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
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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.
Oh yes, dt’s ability to work with these contrast equalizers is fantastic.
But I don’t want to complain, ART offers me so many good options that I haven’t exhausted yet.
latitude is a parameter of filmic, not of contrast equalizer. Please read our replies a bit more carefully.
It’s all a bit too much for me. You experts may take all this for granted. I had never heard of latitude before.
I just wanted to express my delight that I have finally discovered a significant difference between Sigmoid and Log Tone Mapping in ART. And if my statement is not correct, I would be happy to be proven wrong.
It is well known that dt can do great things and works differently. But that goes a little beyond the scope of this topic.
The point is Sigmoid has a straight linear portion and something similar can be introduced in filmic and it explains some of what you are commenting on about contrast. It is is not part of log tone mapping in ART and so for that you also might need to use an addition tone curve… it wasn’t meant to promote one program over the other… just noting this for clarity
You’re getting lots of replies ignoring the ART tag and that you’re interested in ART. People are talking about a different software altogether without even explaining why. In addition they are intentionally or not using convoluted language that should be reserved for development discussions. Just note the names and remember when not to worry to much about their posts.
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We brought examples from another software because sigmoid exists there, too. I do respect that this is an ART topic. The use of the term latitude only came up because that is what one of the filmic parameters is called. I think filmic was mentioned only to point out that the two approaches (a log mapping and then a separate contrast curve vs an all-in-one parametric curve) are not all that different. None of that was done to confuse anyone or to promote darktable in an ART topic.
Just note the names and remember when not to worry to much about their posts.
I think that was uncalled for. Everyone who responded had good intentions.
I’m on the phone now. Once I get to the PC, I’ll try to create a simple, illustrated explanation.
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What wonderful words to my ears.
I often don’t understand what these people are saying and I don’t understand why they are saying it. What you say here relieves me enormously.
Thank you very much for your comment.
Of course I am also open to new ideas.
If someone says to me: Listen, what you want to do here with ART works much better with xy, why don’t you try it out? Look, I’ll show you how it works, then of course I’d be interested in taking a look.
Even if I would prefer to stay with ART. I am deeply convinced that ART can do much more than I can. I’m also convinced that it’s much more important to photograph better than to develop even better.
The sine qua non of photography ! 95% should be done when you press the shutter button. The last 5% can be done in software.
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I’ve seen many examples of various software (FOSS and proprietary) made to “walk and talk” by highly skilled operators. But as impressive as that is, I always have to wonder if / why the shot wasn’t better exposed / composed. Who knows, there may have been good reasons.
Then again, it’s more interesting to watch a miracle being performed than watch ordinary stuff, so… 
That said, I’m glad there are “miraculous” tools available, given how I shoot sometimes! LOL
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If someone exposes badly or chooses an unfavorable section, it is rarely intentional, but almost always an inability. Either the photographer couldn’t do any better or the situation made it impossible to make the necessary adjustments. But sometimes there are also situations where there is no better way if you don’t want to miss the shot.
Good software should and can help you get the best out of your photos.
And that brings us back to an old topic:
ART, RawTherapee and darktable: all three are very good. And if poor developments are made, it is certainly always because the user was too sloppy or too inexperienced. But as far as I know today, it is completely impossible to achieve significantly better results with one of the three than with the others.
The crucial question is: How do I personally cope with the software?
I’ve admired darktable for years - but if I take a break for a few weeks, I’ve forgotten a lot of important things. In short: I’ve always struggled with my understanding and mastery of dt.
RawTherapee was my absolute favorite for years. I could do anything with it and would be happy with it for the next 100 years, but not when the project stalled like this, that worried me a little, especially in terms of support for new cameras or lenses.
And that’s where ART came in: Fresh, clear and intuitive. After several attempts, I manage to achieve the quality of RT. And ART is thoroughly likeable. And then there’s Alberto. I can’t believe how much and how quickly he helps everyone, even beginners. And as soon as someone draws his attention to a bug, it is usually fixed very quickly.
What more could you want? I don’t need any more, ART offers me everything I can only dream of.