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Exposure compensation

 
 
LouisB
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      11-15-2006
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?

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?

If anyone has the patience to succinctly explain this or point me in the
right direction, I would be eternally grateful.

LouisB
------
"I'm a half-wit. I sold the other half on e-Bay"


 
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rowan194
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      11-15-2006

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?
>
> 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?
>
> If anyone has the patience to succinctly explain this or point me in the
> right direction, I would be eternally grateful.


Exposure compensation allows you to "shift" the camera's automatically
chosen shutter/aperture values in order to expose more or less.
Exposure meters are not perfect so there are some tricky (and even not
so tricky) situations where you'll need to help them in order to get
the best possible exposure. Basically anything which fools the
averaging effect of the exposure meter will probably need some manual
adjustment.

I usually use exposure compensation in conjunction with the histogram
which is shown after the shot is taken - the exposure meter makes an
educated guess before the shot, the histogram shows exactly how that
estimated exposure has been captured. For myself, if there seems to be
more than about a stop that is unused I'll retake the shot with
positive exposure compensation. If the histogram (or image) shows blown
highlights then I'll retake it with negative exposure compensation.

By the way, RAW is not magic: if you increase the exposure too much in
post processing you'll bring out noticeable noise. If you have
overexposed the shot then negative exposure compensation may require
some "guessing" by the RAW converter as one or more of the R, G and B
channels are overexposed (clipped) beyond their maximum values.

If you do a Google search for "exposure compensation" you should be
able to find out more.

 
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jeremy
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      11-15-2006
"LouisB" <(E-Mail Removed)> wrote in message
news:qxJ6h.134560$(E-Mail Removed) .uk...

>
> 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?
>



Start here:

http://www.kenrockwell.com/tech/digital-exposure.htm

and here:

http://www.kenrockwell.com/tech/exposure.htm

And, if you feel ready to explore parts unknown, here:

http://www.tofahrn-foto.de/index.php?lg=en&pg=tipps.dri


 
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Charles Schuler
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      11-15-2006

"LouisB" <(E-Mail Removed)> wrote in message
news:qxJ6h.134560$(E-Mail Removed) .uk...
> 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 is metering for neutral gray. When you are in the snow, the
camera will turn the snow to gray. So, you dial in +EV. When you are in
the dark, the camera will change black to neutral gray. So, you dial
in -EV.

When in doubt, you could use EV bracketing (the camera will take three shots
.... depends on your camera).

If you are serious, use a spot meter and the zone system!


 
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Paul Mitchum
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      11-15-2006
LouisB <(E-Mail Removed)> 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?
>
> 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?
>
> If anyone has the patience to succinctly explain this or point me in the
> right direction, I would be eternally grateful.


The classic example is a back-lit scene. Say you want to photograph a
person standing in front of a bright sky, without using fill flash. The
camera's light meter will likely meter for the bright sky, rather than
the shadowy figure, leaving you with an image of an indistinct
sillhouette. So you dial in +1.5 EV (or so). The camera still meters off
the sky as before, but boosts the exposure time and/or aperture
(depending on how the camera figures these things out), and you end up
with a washed-out sky and a well-exposed person.

If your camera allows it, you can do the same thing by just changing the
aperture and/or shutter speed yourself. EV is measured in stops, so
opening the aperture one stop gives you +1 EV. Doubling the time of the
shutter speed is +1 EV. Do both and get +2 EV.

As to when to use EV compensation: That depends on what you're
photographing. In the above example, if you *wanted* a sillhouette
against the sky, then boosting the EV would be a bad idea. It's a
judgement call.

HTH.
 
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John McWilliams
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Posts: n/a
 
      11-16-2006
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?
>
> 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?


No. It's way better to nail the exposure as close as you can, whether or
not you shoot RAW, even though it is more forgiving on luminance and
color balance.

--
John McWilliams
 
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Floyd L. Davidson
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Posts: n/a
 
      11-16-2006
"LouisB" <(E-Mail Removed)> 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) http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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