Light Meter discrepancies!

[DS Williams]

I am a film student, and of course in class the cinematography professor indicates light meters, both incident and reflected, as being calibrated for 18% grey, as it is perceived as mid range tonality by our eyes ect, ect.
But recently someone told me that light meters are in actuality calibrated for twelve percent grey.
Now of course this naturally confused me. Which is it? and is there a way to tell without painstaking testing? This causes me to conclude that in reality your footage would come out 2/3 of a stop or so overexposed against your will...

Any help would be appreciated! I'm looking to purchase an incident meter soon...

 

[Graeme Nattress]

1) ask the manufacturer of the meter
2) rely on in-camera exposure tools for final adjustments

Graeme

 

[Stephen Strangways]

Graeme's answer is really all you need to know. These days, lightmeters are perfect for working out lighting ratios, but final adjustments are easiest and most accurate if you use in-camera exposure tools.

But, to be technically exact, ANSI standards specify light meters should be calibrated for a luminance value approximately equivalent to the reflectance of 12% grey.

A good compilation of all the info is here:
http://www.bythom.com/graycards.htm

 

[Stephen Williams]

I am a film student, and of course in class the cinematography professor indicates light meters, both incident and reflected, as being calibrated for 18% grey, as it is perceived as mid range tonality by our eyes ect, ect.
But recently someone told me that light meters are in actuality calibrated for twelve percent grey.
Now of course this naturally confused me. Which is it? and is there a way to tell without painstaking testing? This causes me to conclude that in reality your footage would come out 2/3 of a stop or so overexposed against your will...

Any help would be appreciated! I'm looking to purchase an incident meter soon...

Hi,

There is a reason that cinematographers have been testing film & other equipment for themselves. Many people overexpose negative 1/3-2/3 stops from what their meter says the reason is many meters are calibrated for 12, 14 or whatever. I generally know what I expect the meter to read & will angle the meter until it matches what I expect! Because film has a huge DR & therefore enough latitude to get away with several stops of overexposure. A meter calibrated to 12% rather than 18% would be underexposing.

 

[Thomas Wright]

So what then is 12% in IRE, ie what would I set my lower zebra to if I wanted it to hatch across that.

And does this mean that all the DR tests done by all the dilligent fellas on this site are wrong by a certain factor. Oh my hat, my world just fell apart.

 

[Graeme Nattress]

Has nothing to do with DR, Thomas. Only spot metering, and only absolute readings. Any relative reading is fine.

Graeme

 

[Thomas Wright]

I just thought that the results would be shifted along the curve, so overall DR would remain the same but when people say things like 2 stops over and 7 under would actually end up something like 2 and a half over and 6 and a half under or the other way around, know what I mean?

 

[Graeme Nattress]

No, because that's still a relative measure, and we don't measure DR with light meters anyway.

Graeme

 

[Stephen Strangways]

So what then is 12% in IRE, ie what would I set my lower zebra to if I wanted it to hatch across that.

Why would you want to? When you're shooting raw, all you really need to worry about is where the highlights clip, and where the shadows fall into the noise floor. The new (Build 30) false color "exposure" mode is perfect for this.

If you are recording RBG video, the false color "video" mode is perfect for that. It will show in green anything that is 18% gray (IRE 44-47) and that's close enough.

See pages 49-56 of the Build 30 operation guide. This is how you should be making final exposure decisions: based on what the camera is actually seeing in the end. A lightmeter is useful when setting up lights and working out contrast ratios, but once it gets into the camera, so many other factors come into play that you should always trust what the sensor is actually seeing, not what the lightmeter is reading.

The difference between 18% and 12% is really splitting hairs unnecessarily, because as far as setting exposure in a practical situation goes, hitting middle gray exactly is not critically important, so the difference between the two is even less so.

 

[Thomas Wright]

Personally when I shoot I use false colour but I get DP's coming in to the place where I work wanting to rate the camera relative to their lightmeter, I understand what Graeme is saying, it's relative so don't worry, but with all this information floating about I'm bound to get one come in with half the story- and you know what they say about a little bit of information: it's a dangerous thing- so they'll sit in the test room scratching their heads and somewhere along the line it becomes my problem.

I hope I can assuage their fears with what you've said fellas, it makes sense to me, ta.