AE-Lock:
Generally, the reason you use AE-Lock is because the main subject is in a
different light than the rest of the scene. You use center-weighted or spot
metering to take a "close-up" reading of that subject, then lock the
exposure as you re-frame the picture. An analogy would be taking a torso
picture of a person standing a few feet away with snow-covered mountains in
the background. The two classic ways of metering this scene would be to go
up to the subject with a hand-held incident light meter and take a reading
directly in front of the person's face, or take a reflected meter reading
from the face and adjust it appropriately. Center-weighted and spot
metering give you the same capability without necessary moving closer. This
is particularly useful for back lit photos of people when the composition
requires them to be placed off-center. You take the spot reading, lock the
exposure, recompose, then shoot. Matrix metering will try to evaluate the
entire scene to give you the "best" overall exposure, which may or may not
expose the main subject correctly.
Film pushing:
These comments are general to _all_ films, but in particular to color slide
and color negative films. "Pushing" film means to give it increased
development time during processing -- not to exposing it with a higher film
speed rating dialed into the metering system. The increased development is
what produces the effect of higher film speed.
Many people like darker, richer colors with their slide films (often called
enhanced color saturation). By setting the meter with a slightly higher
speed rating than on the box, the film is effectively slightly underexposed
compared to the nominal manufacturer's rating. This is the desired effect.
Don't tell the lab you changed the film speed rating. You _want_ the slight
underexposure. If you tell the lab, "Oh, I shot this at ISO 64 instead of
50," you are in effect telling them to "push" the film by giving it
increased development to achieve the result of a normally-rated 64-speed
film. This will negate the effect you wanted, and instead give you the
effect of using a "true" 64-speed film. In addition, some people find that
with their own specific equipment and techniques, they need to assign a
higher film speed rating to achieve proper exposure. Pushing their film
during processing would effectively give them overexposure.
Color negative film generally responds better to overexposure and loses
quality quickly when underexposed (even when pushed). Many people, myself
included, feel most color film (particularly the slower ones) give better
results when slightly overexposed (for example, rating 100 ISO film at 80,
400 at 320, etc.) Pushing color negative film would be useful if lighting
conditions were such that you need a higher film speed rating to capture
some event. In general, though, avoid underexposing color negative film --
it does not give you better saturation -- but if you do need a faster film
speed, then pushing helps you get the increased speed.
--
John Albino
mailto:[EMAIL PROTECTED]
