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spot vs. incident meter


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#1 Christian Janss

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Posted 27 February 2007 - 01:11 PM

First, I do understand the difference between incident and spot metering, generally, incident readings give an over-all measure of the light falling on the dome like that falling on a human face, spot readings give a precise measure of exactly how much light is bouncing off the subject/object and hitting the lens.

My problem is that I'm confused as to which reading to go by when measuring the same subject, or piece of the subject. I may be overly obsessive about this but it's been bugging me for a while.

For example:
There have been times when I'll take an incident reading from right under a light at height X, and then a spot reading of a person's face (say the person's illuminated cheek), also at height X, and the spot reading will be 1 or even 2 stops above the incident reading.

People have always told me to go with the incident reading, since it better approximates the human head, but I'm wondering if the spot reading is more accurate since it measures the precise area I'm looking at (i.e. if I wanted to expose for the highlight on the cheek) and it also takes in to account the distance of the camera from the illuminated object.

Would the answer be different if we weren't talking about a subject's head, say a chair or something less round and dome-like? If for example, if someone is wearing a dark shirt but has pale skin. An incident reading would give you an overall exposure but a spot reading would give you a more accurate idea of the difference between the shirt and skin. (This could be the same for a light picture hung on a dark wall)

And if the incident reading is best, should I just use the spot reading to get an idea of the relative ratios that different areas of the frame have as compared to other areas, and not for the actual f-stop readings? Or should I just not use the spot meter option at all unless I'm only measuring a perfectly flat surface.

Anyway, these questions keep bugging me. I don't want to rely on my meter as a crutch, but I don't feel like I can let it go until I've understood this. Any clarification would be much appreciated.

thanks
Christian Janss
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#2 Matt Sandstrom

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Posted 27 February 2007 - 04:24 PM

it's hard to see from your post what level you're on, so forgive me if this is too basic. try using the incident meter to measure the strength of the light sources rather than as "an approximation of the human head". next to the subject, point it towards a light source, or all the light sources from that direction, shade it from the other ones, read, repeat. then it's just a matter of deciding which light source to expose for. as for getting different readings with the spot meter, that's only natural since different objects reflect different amounts. if skin is lit from an angle where it becomes specular it reflects much much more than medium grey, but usually around a stop less. i only use spot meters to double check problem areas or areas that i need to go black or blown out. i don't think many people use them to set their exposure.

/matt
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#3 Michael Nash

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Posted 27 February 2007 - 05:41 PM

Neither meter tells you how to expose the image. They are only measuring light; one reflected light, the other incident light (this much you know). You select your lens exposure to create a desired brightness on film, based on the information the meters tell you.

A reflectance meter tells you an f-stop that will render the subject a medium gray. Caucasian skin tones are approximately one stop brighter than medium gray under flat light (which you've witnessed). A white peice of paper is about 2-1/2 stops brighter than medium gray. Setting you lens by the spot-meter's reading would render those subjects at a medium gray brightness, which would appear underexposed. So for a "normal" exposure under flat light you might spot-measure Caucasian skin tones and open the lens one stop from the reading. Or read a piece of white paper and open up 2-1/2 stops.

An incident meter measures the light falling on a subject; with the dome taking into account (and averaging) light from multiple angles, such as key and fill. Aiming the meter's dome straight at the camera lens produces a reading that will render a "normal" or average exposure for all the light hitting the subject (at least in the position where you measured it). Of course there are nuances to doing this, but this is the general idea.

But what if you don't want your subject to appear at a "normal" exposure? What if you want your subject to appear as if it's in a shaft of bright sunlight coming through a window? Or in shadow? You wouldn't set your lens to the incident meter's reading then -- you'd use the reading as a basis for deviation, choosing how much above or below that "normal" brightness you want it to appear.

And spot meters do not take into account the distance of the subject to the lens. What they take into account is obstructions between the lens and subject, like smoke and atmospheric haze (when looking at distant subjects). If the angle of measurement is too large (say, 10 degrees instead of 1 degree), then a spot reading may also be capturing lighter or darker areas surrounding your subject, and averaging them in to the reading.
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#4 Frank Barrera

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Posted 27 February 2007 - 06:40 PM

What you should do is gather these four things: a 35MM still camera with manual controls, a grey scale and color chart, a caucasian model with "average" skin tone and index cards with a sharpie. Do identical zone tests with both incident and spot meter. Have the film printed on contact sheets so as to preserve your work.

