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Incident VS Spot: Calculating the Difference

light meter sekonic l-785cine comparison spot incident

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#1 Thomas McNaught

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Posted 02 October 2015 - 10:51 AM

Hi,

 

I'm new to the forum, so I'm sure that my question -- in typical "noob" fashion -- will be an obvious one; I'll ask it, nonetheless.

 

As a young, aspiring cinematographer, I was recently advised by an experienced DoP to invest in a light meter, which I have done. I've found, however, that my readings will differ between incident and spot; the former tending to have my exposure higher than the latter. I know the difference between incident and reflected light, but am I incorrect to presume that the readings should be the same regardless? Or, am I in fact taking the readings wrongs all together?

  • For spot metering, I will stand where my camera is and aim at the subject's face.
  • For incident metering, I will place the meter in front of the subject, with the lumisphere facing my light source: in the provided example, the light was to the left of the subject's (my mother) face.

So, I'm just looking for a bit of feedback regarding my metering techniques. What's the better option? Where should I be aiming the lumisphere when taking a reading? Where should I be aiming the spot?

 

Thank you in advanced,

Thomas.

 

lm-comparison.png


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#2 David Mullen ASC

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Posted 02 October 2015 - 12:36 PM

The incident and the spot meter reading would only be the same if you were spot metering an 18% grey card. A Caucasian face is usually a stop brighter than 18% grey, more or less (human faces have a wide variety of tones.)

You have to factor in the reflective values of the subject when taking a spot meter reading and decide how much brighter or darker than 18% grey you want the subject to be rendered.
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#3 dan kessler

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Posted 02 October 2015 - 12:45 PM

When using an incident meter, you want to aim the sphere towards the camera, not the light source.

There are numerous resources on metering techniques to be found online


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#4 Satsuki Murashige

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Posted 02 October 2015 - 03:38 PM

If you are using one of the Sekonic combo meters (508,608,558,758) you should be aware that they come from the factory calibrated for 11% grey in spot mode, not 18%. Which may be why your incident readings may not match when using the spot with an 18% grey card. If you send the meter out to get calibrated by Quality Light Metric or Spectra, you can request them to calibrate for 18%. I believe with the 758 you can adjust this yourself, but I only have experience with my old 508.

At that point, any variations you are dealing with should be fudge factors - how you hold and aim the incident meter, glare from an off-axis grey card, flare in the spot meter optics. By using the meter consistently and checking how your exposures came out, you should be able to adjust for this and compensate without thinking about it too much.

In incident mode, I always aim the dome toward the light itself and shade it from other sources with my hand since I want to know how much light each unit is outputting rather than getting an averaged reading. That would be the same as retracting the dome, or using the flat disc if you are using a Spectra meter. But everyone has a different method. Apparently, Douglas Slocumbe, BSC (who shot from 1940 -1989) knew the film stocks of his day so well that he never used a meter until he was guilted into buying one, then pretended to use it so nobody would think he was weird, all the while relying on his eye.
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Also tagged with one or more of these keywords: light, meter, sekonic, l-785cine, comparison, spot, incident

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