Adobe editing software the most used in the world for a reason.

I don't trust in-camera histograms to indicate clipping in raw, and most of the cases in low-light conditions (exposure-limited I think would be a better term) I don't have time to check those, or even want them obscuring my viewfinder ;) Fine-tuning ISO setting also takes time which I often don't have.

hmmmm.....

But you in another thread asking the silly questions

and

when neither of the histograms was a raw histogram 🤣 does not qualify you as a 'real' photographer, let alone a supposed professional one you claim to be.

He's not going to, is he. Hundreds of posts on this subject and when pinned down to a simple binary choice, no answer.

Oooh, a DarkTable user. I find it incredibly complex and not the least bit 'user friendly'... and yet, now that I have started down that path, I don't want to move to something easier.

I have been intrigued by the masking abilities built into it, but its not easy to figure out if you don't have the time to commit to digging deep. I understand you have the ability to create a mask in one module and use it in another. That alone seems very useful.

He doesn't know the answer. I don't believe he is a photographer.

I am aware the in camera histogram is not a raw histogram but in my experience is a close approximation and in most cases I know I can about 1/2 stop max extra exposure* above where the camera histogram shows clipping before the actual raw data is clipped.

But yes, to be honest, if I have time I will raise ISO but it's not often.

After much messing about over several years, getting mighty confused which modules to use for what, I eventually worked out a simplified workflow. And the new UI customisation feature lets you set the UI up to hide all the confusing stuff 🙂

Masking is complicated and there are obviously simpler or harder ways of using it.

When you create a mask (eg a circular mask in the exposure module), this mask is remembered and stored in a drop down list in the masking tab. You can duplicate subsequent masks by simply selecting one you created previously from the drop down masks. The new mask won't remember any refinements like feathering, you have to add them again. Masks also get stored in the mask manager module and over there you can combine masks (similar to using intersection in the new LR masking feature).

If you want to take this to another thread, I can share my understanding and how I use dT.

And the answer is......?

Ok, I'll answer my own question. I've prepared this as a practical rule of thumb rather than a technical dissertation. If the experts can confirm the broad understanding before launching into the details, I'd be grateful.

THE ISO KNOB, WHAT IT DOES AND WHEN TO USE IT

This is a summary of what I think I have learned over the last few days about the ISO knob and what it is useful for.

1. The total noise in an image is derived from multiple sources. Without going into the fine detail, I will simplify this by saying that one source of noise comes from the quantum nature of light itself and can't be bypassed or avoided by tech improvements: it's basic physics. We call this noise photon shot noise

2. Another kind of noise is noise generated by the camera electronics: for simplicity I'll group this kind of noise and call it read noise

3. The ISO knob does fancy things depending on the implementation in specific camera models. The primary benefit raising ISO has is it reduces read noise. It does not reduce photon shot noise.

4. One way of thinking of the ISO knob is as an in-camera read noise reduction knob.

5. Older generation cameras (such as my CCD Nikon D100 6MP DSLR) had a lot of read noise. It made a significant contribution to the total noise in images.

6. When shooting in low light, you often risk less than ideal exposure (ie underexposure) if you raise your shutter speed to avoid camera shake

7. In the older cams, underexposure leads to high read noise. Raising the ISO reduces this read noise.

8. In modern sensors, read noise is generally quite low and is overwhelmed and drowned out by shot noise. Raising the ISO still reduces read noise, but because read noise starts out as a minor component of overall noise, reducing it doesn't achieve a great deal.

9. Therefore, depending on the particular camera and how it implements ISO, either boosting ISO does absolutely nothing at all to improve noise or it reduces an already modest amount of read noise. Perhaps equivalent to about 1/3rd of a stop in the deep shadows?

Conclusion: Whether you raise ISO, or just underexpose and brighten, in modern cameras it makes very little difference to the noise in your edited image. You will end up with a shot that looks almost the same.

Pick whatever method you are most comfortable with or if you are a perfectionist, find out what the ISO knob does on your particular camera, and whether it is worth adjusting it to save on that last little bit of noise in the deep shadows.

The only way to significantly reduce overall noise is to increase the exposure by:

• increase the level of the lighting
• increase your aperture
• reduce your shutter speed

to get more light on the sensor and reduce the primary source of noise which is photon shot noise.

The ISO knob is a red herring for most practical purposes when setting exposure unless you are an absolute perfectionist about scrubbing read noise and then only in cameras where the ISO knob is used to reduce read noise.

Can be not very close. Couple of examples:
www.fastrawviewer.com/raw-histogram-for-culling
www.fastrawviewer.com/culling-raw-vs-jpegs
I try to minimize the risks, especially when lights are coloured or some significant in size objects in the scene are strongly coloured.

ok, thanks. Those examples are very helpful.

test shooting 4 stops then raising in post compared to raising in camera. just shot some test shots in my studio.

first shot iso 1250 raw processed ACR no processing
second shot iso100 not quite 4 stops under exposed.

conclussion the images looked near enough identical if anythin i would give the prize to the 100iso lifted in ACR close.
dont know what you wanted but i just dont shoot 4 stops under as i shoot live,.

i am trialing fast raw viewer , its not bad, might purchase it.

Thank you, you have confirmed my previous post. Take ISO out of the equation, it doesn't matter. Just set whatever shutter speed and aperture you need to get the shot and ignore ISO. You'll underexpose and get a dark preview but when you edit and brighten in post, the image will look about the same as it would if you boosted ISO. Or if it makes you feel better to boost ISO, do so but it doesn't matter.

These threads have pretty much all been red herrings, for practical purposes the only thing changing the ISO changes is the brightness of the preview on the back of the camera.

p.s.

It's funny how the key thing that seems to get overlooked in these threads in that the ISO knob only effects the read noise component. And that is drowned out in modern cameras by shot noise, so the effect of the ISO knob in modern cameras in mostly an illusion (potentially a tiny noise improvement for the perfectionist). Just as DPR have said for years.

Since I don't consider myself an expert, I will launch into details.
First and second generations of Nikon DSLRs (up to D3) were nearly ISO-invariant, raising ISO setting helped very little with read noise, half a stop at best. Learned it the hard way, and rather late, on my second year with D2X. Clipped a lot of highlights and missed a lot of opportunities because of my ignorance.

PS. Here is Bill's data:
www.photonstophotos.net/Charts/PDR_Shadow.htm#Nikon%20D1,Nikon%20D100,Nikon%20D1H,Nikon%20D1X,Nikon%20D200,Nikon%20D2H,Nikon%20D2Hs,Nikon%20D2X,Nikon%20D2Xs,Nikon%20D3

pretty much my conclusion from my test shots tonight. BUT i have to use iso all the time as i shoot live monitors. just tested AF and the boost on the a74 is amazing and iso makes no difference.

But doesn't raising ISO also increase the SNR since the signal is larger than the noise?

i will also comment that the colour tones of the underexposed 100iso lifted were more saturated and pleasing even though the noise was the same. good test and we both have learn someting new or at least confirmed some common misconceptions. modern cameras are awesome.