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RE: [OM] exposure

Subject: RE: [OM] exposure
From: Ray Moth <ray_moth@xxxxxxxxx>
Date: Tue, 4 Sep 2001 23:16:13 -0700 (PDT)
Don Gaikins wrote: 

(Snip) . . . I would still like to know just what exposure compensation
is and how it works.  What exactly happens when you turn that knob on
the top of the camera?  Does it just effect the background, or does it
change the aperture?

================================================================
Don,

The exposure compensation dial is mechanically part of the film speed
dial. To set the correct film speed, as you know, you lift the knob,
turn it until the required film speed appears in the rectangular window
in the knob, and then lower the knob and rotate it to line up the
window with the line on the prism housing. If you turn the exposure
compensation dial from that position you are changing the film speed
that the camera uses to gauge exposure. 

Each click of the compensation dial represents 1/3 exposure value (EV).
Such small increments are useful when bracketing with slide film, which
has narrow exposure latitude. It's a bit hit-and-miss until you gain
experience with it and many users of OM-2S, OM-3 and OM-4 cameras
prefer to use spot metering to get the optimium exposure.

Example: if you turn the exposure comensation dial 3 clicks in the "+"
direction, you are telling the camera that the film speed is half of
what it really is: e.g. if using an ISO 400 film, you are pretending
that it is an ISO 200 film. This will cause the camera to over-expose
by 1 EV. You may need to do this when shooting a scene where the
overall reflectance is brighter than "18 0rey", like a subject against
a white wall, a sunlit beach, or when shooting a subject which is in
shadow. Snow scenes typically require compensation of around +2 EV, to
prevent the snow from looking grey in the final slide/print. 

Conversely, if you turn the exposure comensation dial 3 clicks in the
"-" direction, you're telling the camera that the film speed is twice
what it really is. The camera will then under-expose by 1 EV. You might
want to do this when the scene is darker than "18 0rey", such as with
a subject against a dark wall or a close-up of a black cat. 

If you use print film, be aware that all your endeavours to compensate
for bright or dark subjects may be defeated by the processing lab,
whose print machines have their own light metering that may try to
compensate in the opposite direction unless otherwise instructed! 

OM cameras with compensation dials show a symbol in the viewfinder to
remind you that you have dialled a + or - compensation. Even so, it's
easy to forget to reset it, which can be a pain in the *ss, especially
with slide film.

Hope this helps.

Regards,

=====
Ray

"The trouble with resisting temptation is
 you never know when you'll get another chance!"

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