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Expose for more detail at lower ISO


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Hey folks,

I'm hoping someone can clarify for me the idea of lowering Alexa ISO when you want more detail in the dark areas of the image. 
How is this different to filling in the dark area with light? I'm not sure what the process is, for example, I meter at f8 at 800asa. So then I put the camera to say 400 iso and shoot at f5.6, and then I have a stop more detail in the dark areas. But this is assuming that I have enough light in the dark area to read it in the first place no? 

 

I appreciate that this may sound confusing, but that is exactly what I'm trying to figure out ; )
Any responses/answers are much appreciated.

 

Neel 

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The difference is that filling the image with more light in the shadows makes the lighting flatter, lower in contrast. Unless you then increase contrast in the shadows in color-correction.

Not adding more light in the shadows and using a lower ISO on the Alexa does not change the contrast ratio, it just lowers noise and adds more shadow detail at the expense of overexposure detail. However in real-world situations, how much you see that increase in shadow detail or decrease in highlight details depends on the subject.  Outside in daytime where most shadows aren't very dark, you may not see much benefit in getting more shadow detail but see the detriment of getting less highlight detail.  At night on dark subjects, the opposite may be true. But technically the contrast of the image and dynamic range (14+ stops) is the same at different ISOs on the Alexa within practical real-world limits.

The real-world situations are important to take into account. For example, we often rate the Alexa faster at night in order to increase the brightness of the subject, the highlights, in low-light. But we won't see an increase in detail in the shadows, we just see more noise in the shadows, so we may need to add some fill light there because we have no latitude in post to lift up any detail, we've already lifted the whole signal by using a higher ISO. That doesn't mean to light flatter or to always use fill light, it just means you have to consider you have less post flexibility to work with darker areas of the frame due to noise so the lighting is more critical. Whereas in a low ISO situation in daylight, it is no big deal in post to bring up a face that goes a bit dark at some point.

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Hello David, thank you for the response. 

So would it be correct to say that in real world examples, for instance a house interior (without seeing windows), you would lower the ISO to say 400/320ish but still expose "normally" (with an incident reading for example) and then have 1 stop of detail more in the shadow areas; 
or as another example, say you are shooting a day exterior in bright sunlight, with cloudy skies. You push the camera to 1600 iso, expose "normally" and then you have a stop more info in the highlights. 
That is the way I understand it currently, but in reality it feels counter-intuitive. 
A more extreme lighting situation, say for example a night street scene, with the possibility of lighting it, would you lower the ISO (assuming of course that you have some lamp lights/windows in the frame and you loose info in these specular highlights)? 

Final remark - I think the only time I can see doing this is if I cant light it properly. Or would you use it even if you do have the possibility of lighting the scene to your taste?

Kind regards
 

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Whatever works for you is what you should do.

I shoot the Alexa at ISO 500 most of the time, if I don't have enough light, I start to creep up to ISO 640 and then ISO 800.

That's mainly to keep the noise down. I don't play with the ISO to control highlight versus shadow response usually, I do it with lighting. Plus there's nothing to stop you from keeping the ISO the same but under or overexposing a stop in certain circumstances to hold detail where you need it.

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  • 11 months later...
On 10/18/2021 at 3:55 PM, David Mullen ASC said:

Whatever works for you is what you should do.

I shoot the Alexa at ISO 500 most of the time, if I don't have enough light, I start to creep up to ISO 640 and then ISO 800.

That's mainly to keep the noise down. I don't play with the ISO to control highlight versus shadow response usually, I do it with lighting. Plus there's nothing to stop you from keeping the ISO the same but under or overexposing a stop in certain circumstances to hold detail where you need it.

I see some cinematographers use ISO 1600 with 2 stops of ND, even in a low key  lighting scene, is this something you do too if not do you have any idea why it’s done instead of using ISO 400 without the ND?

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  • 2 weeks later...
On 9/24/2022 at 3:21 PM, David Mullen ASC said:

High ISO gives you more noise and more highlight detail. In some ways, it feels more film-ish due to the texture and the long roll off to white.

what about the ones that uses low iso like ISO 400-500 it seems they light their scene brighter and then compensate with 2 stops of ND, this is actually what I see the most by many pro DPs

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If you're lighting brighter for a lower ISO, then you don't usually need ND as well, lowering the ISO is already lowering the exposure.  But you might be using a lot more light due to balancing with a bright view outside of a window on location and then using ND to get the f-stop / depth of field you want.  Or you lit the set to f/2.8 at ISO 500 but want to shoot a close-up at f/2, then you might use an ND.  Or maybe you plan on some slow-motion work so you light the set to f/4 at ISO 500 and use an ND6 to shoot at f/2 but then when you run the camera at 96 fps, you just pull the ND6 and keep the stop at f/2.

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