View Full Version : Exposure control Vs T-stop
Say we have a scene lit. The DoP wants it dark and sets the lens to 5.6
The histogram shows that we could open up 2 stops before the traffic lights come on. What would happen if we opened up to 2.8 and set the in-camera exposure to -2?
We are now exposing to the right which should gives us a better image, but do we change the contrast ratio? Should the image look exactly the same apart from being cleaner and having less DoF?
Graeme Nattress
02-07-2009, 07:44 AM
Ideally, you'd choose aperture based upon creative control of DOF, and change the light, or use ND to always ensure the sensor is exposed as best as you can. But I'm not a DOP.
Graeme
Ideally, yes. But that's often not possible. Ignoring DoF, do we change anything?
Graeme Nattress
02-07-2009, 08:08 AM
Ignoring DOF, you will get less noise by exposing to the right.
Graeme
Sure, but will I get the same sort of brightness and contrast levels if I open up two stops and set the camera exposure to -2?
Graeme Nattress
02-07-2009, 08:45 AM
Yes you will!
Graeme
Interesting, I'll have to do some tests.
Thanks Graeme.
Shawn Booth
02-07-2009, 09:07 AM
Say we have a scene lit. The DoP wants it dark and sets the lens to 5.6
Should the image look exactly the same apart from being cleaner and having less DoF?
Did you talk to the DP about it? About his/her decision to set the stop at 5.6 (sweet spot on most glass)?
Perhaps he/she was thinking of his/her 1st...
Perhaps DoF was needed?
It was a hypothetical question.
BradWright
02-07-2009, 09:13 AM
Say we have a scene lit. The DoP wants it dark and sets the lens to 5.6
The histogram shows that we could open up 2 stops before the traffic lights come on. What would happen if we opened up to 2.8 and set the in-camera exposure to -2?
We are now exposing to the right which should gives us a better image, but do we change the contrast ratio? Should the image look exactly the same apart from being cleaner and having less DoF?
You really shouldn't under expose video if you can avoid it. It's usually a better option to darken the image in post than to try to under expose/light. All the noise is at the bottom end of the waveform, so keeping video out of this region is usually a really good idea.
Shawn Booth
02-07-2009, 09:15 AM
Gotcha Irri -
You really shouldn't under expose video if you can avoid it. It's usually a better option to darken the image in post than to try to under expose/light. All the noise is at the bottom end of the waveform, so keeping video out of this region is usually a really good idea.
I agree.
I am often in a situation where the DoP is setting their exposure based on the image on the monitor, opening and closing the iris until the image brightness looks right. This can sometimes be a stop or two below "exposing to the right."
There isn't always time to convince a long time film DoP of the theory of ETTR. I was just thinking that maybe I could ETTR and the DoP could set the brightness with the exposure control.
Peter Strietmann
02-07-2009, 09:59 AM
How about turning down the brightness level on the monitor when the DP is away?
David Mullen ASC
02-07-2009, 10:15 AM
Any DP would understand the concept -- negative film works the same way, it likes density (exposure).
But while always going for the most exposure would reduce noise, that's not necessarily the defining principle behind how you expose a scene creatively.
For one thing, some DP's prefer the look of a "thinner" negative or in this case, a slightly noisier digital image -- maybe it's part of their creative concept for the film.
Secondly, shots are not out of context, they are part of a sequence that is cut together. So a DP picks a sensitivity rating in general that they prefer for the look they want in terms of noise, black levels, etc. and then they expose the shot within that base. So if the base look is 320 ASA, for example, and within that sequence, you have a camera angle that is two-stops underexposed, maybe because someone switches off some lights in the scene, then two-stops underexposed is the correct exposure for the scene, it really isn't "underexposed" because the image is not going to be lightened in post, it's going to be at the same levels as the other footage, at the same noise level and same black levels, etc.
Noise would only be a problem if either (1) the base rating chosen was noisy in general and having a lot of black in the frame now made that more visible, or (2) a decision was made in post to brighten the dark shot.
Otherwise, don't think of a darker exposure as necessarily being the "wrong" exposure, or that every shot has to be exposed at maximum value to minimize noise -- this would basically be the same as re-rating the sensitivity shot-to-shot and thus having mismatching in base noise levels.
Think of it this way -- if in the scene, all the lights were turned off and the room went pitch black, then the signal on your waveform would hit "zero", right? Is that the "wrong" exposure? Would you change the exposure so that the pitch-black room was still registering a normal level of signal? No, a low level, or no level, is now the "correct" level.
Shots are cut into a sequence and what matters more is that the exposure technique is consistent within a sequence so you aren't radically changing levels once you get into post color-correction.
All that said, when it comes to really dark scenes, it's generally better to be conservative and only underexpose partways, figuring you can always darken it further in post.
That's not the same thing though as saying that you should just expose every shot normally no matter what the mood or look you are going for and do all of the correction in post. If you have a clear creative concept for the look of the scene, then expose for it.
Now if you think your DP is making choices based on a monitor image that is too light, then adjust the monitor or the output of the camera so the recording is biased towards more exposure in general.
In film, it works the same way -- black level is a function of printer light values. In other words, you could shoot with the lens cap on, or send the unexposed negative to be processed normally and printed. Now how black the blacks look will just depend on the printer light level used. Printed in the teens, it looks grainy and milky. Printed in the 40's, it looks very dense and fine-grained.
It is similar with digital -- how clean and black the shadows look is dependent on the final level you choose to set for the color-corrected scene. So maybe a shot that is five-stops under is set to the same levels as a normally exposed scene -- it will end up looking very dark, but it would have the same blacks levels and noise as the normally exposed shot would because you haven't really done anything to "lift" the low signal. So if that's the look you wanted, then the dark shot wasn't really exposed incorrectly.
The danger is that if someone changed their mind in post and now wanted to lift the dark shot. Then you're going to get noise problems. Hence why you want to be a little conservative and give yourself some wiggle room.
Now the other danger though is in dailies and editing, they are working with a shot that is brighter than the look you intended, they will get used to that look and not let you darken it later in post color-correction. So you need a way to make sure that what is used in editing has the correct look.
Chris Parker
02-07-2009, 10:37 AM
good post david. your generosity on here is always appreciated.....
Thanks for the insightful post David. Not all photographers are as progressive as you when it comes to digital.
It’s nice to be reminded of shot continuity; perhaps I concentrate on the perfect exposure for each shot a little too much. Maybe that’s a commercials thing.
I would explain in more detail, but I would risk people knowing what shoot I was talking about which I consider very unprofessional.
Basically, the histogram was in the bottom eighth for every shot. The 1st AC didn’t like anyone steeping on his toes and wouldn’t let me speak to the DoP. I guessed quite early that the DoP was going to expose on the low side so I set the camera to REC709. The Focus Puller actually turned the brightness up on the monitor when the DoP said it looked dark.
Dylan Macleod, CSC
02-07-2009, 01:22 PM
Then there is the infamous story of Gordon Willis who basically exposed the Godfather (can't remember which one) at the very edge of what the negative could handle in terms of underexposure. He was doing this to maintain creative control over the look. So that the studio couldn't "print it up".
There is a great scene in Masters of Light when he is talking about this over a clip from the movie that is horribly underexposed and he says something like "...and sometimes I went too far".
Apparently Harris Savides is known to underexpose quite a bit.
Well, as someone once said "There's nothing wrong with rebelling, as long as you know what you're rebelling against."
Unfortunately, I doubt this project will make an appearance in Masters of Light Volume II :)