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Cristian Vega

Alexa Mini sensitivity compared by a light meter

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Hi guys. 

I was doing some tests to determine what ISO to use in a light meter to be able to prelight without using the camera as a tool.
At this point I am not interested in the noise level, nor the dynamic range.
I just want to have a reliable instrument that when you expose an 18% gray and analyze it through the False Color of the Alexa Mini, get the green color that corresponds to your scale.
Perform the tests with two different camera bodies and with two different lens sets. Before the tests, perform a factory restet of both camera bodies. I used a spotmeter Minolta F and  the Sekonic L758Cine light meter with factory profiles and also with the profile created for Alexa by Ryan Waters (http://www.ryanewalters.com/SP/sekonicprofiles.html) and original (and clean😀) gray card by Kodak.
In all cases to obtain a parity between the light meters and the Alexa Mini, I had to use the light meters with a sensitivity between a stop and a stop and one third above what I used in the camera.
For example, if an EI 800 was set on the camera in the light meters to match the reading, I had to set an ISO between 1600 and 2000.
I wanted to know your opinion, if this is the usual thing or I have missed something in the tests.
Regards

Edited by Cristian Vega

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I've shot on my fair share of Alexas over the years, and though I still always do a quick grey card check to calibrate the cameras to my meter, I'm yet to encounter an Alexa that hasn't returned a nice, clean green false colour metering over a grey card exposed per the camera's rated EI. 

800 on my meter has always matched 800 on the camera.

I personally tend to rate the camera at 400 ISO most of the time (unless I'm short on light). But I'm setting my meter to 400 when I do so.

There's always a chance one or the other (the camera or your meter) are out of spec/calibration. But it doesn't really matter so long as you calibrate them to each other, and can get consistent, repeatable results from doing do.

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11 minutes ago, Mark Kenfield said:

I've shot on my fair share of Alexas over the years, and though I still always do a quick grey card check to calibrate the cameras to my meter, I'm yet to encounter an Alexa that hasn't returned a nice, clean green false colour metering over a grey card exposed per the camera's rated EI. 

800 on my meter has always matched 800 on the camera.

I personally tend to rate the camera at 400 ISO most of the time (unless I'm short on light). But I'm setting my meter to 400 when I do so.

There's always a chance one or the other (the camera or your meter) are out of spec/calibration. But it doesn't really matter so long as you calibrate them to each other, and can get consistent, repeatable results from doing do.

Thanks Mark,

As I said before, I don't know that it might not match me. I tested it with two different light meters and two different camera bodies. In different EI values (from 160 to 3200) and I always got the same difference from a stop or a stop and a third.
Regards

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One more thing to add. The measured values of False Color were always checked with the waveform monitor, to reduce some other error factor.

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So long as the results are consistent, I'd say you're good to go.

Alexa's a bit unique/painful in that the Log image is so damned flat, that I find it really hard to use with a waveform - everything looks like a midtone. Kind of forces you to stick to your meter (which is why I'm always checking that the cameras calibrate with mine).

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The meter will “match” an Alexa set to the same ISO.

Bit I do find, like you, that I light a bit darker for the Alexa. 

I would though, if you have enough light, set the Alexa to ISO 640ish and meter at 800.

Alexa at 2000 is kind of noisy...

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13 minutes ago, Bruce Greene said:

The meter will “match” an Alexa set to the same ISO.

Bit I do find, like you, that I light a bit darker for the Alexa. 

I would though, if you have enough light, set the Alexa to ISO 640ish and meter at 800.

Alexa at 2000 is kind of noisy...

Hi Bruce, thank you for your response.  

But I am not referring in this post to how I should expose.

I don't know why it is such a big difference between the cameras and my light meters.  I would like to know if someone can guide me to find out what it can be.

Regards

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It could be that the camera display output assumes gamma 2.4 and you are viewing on a gamma 2.2 display?

 

That would make everything look a little bi lighter on the display.

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That could be a possibility, if I had modified some values in the camera menus. But you remember that I made several factory resets on both camera bodies to rule out any configuration errors that might exist.

On the other side I also probe changing the values of Look: LCC709, 709, LogC, etc and nothing happened.

I'm really freaking out 😀😀😀

Edited by Cristian Vega

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A test that I forget to mention is that at the same time I checked the Alexa Mini, I also used a Sony Venice camera body, and with it I obtained exactly the sensitivity indicated by the manufacturer, ISO 500 and ISO 2500 which are its native ISO, and also test your intermediates.
In all cases, the ISO sensitivity values of the Venice corresponded with my light meters, which are exactly the same light meters that I used for testing with the Alexa Mini.

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