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Exposure
Painful Details
Point, meter, close 1 stop to move the lava from
gray to black, click. From Hawaii.
There are some details that can make life painful when setting exposure. As you focus closer to a subject you are
moving the lens farther from the film. The lens is throwing the same amount of light in a larger and larger circle
of which the film intercepts a smaller and smaller fraction. For small format (35mm) cameras this effect is not
significant until you get a macro lens and start taking pictures of things comparable in size to the 24x36mm
frame itself. However, if you are taking macro photographs and following the recommendations of a handheld
light meter you will find that your pictures are underexposed by 1 or 2 f-stops.
The handheld meter, whether reflected or incident, can't know what impediments there are to light reaching the
film. The meter manufacturers assume an ideal lens. Your lens may be covered with a fine coating of dust. Your
lens's internal elements will not be perfectly transmissive; some light will be lost each time it goes through a
piece of glass within your lens. You may have stuck a filter in front of the lens.
A good way of sweeping away all of these details is the through-the-lens meter. Necessarily a reflected light
meter, the metering cells are placed behind the lens and in front of the film, oftentimes built into the viewing
system. These cells see what the film will see and therefore if light is getting blocked for any reason the meter
simply sets the exposure as if there were less overall scene illumination.
More
● archived Q&A threads on exposure
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http://www.photo.net/making-photographs/exposure (12 of 13)7/3/2005 2:22:14 AM