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Avoid light trapsin Tutorials |
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By lanycosta/Lany (2,247) 
on February 5, 2003 6:21:54 PM CST
Avoid light trapsCAMERA CELLS. WHY SOMETIMES THEY GIVE WRONG INFORMATION.For beginners.
So in spite of my bad English, (My mother tongue is French) I'll try to write a bit about cells in reflex cameras (Spot, Matrix, Center Weighted etc). I hope you will forgive me about the "Linguistic massacre". Some times we shot accordingly to the camera cell mesurement and the result is under-exposed or over-exposed, generally the first reaction is to say "The battery is out", "the camera is a piece of junk", "I wish I had an F5", " I respected the cell indication, my cell is dead" or " I don't understand what went wrong" The cell was right, but the poor thing was completely bluffed by the light from the background or by the contrasts of light. Why? Let's try to keep it simple; the cell in your camera measures the light reflected by the subject you are photographing. Moreover, like any measuring device your cell needs to have a standard, a sort of reference.This standard has been defined, as being the average light reflected by an "average" subject. It is 18% of the solar light illuminating the subject. This coefficient corresponds to a "gray tonality " also called "average gray". This is also the value reflected by the "gray Kodak card" wich is a gray piece of thick paper that reflects 18% of the solar light. You camera cell is gauged to this value. In the majority of the cases, the main subject (witch represents less than 20% of the surface of your photo) reflects more than 18% or less light than 18% of the incident light: The cell will give you a wrong measurement. In addition, the background has a different amount of light reflected (high light or low light).
There are three types of measurements The center Weighted cell will measure about 60% of the light coming from the center of the picture, the other 40% come from the remaining surface of the picture. If your main subject let's say a model's face represents only 20% of the all picture, your cell will measure the reflected light from the face plus the reflected light from "around" the center plus the rest of the light; the result will be an over or under exposure. In case of a Matrix Measurement the image is divided in 5 to 25 (or more) parts which are analyzed separately and the result will be compared to a "library" contained in the processor of the camera and the best exposure will be chosen.But if the contrast is too strong, your camera processor will not "know" what to do. In case of Spot Measurement the results will probably be acceptable -because you measure the light reflected by the main subject- but still perfectible. For instance: q Lets take the classical landscape full of snow, much lighter than the average gray, with a small subject in the middle.What will your cell do?It will measure the reflected light- consider that its an average gray- determine the Aperture and Speed and give you the indication that will under expose your photo, in order to obtain a gray snow. q If you have a model, wearing a dark dress on a dark background, the cell will over expose and your model's face will be "burned"; meaning totally white. q If the model is on a very bright background and you want a portrait, probably the model's face will be under exposed, and the back ground a bit gray. q If you have a landscape with a very large potion of light blue sky, probably the ground in the bottom of the image will be underexposed.
How can you avoid this? WITH A MANUAL LIGHT METER. Why all the pro's use one? Some years ago I was at a Tennis Tournament in Paris with a professional sport photographer, he had 2 Canons EOS 1 (!!!) a 300 mm F2.8 and a Zoom, before the match he went down to the court and made a measurement with his cell. When he came back, I asked him why was he doing so in spite of the fantastic camera he had. -I always trust my manual cell; -he said- what counts is the incident light. If the camera gives a different measurement than the manual lightmeter, I'll always trust the lightmeter. Then I generally compensate in the camera's compensation button. I tried to follow his advise, and believe me it works!!!
How to proceed. First you need a Lightmeter. With a diffuser for incident light measurement. Step one. You set the film speed and the aperture (or shutter speed) you want, you put the diffuser on Step two. You get as close as possible from the subject. (I its a portrait, as close as possible from the face) Step three. You turn the diffuser of the lightmeter to the source of light (sun, spot, or even flash) Step four. You read the measure Step five. You shot with the parameters (Aperture and Shutter speed) given by the Light meter. That's all.
I have two light meters and they both give excellent results. A Sheperd Polaris (Flash meter and Light meter) A very old Lunasix (without flash meter, but indestructible) If you found this article of some interest, I' will write about some other practical things in a near future, in spite of my bad English, and please CRITIC THIS ARTICLE so that I can improve with your feedback. Merci beaucoup Lany.
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