At 03:16 4/5/01, Jim Teo wrote:
Using my OM camera for correct exposure at F2.8 (max. aperature) it reads
1/8sec using ASA200.
In order to have a speed of 1/30sec to prevent camera shake, can I set the
camera manually to 1/30sec and increase the exposure +2 to have a correctly
exposed picture?
From how you describe this, the answer is no.
In order to increase the shutter speed from 1/8th to 1/30th, you must
somehow gain two stops . . . and I think you've got this part
right. However, unless you somehow add more light (lamp or flash), get the
aperture two stops wider (f/1.4), or use a film two stops faster (ISO 800),
or a combination of these that adds up to two stops more (i.e., f/2 and ISO
400), you're not going to get a "correct exposure."
The +2 you mention is on an "exposure compensation" dial. It's used in
very unusual (and/or difficult) lighting situations when when you want to
deliberately shift your exposure to something more or less than what the
metering is telling you. When you shift to the +2 (add two EV or "stops"
of exposure) with the ASA dial on 200, it's exactly the same result as
changing the ASA dial to ISO 50 and leaving the compensation dial at
Zero! That's why the compensation dial is concentric with the ASA
dial. You're actually working the same control by turing either one.
--------------------
Longer answer with explanation of EV: A Crash Course on "EV"
EV = Exposure Value, a.k.a. Additive Photographic Exposure System
It has two pairs of values (four total). The sum of either pair is the EV,
or Exposure Value:
EV = Brightness Value (BV) + Film Speed Value (SV)
and
EV = Aperture Value (AV) + Time Value (TV)
When the sum of one pair equals the sum of the other pair, you have a
"proper" exposure.
In the situation you described, the two are not equal. The first equation
has an EV lower than the second equation for the aperture and shutter speed
you want to use. Only three ways to change this inequality:
(1) Make the EV from the first equation bigger so it equals the second
by somehow adding light or increasing film speed, or both.
(2) Make the EV from the second equation smaller so it equals the first
by increasing aperture or decreasing shutter speed.
(3) Some combination of (1) and (2) to make them meet "in the middle."
You've ruled out (2) as your lens is already wide open and you don't want
to use a longer shutter speed than 1/30th. That leaves (1) as the only option.
When turning the compensation dial, you're not changing actual film speed,
you're only changing what the camera "thinks" your film speed is. Going in
the the plus (+) direction is the same as a lower film speed; in the minus
(-) direction is the same as a higher film speed.
-- John
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