1 has curves and tonal sliders as do they all. Interestingly, ON1 has Curves as a "Filter" attachment and this is as good as any of the others.

I'm going to make a suggestion: until you've really mastered this, do your tonality adjustments in one software suite so that you get a really solid understanding of what each tool is doing.

What I've found in teaching this stuff is that for most people, the key is learning to think about how the histogram relates to the image and how they want to change the histogram. The choice of tools should follow that. Some people find that this sounds too technical, but in my experience, it's the key to gaining control.

For example, you may decide that for a given image, the brights are too dark. So you start by pulling up the bright end of the histogram, which you can do several ways. In Adobe software, the cleanest is to move the white point either by using the input adjustment in the levels tool or by moving the top anchor point to the left in the a curves tool. ACR and Lightroom also have a "whites" slider, but if you play with all three, you'll see that they don't work the same. The ACR tool is designed to move a region of the histogram, stretching the rest as needed, while the other two tools move a point on the distribution and then stretch the histogram. The results can be noticeably different.

This gives you a new image and new histogram, so you move on from there. For example, how are the midtones once you've stretched the histogram? Stretching the histogram will increase global contrast, but is the image still lacking in contrast? The answers to these sorts of questions determine which tools are best to use next.

The curve in particular can be used in very sophisticated ways. For example, a student recently sent me an image that was lacking in contrast but that had areas that were about as dark as one would want as well as a few areas that were if anything too bright. An S-curve over the entire histogram would increase midtone contrast but at the cost of making the brights too bright. What I suggested was to impose a curve on the bottom half or so, only slightly darkening the bottom and then raising the midtones to get contrast in that region, and then adding an an anchor to leave the top third unaltered. I sent it back to him with a screen shot of the curve and asked him to think about what he would want. There's no end to the ways you can fiddle with this, but the key is being able to think about what parts of the histogram you want to affect, and in what ways.

[xpatUSA]

For example, a student recently sent me an image that was lacking in contrast but that had areas that were about as dark as one would want as well as a few areas that were if anything too bright. An S-curve over the entire histogram would increase midtone contrast but at the cost of making the brights too bright.

Good point. Upon occasion, I have also imposed a reverse-S curve in, for example, a landscape with bright clouds and dark terrain, so as to get more shadow and highlight contrast, of course at the expense of any mids.

Drifting off a bit, but related, if one looks at DPR's plots of camera dynamic range, one will see that many cameras have a S curve in their plots of image tone versus input EV. This increases the dynamic range (measured that way) of the capture - but at the expense of shadow and highlight contrast. A bit of a fiddle, IMHO, and I personally will select the straightest line possible.

https://www.dpreview.com/reviews/nikon-d800-d800e/19
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[Manfred M]

Good point. Upon occasion, I have also imposed a reverse-S curve in, for example, a landscape with bright clouds and dark terrain, so as to get more shadow and highlight contrast, of course at the expense of any mids.

Drifting off a bit, but related, if one looks at DPR's plots of camera dynamic range, one will see that many cameras have a S curve in their plots of image tone versus input EV. This increases the dynamic range (measured that way) of the capture - but at the expense of shadow and highlight contrast. A bit of a fiddle, IMHO, and I personally will select the straightest line possible.

https://www.dpreview.com/reviews/nikon-d800-d800e/19
.

Ted - a little caution here. The article discusses JPEG output and camera manufacturers have always applied some level of contrast control, lens profile corrections and sharpening in the JPEG images. This is (hopefully?) not the case with raw data.

[xpatUSA]

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