Using a light meter for different camera settings (eg. codec

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saleh3z

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Using a light meter for different camera settings (eg. codec

PostSun May 02, 2021 9:36 pm

I just got a light meter. I understand it's original main functionality is to set proper exposure. I plan to use it for lighting ratios, but, I would help understand how accurate it is and see if it returns "proper" exposure values.



When I input my shutter angle, ISO, and fps, it returns an F-stop correlating to proper exposure. It's simple.



However, each camera reads middle grey, or proper exposure differently. In fact, each codec and ISO settings within a given camera affect IRE levels/exposure.



My question is, how does the light meter know to return results appropriate to the specific sensor ur using, and even further, the specific settings beyond shutter angle, ISO, and fps, (such as codec or even native/dual native ISOs...etc)?

Is there some sort of calibration? If so, I often shoot dual cam setups with different sensors. Do you calibrate for two sensors?



Thanks alot!
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Howard Roll

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Re: Using a light meter for different camera settings (eg. c

PostSun May 02, 2021 10:10 pm

For BM cameras middle grey is the same for every ISO, it's the distribution of stops above and below that change. BM & Arri both plant mid grey around 40%, I think Slog is ~32%, Rec709 is 40%. The lightmeter is going to give the correct exposure for mid grey at a given ISO, how the data a managed around that point is camera, user, and color space specific.

Good Luck
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Hendrik Proosa

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Re: Using a light meter for different camera settings (eg. c

PostMon May 03, 2021 7:06 am

It is camera manufacturers job to make it output expected values per some exposure. If one makes a camera that underexposes by 10 stops for settings that work as expected in other cameras, users would be annoyed. Codec has no effect on exposure, encoding comes after image has been captured and processed. Light meters measure scene-linear light, where mid gray or some other reference luminance lands after some encoding schema x is irrelevant (what scenelinear value 0.18 is encoded to in some wonky log curve), because you can always swap out the transfer function for something else in post.
I do stuff.
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John Brawley

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Re: Using a light meter for different camera settings (eg. c

PostMon May 03, 2021 9:09 pm

There are some companies that allow you to profile a camera.

Sekonic have an exposure target you photograph and it allows you to create camera specific profiles.

This was a thing in the early 2010's but I see far fewer doing it.
https://www.ryanewalters.com/SP/sekonicprofiles.html

Codecs won't so much affect your dynamic range but the bit depth *CAN* and these are often tied to codecs.

JB
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Howard Roll

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Re: Using a light meter for different camera settings (eg. c

PostTue May 04, 2021 4:43 pm

Other than the actual sensor, most of what makes up a basic lightmeter amounts to a fancy cereal box decoder. There's an absoute value for exposure that the calculations are based upon. I've always been curious but never looked into it. Turns out the number isn't 42, rather 25.

Incident exposure calculations are based on an input value of 25 footcandles when the aperture is f1.0 and the ISO and exposure times are reciprocal.

This makes for quick mental calculation because you're only adding and subtracting stops. It's like counting cards, not that I would know ;) Shooting at 1/50th, an ISO of 400 would be -3 stops, a shooting aperture of f2.0 would be +2 stops. 25 footcandles -1 stop = 12.5FC.

Pcam, Arri, and the Set Lighting Tech's Handbook all return a value of 25, if this is arbitrary or rounded from other math I don't know.

Good Luck
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askomiko

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Re: Using a light meter for different camera settings (eg. c

PostTue May 04, 2021 6:11 pm

saleh3z wrote:
Is there some sort of calibration? If so, I often shoot dual cam setups with different sensors. Do you calibrate for two sensors?


Yes, ISO itself is the calibration:
https://en.wikipedia.org/wiki/Film_spee ... ystem:_ISO

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