measurements spotmeter/exposure meter

[Manolo S. Navazo]

hi,

perhaps somebody could me explain waht i was doing wrong.

i was checking if the internal measurement tools from red one would show me the same as a spotmeter with a greycard and/or a incident exposure meter.

- i have the Sekonic L 758C Cine Light Meter(double checked it with an other light meter, to be sure that this lightmeter is calibrated)
- a greycard (doublechecked the greycard with spotmeter measurement and incident measurement, to be sure that this card got true 18% reflection)

settings in camera:build17, 25fps, 180°shutter(1/50), 250 ISO ....(5000°Kelvin).
tungsten light 1000Watt in room

with spotmeter and incident light meter, i am getting a value about f 5,6
in camera with raw i am getting a value f 2,8
in camera with redspace i am getting a value f 2,8 + half

i measuredin camera with histogram, zebra 50%, internal spotmeter 50 ire
..... so all internal measurement tools from red one where showing me that i was in redpsace 1,5 stops away from the measuremnet with an exposuremeter.
in all measurements i havent moved the greycard neither the camera neither the 1000w light, neither something else

what i was doing wrong?
i am wondering that the native ISO from the red one chip is in my calculation something like 80 or 100 ISO. and the redspace isn't also showing that what it should. or does redspace isn't showing me a value of the asa settings?

perhaps somebody could me explain what i am doing wrong.

thx, manolo

 

[Richard Goodwin]

I think the Sekonic's are not calibrated to 18% grey but if I remember 13% grey (you will have to verify). The main thing is they don't use 18% grey as their reference grey.

I am not sure what the reference grey is for the measurements the RED provides.

That may be one reason for the difference. (I realize you mentioned a second light meter, but that may be calibrated for something other than 18% as well).

One additional thought: you might want to measure a range of light values with both "meters". If they difference stays fairly constant its a good bet that they are calibrated differently. Just doing one measurement is limited in terms of a sample.

 

[Manolo S. Navazo]

sorry, but what you are saying doesn't make sense to me.

if a exposure meter is not calibrated that he reacts like an 18% grey reflection,.... it is definitley not an exposure meter.

 

[Manuel Wenger]

I´m quite sure that IRE of 50 doesn´t represent 18% Grey. As far as i can remember it´s more at the IRE 40 mark. Did you try it out with false color ? if you 18% grey goes green, you´re there.
The Sekonic is set to 18%, just returned from a shoot where i was DIT, the DP measured with his Sekonic and i did via RAW view and Histogram, we always came to the same stop within +/- 1/3rd of a stop. Camera was at 25fps 180deg shutter, 320 ASA.

 

[Manolo S. Navazo]

hi manuel,

nice to hear that you have done this test.
i haven't tried the false color measurement, i will do this next time. do you think that 50% zebra also doesn't represent 18% grey reflection? the 18% reflection is more or less equivalent to a middle european skin tone.

setting the 5,6 on the lens(50mm zeiss mkII orIII, T2,1) the histogram mountain was on the 1/3 left side, ire like 27 - 30
this gave me headache, so i have put the histogramm to the middle, the ire was on 50, and the 50% zebra was as well markin the grey card.

so like i said in raw it was f 2,8
in redspace it was f 2,8+half stop

next time i will do this test again with false color, looking for some green

 

[Richard Goodwin]

sorry, but what you are saying doesn't make sense to me.

if a exposure meter is not calibrated that he reacts like an 18% grey reflection,.... it is definitley not an exposure meter.

There are a number of references on this (search on sekonic, 18% etc). You can offset the Meter or have it re-calibrated. But, 18% is not necessarily the reference standard used by the meter manufacturer when calibrating at the factory.

This is a whole set of arguments about what a "mid-tone" really is. I'm not trying to throw my hat in there. Just to provide some possible reasons you are off in your measurements.

If you want to do an real test you need to put a card fixed on a stand, light it evenly, and vary the light in 1/3 stop increments over a range of stops (or get a exposure chart, with patches of grey, and a MacBeth card and measure those, ideally you would do it over the whole DR of the sensor). The meter and the RED would need to be at the exact same point relative to the card (mount each on a tripod etc...). To vary the light you could also just ND filters over the cam and meter. Basically, you would graph out how the meter in the RED and the meter in the cam react to the same variation in light.

The other issue (and I think there are thread on this) is that the results may depend on what color/lut space you are using in the cam: ie REDSPACE etc... as that will affect response. I am not sure how that affects the zebra and metering. (That may explain why your RAW and REDSPACE measurements did not agree. The response is not linear, and the curve is different for the meter and each "space".)