This should clear up all of your confusion.
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#5 Jonathan Bowerbank

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Posted 27 February 2007 - 07:12 PM

I stick to incident readings mostly, as I can basically eyeball my reflected readings just by knowing the incident reading from any light.

For instance, if I have a kicker or backlight that's shining light on the rear half of the side of someone's head, I can usually assume THAT reflected highlight is roughly 1 to 2 stops over my aperture setting, even though my incident reading from behind my actor's head is probably the same as my key.

What you need to do is get familiar with your light meter, shoot some tests, and see how various lighting setups correlate with the readings you got while shooting. Be sure to take notes on every take, this can be a quick reference that you should keep plugged into your brain for future usage.
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#6 Davon Slininger

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Posted 27 February 2007 - 09:23 PM

I saw a lighting demo by Christian Sebalt ASC once and he said that he never uses his incident meter. He prefers to be by the camera so that he can evaluate the scene as a whole and fire off spot meter readings from there. He also boasted that the lab he works with told him once that his negatives are always consistent with their exposure.

He justified metering this way by saying that if he meters the incident light he feels he is just getting an approximation of light values. If he spot meters from the camera he feels like he is reading light the same way the film will since the film is exposing what is reflected at it. He also said that by no means was this a scientific justification but it works for him.

Of course you still have to pick an exposure based on your spot readings.

"The Negative" by Ansel Adams has great info on this as well as some info on the Zone system that may help round out your choices on exposure.
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#7 Sam Wells

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Posted 27 February 2007 - 10:19 PM

I got into using the spot meter heavily on a film where I had things like pyro effects on construction cranes 30' up - not easy or wise to use incident ! - or otherwise lighting inaccessible objects - also stuff like things reflected in water and so on. So I turned into a "spot meter only" person for awhile (also saved on shoe leather !)

But sometimes, like in many daylight situations, incident is faster, I don't like being an obsessive meter reader.

I'm not sure this helps you but....

-Sam
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#8 Matt Sandstrom

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Posted 28 February 2007 - 07:34 AM

He justified metering this way by saying that if he meters the incident light he feels he is just getting an approximation of light values. If he spot meters from the camera he feels like he is reading light the same way the film will since the film is exposing what is reflected at it.

if it works for him, fine, but isn't it just a question of how you've learned it and what you draw from experience? i know extremely well how a face looks on neg, reversal, video, hd whatever on this and that lighting setup with this ratio. do i know how a face will look if one side of the face spots two stops under the other, and how this will look in front of a green wall another stop further down? not really. i'm not lost of course but it doesn't come naturally. i'll still use my spot meter to check so that wall doesn't become too dark though.

/matt
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#9 Karthik Ganesh

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Posted 28 February 2007 - 08:49 AM

Basically the incident meter as the name suggests, just takes into account the light incident on the subject, and it doesnt take into account the reflectivity of the subject. For eg, for the same source, - Black, or Mid-Gray, or White everything will give the same reading. While the spot meter will take into account the reflectivity of the material into account and give u a reading, by which if u expose, you will get mid gray. As in, if u meter and expose white or mid gray or black with the help of a spot meter, u will get mid gray in all the 3 instances. If u use a spot meter on a gray card, that will match the incident meter reading for the same scene...hope i am clear...

A clarification, regarding this...

"saw a lighting demo by Christian Sebalt ASC once and he said that he never uses his incident meter. He prefers to be by the camera so that he can evaluate the scene as a whole and fire off spot meter readings from there"

I have used a contrast filter, determined roughly my mid gray in the frame, took reading from spot meter and exposed, it was fine. Can u please comment. I will definitely try more number of times, but pl do share ur thoughts. Thank you.

- Karthik Ganesh
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#10 Sercan Tekin

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Posted 28 February 2007 - 09:19 AM

I was also recommended to use spot reading from one of my tutors at the school.

I experienced the advantages of spot reading in close-up shots. But it's difficult to maintain the continuity with spot reading.
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