Proper exposure in low light — an "expose to the right" question

[jimbee]

I am wondering if "expose to the right" should affect the exposure choice in low-light scenes.

I tend to think the "best" exposure should have shadow-to-highlight tones in all five bars of the histogram display, but not clip on either end. And reading the T2i exposure meter, the "best" exposure seems to be roughly the middle of the scale, which gives a good distribution of tones in the histogram.

But if the scene is fairly dark to begin with, does that exposure theory still make sense? Exposing a dark scene with the needle in the middle makes the scene brighter than what my eye sees, and also means the ISO is cranked up two or three stops, resulting in more noise.

I'm now thinking it makes the most sense to expose a scene to render it close to what the eye sees, even if that means the exposure meter is telling you the shot is two or three stops underexposed, and the histogram shows tones in only the two left-side bars, or even tones in only the first bar. But am I missing something? Does that mean you get poor image quality because you did not "expose to the right"?

Thanks for any advice,

jimbee

 

[c3hammer]

I've found that what ever you do, you don't want to have to bring up the levels in post or the noise becomes completely unmanagable.

I've done some tests with the T3i and found that I actually achieved a better "look" and less visible noise by going to ISO 2500 with ML than the exact same scene shot at ISO 800 and boosting in post. I also found that Cinestyle doubles down this problem from my workflow in Vegas. Exposing to the right and crushing in post was significantly better than exposing correctly from my experience. Just simply applying the LUT to Cinestyle in these "on the margin" lighting conditions is a receipe for disaster with that picture style.

I would try some tests of your own with your workflow. You might find some surprising results, depending on how your path effects the image.

Cheers,
Pete

 

[gonzo_entertainment]

Generally better IMO to overexpose (as long as nothing blows out) than underexpose. You can bring that overexposed down in post much cleaner than you can try to bring up the under exposed.

What I have found with DSLR is you generally expose for the highlights. Blown out highlights look so bad that avoiding them tends to take over.

 

[AndrewGentle]

Yes, I wouldn't expose to the right at a cost of increasing the ISO higher than you're comfortable. The point of exposing to the right is to put the detail closer to the most significant bit of each binary luminance value and further away from the noise floor of the sensor. That way, when you divide the values in post, you are reducing the noise floor even further. When you increase the ISO, you're not changing what's hitting the sensor, you're just multiplying the values mathematically and bringing the noise up with it. Dividing it later is usually going to be a pointless exercise. The only advantage to doing this would be if your codec is damaging the information if the levels are too low. That could explain c3hammer's results.

-Andrew

 

[a1ex]

http://www.guillermoluijk.com/article/isos5dmkii/index.htm - see Fig. 6.

You should expose to the right, ISO max 1600, then darken in post.

When you increase the ISO, you're not changing what's hitting the sensor, you're just multiplying the values mathematically and bringing the noise up with it.

This happens only above ISO 1600.

 

[Samuel H]

what a1ex said - and I think the reason pushing ISO up actually works is that it's not "multiplying the values mathematically": it's a change in the analog-to-digital conversion, and it does bring up noise, but slightly less than what you get with a digital push