Question about incident light meter

Started Feb 28, 2013 | Discussions thread
chkproductions Senior Member • Posts: 1,104
Re: Question about incident light meter

This is meant as an illustrative and not a technically pure explanation of metering. And my apologies if it confuses the issue more than simplifies it.

A hand-held meter is calibrated, regardless if it's a reflected or incident light meter. By default they generally come in calibrated to 18%, meaning they read light reflected to it or falling from a light source on it in a middle gray or the middle of the 10 steps of grays from pure black to pure white. In-camera meters have changed the calibration point, varying from 12% up to 18% depending on the manufacturer.

When you take a reflected light reading from a subject, the hand-held meter will take that reflected light and read it either down or up to that middle gray that the meter is calibrated for. So if you took a reflected reading off of a white card, and used the indicated (indicatedbeing a very important factor) exposure on the meter, the white card would look gray and not white. The same would conversely happen taking a reading from a black card. The meter sees everything as that calibrated gray.

When you take an incident reading of the light falling on your subject, the meter will only see the light or lights that are illuminating the subject(s), and not how the subject(s) is reflecting it. And because the meter is calibrated for the middle gray, anything that is lighter than or darker than that middle gray will be exposed in proper relationship as either lighter or darker to that middle gray of the meter.

If you can calibrate a meter, you can calibrate it to anything you want. The only thing you're trying to do is create a constant with which to make adjustments to your exposure. So if you took a incident meter and measured the light falling on a subject ( a person's face as an example) and you use the indicated meter reading for your exposure, you may or may not end up with an accurate exposure of the person's skin tones. It all depends on how you calibrate your meter in conjunction with the copy of the lens and camera you are using. What is important is to calibrate your meter so that it creates for you a consistent and accurate exposure of a know object (face, rocks, whatever) and that you can use that constant consistently with your copy of a lens and camera that gives you accurately exposed files for the subject at hand.

Nomenclature in photographic exposure is arbitrary at best. The 18% nomenclature came about in the B&W world of film and printing as it struck the approximate middle of the range grays that could be captured on B&W film at the time, and then transferred to the print where the grays ranged from paper-base white to paper-base black. It was chosen by using a densitometer that accurately measured the amount of light that was transmitted through the film base, plus the fog of the film, when the film was exposed in steps from clear to totally black. 18% turned into Zone 5 of the Zone System. Zone 6, one step lighter, was chosen as it also approximated the gray of Caucasian skin tone when photographed in open north light (the light coming from the North without any direct influence from the sun itself) when printing a B&W print. What was trying to be accomplished was to give some “constant” from which to make calculations for the photographic process from shooting to the print.

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