How does exposure compensation *actually* work.
Erez 'HWank1' Zadok
does exposure compensation in various modes (manual, shutter priority,
aperture priority, and all automatic.)
It is especially unclear to me what might happen in manual mode. I mean, in
that mode you set the speed and aperture yourself. Now you can adjust
exposure in steps of 1/3 or 1/2 stops (depending on your camera.) What
happens next? Does the camera adjust the shutter speed or aperture
slightly? Well if it did then it may introduce unwanted blurriness or take
away one that's wanted. If it changed the aperture then the DOF changes and
that too may be undesirable? So what *really* goes in there? Is there some
other factor that gets adjusted internally? (It cannot be the film speed
since that is only advisory as to the aperture/shutter curves.)
Thanks,
Erez.
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David Jacobson
>I was wandering what exactly
>It is especially unclear to me what might happen in manual mode.
I speak as the owner of a Nikon 8008. I'll assume that other cameras
are similar. The short answer is that the effect of exposure
compensation is exactly the same as though the film speed were
changed.
In manual mode, for any particular f stop and shutter speed, it
changes the position of the "bar chart" display. So if you adjust f
stop or shutter speed to get the bar to zero, *you* will have selected
a different aperture or speed than you would have without the
compensation. The f stop and shutter speed are exactly what you set.
In the automatic modes it affects the dependent setting(s).
In all modes it changes the sensitivity of the TTL flash sensor.
Flash compensation, as opposed to the normal exposure compensation,
changes the sensitivity of the TTL flash sensor, with is changes
adding to the changes from the regular compensation control. (The
8008 and 8008s do not have a flash compensation control on the body;
it is on the SB-24 flash and couples to the body via a microprocessor
link.)
-- David Jacobson
Rob Hasker
>I have a question to the techies out there. I was wandering what exactly
>does exposure compensation in various modes (manual, shutter priority,
>aperture priority, and all automatic.)
All cameras I know of do exposure compensation by changing the film speed
used to compute exposure. This affects the curves used in the automatic
and semi-automatic modes, but has no effect on manual mode.
--
Rob Hasker University of Illinois at Urbana-Champaign
has...@cs.uiuc.edu {uunet|convex|pur-ee}!uiucdcs!hasker
Internet->I18BC@CUNYVM.CUNY.EDU
Changing the exposure compensation while in manual mode
shouldn't change the shutter or aperture you selected. However,
if your camera has TTL flash metering it could affect the flashes
output. This can be helpful if your flash does not have
exposure compensation. I use the exposure compensation for one to
two stops under for fill-flash on sunny days.
Allan Armstrong
>I have a question to the techies out there. I was wandering what exactly
>does exposure compensation in various modes (manual, shutter priority,
>aperture priority, and all automatic.)
>
>It is especially unclear to me what might happen in manual mode. I mean, in
>that mode you set the speed and aperture yourself. Now you can adjust
>exposure in steps of 1/3 or 1/2 stops (depending on your camera.) What
>happens next? Does the camera adjust the shutter speed or aperture
>slightly? Well if it did then it may introduce unwanted blurriness or take
>away one that's wanted. If it changed the aperture then the DOF changes and
>that too may be undesirable? So what *really* goes in there? Is there some
>other factor that gets adjusted internally? (It cannot be the film speed
>since that is only advisory as to the aperture/shutter curves.)
Exposure compensation does not operate in manual mode. I have, as proof,
three recent shots that I exposure bracketed not realizing I had already
set the camera in manual mode. As I expected, all three came out exactly
the same. This was a shot I took very seriously. It was a closeup of a
very unusual mushroom. I spent much time setting up the tripod, choosing
f22 for best depth of field, metering off a background that was much
closer to 18% grey than this white mushroom and setting the appropriate
shutter speed, focusing carefully, and then exposure bracketing just in
case my background wasn't really 18% grey. To exposure bracket, I should
have changed the shutter speed instead of the compensation dial. Oh
well. My guess that the background was near 18% grey was pretty accurate
as all three came out properly exposed.
In shutter priority, exposure compensation adjusts the f-stop.
In aperture priority, exposure compensation adjusts the shutter speed.
Your comments about changes in depth-of-field or blurring moving
subjects are accurate. Look at the readings your camera shows you to
judge how acceptable this is.
Try a simple test. Compose a shot in aperture priority. Press the
shutter halfway to see what shutter speed the camera selects. Now move
the exposure compensation dial and see how the camera selects a
different shutter speed.
In the program mode, the camera adjusts both aperture and shutter speed
according to the measured exposure value. The curve is shown in your
camera manual. If exposure compensation is selected, the camera just
adjusts the exposure value to reflect this tweak and the adjusted value
is used for the index of the table. Typically the way these curves work
is that for very bright subjects the camera is set to 1/1000 shutter
speed and the aperture is adjusted to get the exposure correct. For dim
subjects, the camera is set to maximum aperture and shutter speed is
set. For intermediate subjects (most shots) both aperture and shutter
speed are adjusted to slowly move from 1/60 second at f2 (example lens
f2 to f22) to 1/1000 second at f22. In the case of an intermediate
subject, exposure compensation will only move one of the controls
(selected by which is closer to the quantized step) for abs(
compensation ) <= 1 and both of the controls for abs( compensation ) >
1.
You shouldn't be using "P" anyway. That feature is only used for
snapshots and loaning your camera to friends. I'm glad that my friend's
cameras had that feature. Otherwise, I'd have never started.
One thing that's really handy is to train your eyes to recognize 18%
grey. I can roughly gauge that in a forest scene, but I expect as I get
more experience, I'll even know what 36% and 9% looks like and I won't
have to exposure bracket anymore. I spent the weekend skiing and took
many snow photos. Exposure for them is tricky as snow looks to be at
least 72% grey or at least 2 stops different. Unfortunately, many
photographs contain a mix of grey rock (probably ~18%) and snow or dark
pines and redwoods (probably ~12%) and snow. It can be difficult to
judge exactly what exposure is appropriate in these conditions, so I've
exposure bracketed. When this film comes back, I'll know more.
Does anyone have much experience taking wintertime snow landscape
photos? Anything to share about getting exposure correct the first time?
What about situations with extreme glare?
Another thing I noticed was that as I rotated the polarizer, in some
situations it would work just like the textbook by darkening and
lightening the skies, with the dark 90' away from the light. In some
situations, however, it didn't seem to darken or lighten, but the color
would change from grey to blue. What happens here? Also, why don't they
just mark vertical and horizontal polarization on the dial so we don't
have to fiddle with it? Aren't skies always vertically polarized, so all
you'd have to do is rotate 90' when you change from lanscape to portrait
mode? Then they could install a couple of detents.
Allan
matthew.noreen
Any experiences out there with the aforementioned "LightDome". The December
Shutterbug said that it was excellent at softening the flash and
reducing redeye.
P.S. Most registered Nikon owners probably got the SB-24 Flash "Video"
flyer in the mail. Anyone buy this video, describing SB-24 flash "tips".
Matthew Noreen
m...@cbnewsg.cb.att.com