ETTR & under-expose: darktable manual inconsistency?

After editing my shots with OM Workspace for a few months, I'm learning darktable, and it seems promising, I like it. I've watched several video tutorials, and now I'm reading the user manual.

I have a newbie question regarding a sentence I find in the introductory section--darktable 4.0 manual suggests to take a well-exposed photograph, and states that:

Member said:

Where possible, you are advised to use exposure to-the-right (ETTR) techniques to maximize the amount of data available for processing. As a general rule of thumb, it is safe to under-expose all images by 0.5 to 1 EV (by reducing the ISO sensitivity if possible) even if the in-camera preview looks darker than expected (the preview is not the raw data).

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I don't get this. The two points seem contradictory to me. ETTR would imply over-exposure. Is the manual suggesting to over- or under-expose?

You are right the two phrases are contradictory, maybe the intent was to

- suggest to ETTR;

- relief that if you're not, and even underexposing a bit, you're still good.

But I don't edit their manual, you may want to write them, all in all it's an open source project and outside help is surely welcome.

There's much more than coding in an open-source project, writing a good manual is also very important! (and as difficult)

cheers,

Danilo

I've felt for some time that ETTR is a little bit of a confusing term. In some case, such as taking a photo of a landscape, exposing to the right to retain the highlights in the sky, will result in a darker looking image. Left to it's own devices, the camera might well overexpose the image and blow the sky out, so you need to add some negative exposure compensation to underexpose it, and keep the histogram stacked to the right.

So, whilst you're underexposing you're still practicing ETTR. IMHO ( and completely rank amateur opinion) a better way to think of ETTR is this is the area to watch for highlight clipping.

I get what you mean, Greg.

== EDIT ==

-- Following part should be ignored, because I was wrong. Compensation value depends on what value was set on camera. If you chose -0.3 EV, darktable would compensate with +0.3 EV. If you chose +1 EV, it would compensate with -1 EV. And so on. --

I suspect darktable developers see ETTR generally as over-exposure, though. In the exposure module, by default checkbox compensate camera exposure (-0.3 EV) is selected. When hovering over it, a hint is displayed:

Automatically remove the camera exposure bias. This is useful if you exposed the image to the right.

--
https://www.youtube.com/@solitone

daniloleica said:

you may want to write them, all in all it's an open source project and outside help is surely welcome.

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OK, I'll do it. I wanted to be sure to not be mistaken.

I'd be a little kinder to the manual writers who are, I think, trying to help in a practical way. ETTR is an aim or objective, which conflicting demands (including time) may make difficult to achieve.

Modern cameras in most lighting conditions have DR such that loss of 1eV is normally acceptable, particularly in relation to screen or print DR. So their rule of thumb is reasonable enough, protecting highlights more and not damaging the shadows too much.

In high contrast situations, I find Nikon's Highlight weighted metering quite valuable because I dislike applying EC which I often forget to restore.

JohnnyLuddite said:

I'd be a little kinder to the manual writers who are, I think, trying to help in a practical way. ETTR is an aim or objective, which conflicting demands (including time) may make difficult to achieve.

Modern cameras in most lighting conditions have DR such that loss of 1eV is normally acceptable, particularly in relation to screen or print DR. So their rule of thumb is reasonable enough, protecting highlights more and not damaging the shadows too much.

In high contrast situations, I find Nikon's Highlight weighted metering quite valuable because I dislike applying EC which I often forget to restore.

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Oh but the manual writers are only to be thanked, surely you don't want to be rude !

But pointing out confusing passages can only help them writing a better manual, I mean people are usually thankful to anyone contributing to make their work better (or at least they should be... let's hope that, at least in the open source world, they are )

--
Instagram: https://www.instagram.com/daniloleica/

I guess I'm encouraging balance here, certainly good intent in something that is free.

It's also the case that ETTR tends to encourage rather idealized stances at times.

solitone said:

After editing my shots with OM Workspace for a few months, I'm learning darktable, and it seems promising, I like it. I've watched several video tutorials, and now I'm reading the user manual.

I have a newbie question regarding a sentence I find in the introductory section--darktable 4.0 manual suggests to take a well-exposed photograph, and states that:

solitone said:

Where possible, you are advised to use exposure to-the-right (ETTR) techniques to maximize the amount of data available for processing. As a general rule of thumb, it is safe to under-expose all images by 0.5 to 1 EV (by reducing the ISO sensitivity if possible) even if the in-camera preview looks darker than expected (the preview is not the raw data).

Click to expand...

Click to expand...

Comments like that are very dependent on the metering on the camera. The cameras I have used vary quite a lot in this respect. They also vary a lot according to the type of scene being photographed.

My advice would be to learn ETTR for yourself and see how it relates to the metering on your camera and just ignore that last sentence.

As others have said ETTR does not mean overexposure. It means placing the btightest part of the scene at the highest exposure you can without blowing out. Sometimes it takes negative exposure compensation or exposure lower than what the meter indicates to do that. I use the camera blinkies to do that. I find the highest non-blinkie exposure and then add 2/3 of a stop to that. But the result is often a lower exposure than what the meter suggests.

That said, ettr is not as useful on modern cameras with 14 bit sensors and great dynamic range. As long as you take reasonable care in exposure not to blow out the bright parts.

solitone said:

After editing my shots with OM Workspace for a few months, I'm learning darktable, and it seems promising, I like it. I've watched several video tutorials, and now I'm reading the user manual.

I have a newbie question regarding a sentence I find in the introductory section--darktable 4.0 manual suggests to take a well-exposed photograph, and states that:

solitone said:

Where possible, you are advised to use exposure to-the-right (ETTR) techniques to maximize the amount of data available for processing. As a general rule of thumb, it is safe to under-expose all images by 0.5 to 1 EV (by reducing the ISO sensitivity if possible) even if the in-camera preview looks darker than expected (the preview is not the raw data).

Click to expand...

I don't get this. The two points seem contradictory to me. ETTR would imply over-exposure. Is the manual suggesting to over- or under-expose?

Click to expand...

The manual is properly written to the real definition of ETTR.

"Normal" exposure anchors the middle gray of the scene to a midpoint in the camera's tone range. ETTR anchors the brightest part of the scent to the camera's sensor saturation point; depending on the scene, this could be either under- or over-exposure as related to "normal".

Now, the manual's direction to under-expose is related to the overwhelming majority of scenes, where an under-exposure would be required to pull the scene's highlights under the sensor saturation point. Cases where over-exposure for ETTR would be prudent would be foggy morning light scenes or some studio setups,

The fundamental objective of ETTR is to not blow the highlights. Secondary objective is to put the highlights as close to the sensor saturation point as possible, but metering that can be a royal pain. Me, I use my camera's highlight-weighted matrix metering; usually leaves a stop or so of headroom between highlights and sensor saturation, with the Z 6 the shadows can usually be pulled up without much if any noise...

The aim of ETTR is to get as much light on the sensor as possible within your creative constraints ( DOF and blur) without clipping important highlights. This maximises the SNR (since the higher the exposure, amount of light striking the sensor per unit area, the higher the SNR) resulting in minimising potentially visible noise in the photo. It is a low SNR that makes noise visible.

Assuming there is no movement in the scene, the most benefit of ETTR will be achieved when shooting raw at base ISO, the widest aperture (smallest f-number) that will give the dof you want and then slowing the shutter speed until the camera's histogram begins to touch the right side.

Out of camera the image will most likely look too light but you then dial back the image lightness to suit in the raw converter.

--
Danno
Canon 90D, 600D, Photoshop Elements with Elements+, Elements XXL