I think if you have weird lighting sigmoid is awesome and it might also be nice for things like flowers and other images like sunsets and the like so lots of applications where it will shine and not much to learn about it … so I say use it when needed and learn Log tone mapping as it is pretty powerful…
You already have a good start…lots of good material in this thread you started…
I find Sigmoid (both ART and dt) amazing in how it makes EXPOSURE into this incredible tool.
That is really just it… Sigmoid is about great midtones, well saturated… then a nice roll off to white and black…
The thing is with all this sort of adjustment is for example my monitor has a value that is middle gray for its calibration and yours has one as well based on the same but they can be different … middle gray as a “broad” concept can really be thought of as a region where the your image is nicely exposed. So very very loosely based on our taste and hardware we are going to add or subtract exposure our images to make the “gray” in our captured data match what our screen will display as gray… Then with sigmoid that is set and contrast is simply a rotation around that making the curve steeper and compressing more the highlights and shadows. The skew gives you a way to offset how harsh that can be on highlights and shadows a bit at least so that you can come to balance between any compression you have introduced and to some degree the roll-off compensation using skew… With the log tone mapping you can move “middle gray” around in the module but as Alberto says changing the source gray is like changing exposure compensation so with sigmoid then you need to use exposure to move things not a setting in the module itself… so a bit of a nuance…
ART’s Log Tone Mapping is a simplified UI version of darktable’s Filmic RGB.
And darktable’s filmic RGB (or ART’s log tone mapping) and sigmoid tone curves are different. You can compare the differences between Sigmoid and Filmic RGB in the following article by J. Andren, the original author of the Sigmoid Tone Mapper.
The “log-logistic” curve in his article is the same as a sigmoid curve, and darktable filmic may be the same as ART’s log tone mapping. Filmic has a steeper curve in highlight areas than Sigmoid, but a gentler curve in shadow areas.
J. Andren claims that sigmoid has a more filmic curve than filmic RGB.
That’s (quite) a bit misleading. See also my reply to the thread you linked. Sigmoid is an S-curve (as the name says). Log tone mapping has no S at all 
I recall going back you explained things nicely here…
Thank you for correcting my misunderstanding.
Now I have observed a big difference between Log Tone Mapping and Sigmoid:
Log Tone Mapping creates very even brightness gradations across the entire range. Even in the shadows and highlights:
Whereas Sigmoid produces much more contrast and clearer gradations in the mids. However, at the cost of less contrast in the shadows and highlights:
You can achieve similar results with both tools, but if you want attractive dynamics and contrasts, especially in the mids, then Sigmoid is better suited.
However, if you also want brilliant details in the dark and light areas, Log Tone Mapping is the better tool. I also prefer it because I have many more setting options than with Sigmoid.
A request to all connoisseurs: If what I am writing here is not correct, please correct me.
When you are mapping a camera-native dynamic range, say 14 - 15 EV to a display-native dynamic range, for example ~10 EV, you either map it ‘straight’ (for an input change of ~1.4 EV you apply an output change of 1 EV), which compresses contrast uniformly everywhere, or you pick an area (or more, but usually just 1, in the part of the input dynamic range that is most important for you), keep/enhance contrast there, but then compress / lose contrast elsewhere. That is what you see above. It’s not just sigmoid, it’s all curves. You have a (contrast) budget, and you decide how you spend it.
Sigmoid is/has a curve. When using a ‘pure’ log you have to add the curve/contrast by other means. Say the tone curves for instance.
Using Simoid and a tone curve will over do you contrast in most cases.
You should be able to basically repeat the sigmoid example by adding a tone curve. Sigmoid does clever and helpful things to make things look good but it shouldn’t be impossible to get the same result.
Hello István,
That sounds very interesting. But how do you go about selecting an area that you want to keep in contrast and sacrifice the others for it? Is there a special technique for this?
Do you know whether the results of Log Tone Mapping can also be achieved with the curves?
You can’t compress dynamic range with curves*. Curves just shuffle around what’s between those endpoints. You can’t so to speak move the right part of the curve even further to the right if you see what I mean. Log pushes the data beyond the “ends of the curves” into the scope of the curves to that you can shuffle the tones around.
The exposure tool will move that end point but only in one direction at a time. Increase exposure > loose highlights. Log and some other tools allow you to push data from both the black and the white side into the “view” of the curves.
*not great wording actually but simplified
Wonderfully explained, I understood it perfectly.
With sigmoid, you can adjust the contrast and slide the skew.
As @nosle explained, you can use e.g. a log curve to map the unbounded brightness range to 0 … 100% (screen brightness) range, and then use a ‘bounded curve’ (one mapping 0…100% to 0…100%) to distribute it tonal range any way you want.
Sigmoid and darktable’s filmic do both (that’s why they have settings for contrast and related controls like ‘skew’ (sigmoid) or ‘latitude’ and ‘shadow - highlights balance’ (filmic).
Bottom: your ‘sigmoid’ image; top: your ‘log’ image with the tone curve on the right applied in darktable. Not a 100% match, but it could be refined. I hope this provides a visual explanation.
In DT this is what the latitude is for…you can introduce a strait segment to the curve and adjust the slope of that… the compression each way happens outside those boundaries… I think you can use the regularization a bit in ART for some local contrast but I think you have to add it back with a supplementary tone curve after to areas or a zone where you want contrast…
Have you really placed the two grayscales of mine on top of each other here? They look practically identical.
Which tool did you use to match them so perfectly?
He has just put a tone curve on it… using the picker on your image bars you could adjust points to match visually once you have the spot on the curve from the picker…
A tone curve that you see on the right, next to the two grey scales.
