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Shooting a Grey Card


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#1 Vivek Venkatraman

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Posted 18 May 2016 - 01:42 AM

Hello,

 

This is a bit of a complicated topic for me and I am not sure how to word this even but I will have a go.

 

How does one shoot a grey card and use this to calibrate ones light meter to the camera sensor [in digital]?


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#2 Mark Kenfield

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Posted 18 May 2016 - 02:55 AM

You need either a waveform or false colour monitoring for your camera.

Light the grey card evenly and take a meter reading at the camera's rated sensitivity. Then check your waveform to see if the grey card reads at the IRE value stated for middle grey in the particular gamma you're recording with.

If it reads true, then you're good to go.

If not? Then adjust your iris until the grey card reads at the appropriate IRE, the difference between the stop for the 'official' ISO of the camera and the actual rating required to expose middle grey correctly for the gamma you're using, is the amount of exposure compensation you need to apply to the 'official' ISO in order to have it properly calibrated to your meter.

Some current cameras on the market (I'm looking at you Sony) can be as much a full stop off from their official sensitivity rating.
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#3 John E Clark

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Posted 18 May 2016 - 12:46 PM

On the topic of 'what value' as read from the Waveform display...

 

There are various values depending on whether one is dealing with say Rec 709 representation, vs the various 'log' formats of the camera being used.

 

For Rec. 709 I have seen the values of say 45-50%... and for log representation, say 30-40%. For my Blackmagic Pocket, 38.4%... is what has been stated as the design goal.

 

Most people can't read off better than +/- 2-3%... so it is not an absolutely precise value.

 

In any case one also needs a step wedge to show how 'low values' and 'high values' are represented and will indicate how many stops of 'dynamic range' one may have.

 

Exposure is a compromise, especially when shooting 'available light'. With additional lighting some conditions of the shot may be mitigated, such as fill light to lessen contrast, or accent various areas in the frame. But at some point there will be compromise.


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#4 Dylan Sunshine Saliba

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Posted 18 May 2016 - 01:10 PM

Exposure is a compromise, especially when shooting 'available light'. 

Well said!


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