"LouisB" <> wrote:
>OK, I'm a little confused. It has been years since I did any serious
>photography and I'm staggered at what I have forgotten.
>
>These new digital cameras have lots of easy ways to dial in +ve or -ve EV in
>increments (mine is 0.3, 0.5 or 1 depending on how you customise it). I can
>even bracket in any of those ranges.
>
>I'm just a little unsure as to the rules here. How exactly is the exposure
>altered by using positive or negative compensation? What are the basic rules
>for deciding when and how to compensate?
The camera's light meter is calibrated to assume a scene
averages at about 14-18% gray. If the scene does average that,
your exposure will probably be about right; if a scene doesn't,
then you can use exposure compensation to re-calibrate the light
meter.
But if you shoot a scene that is mostly white (on a winter day
outside in the snow, for example), the scene's average will not
be middle gray. If you do not adjust the meter, it will meter
it for exposure as middle gray, and the resulting images will
have gray snow and be very drab.
Instead, what you do is dial in +1.5 EV, which tells the light
meter than the average is going to be about 1.5 fstops brighter
than middle gray.
Likewise if you take a picture where the background is very dark
it may be necessary to dial in a negative EV number.
All it does is calibrate the exposure meter. The specific
effects from that depend on the various exposure modes available
on your camera. For example, if you use "manual mode" the
immediate affect is only to tell you that you are over/under
exposing by the number of fstops you have dialed in as EV
compensation. If you don't adjust the exposure, nothing
changes... You could just as well leave the camera set to 0 EV
and simply set the aperture/shutter to over/under expose.
But with any kind of "automatic" exposure mode, something will
change, so setting meter calibration with EV compensation is
nice.
>I shoot mainly in RAW mode so I can play around with the compensation when I
>post process but I'd like to know whether that absolves me from even
>worrying about exposure compensation, at all?
Setting exposure compensation is the right way to adjust
exposure when you use any "automatic" exposure mode in the
camera. It is perhaps a waste of time (but may be easy just
because it is commonly used) when shooting in "manual" exposure
mode.
The trick with digital and RAW mode though, is to *ignore* the
meter anyway! (Okay, not totally... but don't use it for
anything other than an quick way to get an initial starting
point for exposure.) Set your camera to either show a
histogram or better yet a "blinking over exposure" LCD display.
Use the exposure meter to determine initial settings, but look
at the histogram/blinking-display to determine if exposure is
right. Between the two, what you want is highlights that should
not be blown to be almost up against the right side of the
histogram. If there are some highlights (light sources, for
example) that you are going to allow to be blown out, a blinking
display is the only way to know what is over exposed and what is
not.
Just increase exposure until the desired highlights blink,
and then back off until they stop. It is fairly easy to get
within 1/3 fstop.
The essential effect of setting exposure as above is that the
image will record the maximum dynamic range possible. It might
well be that the "highlights" are NOT white! But if the
highlights are actually gray, you will record data well down
into the shadows that would be lost if those gray highlights are
set to expose in mid range. With post processing you can adjust
to restore the highlights at middle grey... *and* if you want
it is also possible to "pull" the shadows up too, without the
noise and posterization that would result if the gray had been
exposed as gray! (All of which is part of what makes digital so
much fun. That can be done with film too, but it is not so
easy.)
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)
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