To Flat Disc, or not to Flat Disc?

tom meyer · July 16, 2009

Hi all

I'm a guy just starting out in cinematography - this causes me to ponder all sorts of issues that old hands probably don't lose any sleep over ;)

Bear with me if you will:

The issue at hand is that I feel quite uncomfortable metering with my incident meter's dome. (I own a Spectra IV for that matter.)

The reason for this is that I consider the dome nothing less than a Voodoo device that manages to provide one with a reasonable *interpolated* exposure - but at the loss of precise control over the placement of tonal values on the characteristic curve.

Now I know that above statement is strictly true only in the case where the dome is used as prescribed (point dome towards the camera from the subject position). I also know that precise, non-interpolated readings can be obtained by pointing the dome more or less towards the light source while shielding light sources that aren't exposure relevant from striking it.

But, I ask myself why I would want to to all this shielding when metering with the flat disc will give me a precise reading of the light incident on a given plane (let's say the key side of a face) without me having to shield backlights and whatnot from the meter? From a physical perspective (law of cosines and all that) metering with the flat disc seems the way to go...

As I'm pretty convinced of being on the right track (at least for myself) with this, I will start shooting using this metering technique. While doing this, I'll obviously gain experience and I'll see what works and what doesn't. Nevertheless, I'd love to know what some of the more experienced people on this site have to say about this issue. Not to belabor the point, but as I see it right now, I'd slap on the dome if I were *really* pressed for time and it were necessary to come up with an exposure without the luxury of carefully considering what's going on in the scene in terms of individually metered light levels. Under those circumstances, the dome and its integration/interpolation properties would be a blessing, of course, as it could be expected to deliver a reasonable exposure under duress.

Finally, one last question: The flat disc on my Spectra inexplicably reads up to 1/3 stop lower than the dome - this in a situation that should read identically (head on and homogeneous illumination of the metering cell). Can anybody suggest why that is so and what I ought to do about it? I figure, I'll just "get used" to it and learn to interpret exposure from this new baseline, right?

Thanks a lot for any insights!

Tom