What exactly does "exposure compensation" do on a Rebel XT?

In the "ISO setting vs Noise" thread, I mentioned a co-worker's
dilemma when taking pictures of his son playing hockey indoors. He
wanted fast shutter, smallest aperture (most DOF), and lowest ISO, all
at a reasonable price.

The lady at Henry's suggested that he first try increasing the
"exposure compensation" setting a bit to move the histogram to the right.
The situation was so badly underexposed that pushing compensation plus 2
full stops took it from badly underexposed to merely slightly
underexposed. The result was a vast improvement, i.e. a lot less noise
after the raw file has been brightened to look reasonable.

While he's not complaining at all, he's mystified, and so am I. The
camera's manual says that upping exposure compensation a full stop is
equivalant to opening up the aperture a full stop, or doubling the
length of the exposure. This is done by "increasing the sensitivity" of
the sensor. Huh??? Looking at photos take with +2 stops exposure
compensation...
- the players were not blurry, so the shutter wasn't slowed down
- DOF was unaffected, so aperture was not reduced
- the pictures (ISO 400 like previous shots) were *LESS GRAINY* than
before, and definitely nowhere near as grainy as he has seen with
ISO 1600 in the past

So where does this mysterious improvement come from? He expected that
upping exposure compensation would "borrow from Peter to pay Paul", but
that's not the case. If so, shouldn't he run around with exposure
compensation cranked up all the time, and lower ISO or cut down aperture
or faster shutter, in order to get optimal pictures (except when he
wants certain special effects)?

--
Walter Dnes; my email address is *ALMOST* like
Delete the "z" to get my real address. If that gets blocked, follow
the instructions at the end of the 550 message.

While he's not complaining at all, he's mystified, and so am I. The
camera's manual says that upping exposure compensation a full stop is
equivalant to opening up the aperture a full stop, or doubling the
length of the exposure. This is done by "increasing the sensitivity" of
the sensor. Huh??? Looking at photos take with +2 stops exposure
compensation...

EC doesn't increase the sensitivity of the sensor, it tells the ounboard
computer to meter, then use exposure settings that are above or below that
value based on what you told it to do. If you're in Av mode, you'll get a
faster or slower shutter. If you're in Tv mode, you'll get a larger or
smaller aperture. I haven't watched it in P mode, but it will do shutter,
aperture, or both. It doesn't work in manual mode, because it does't make
any sense in manual mode (YOU do the metering analysis).

- the players were not blurry, so the shutter wasn't slowed down
- DOF was unaffected, so aperture was not reduced
- the pictures (ISO 400 like previous shots) were *LESS GRAINY* than
before, and definitely nowhere near as grainy as he has seen with
ISO 1600 in the past

Look at the EXIF data, and see what has changed. With the same ISO,
aperture, shutter, and lighting, there won't be any difference in exposure.

So where does this mysterious improvement come from? He expected that
upping exposure compensation would "borrow from Peter to pay Paul", but
that's not the case. If so, shouldn't he run around with exposure
compensation cranked up all the time, and lower ISO or cut down aperture
or faster shutter, in order to get optimal pictures (except when he
wants certain special effects)?

Ideally, the camera's metering system should do a fairly good job of
getting it right. It takes *really* extreme situations for me to need +2 on
my RebXT, much more than just shooting on a white hockey rink. It sounds
like either something in the scene is really fooling the camera, the players
are lit from very bad angles (almost backlit), he's hit a limit of the
camera/lens (can't do a larger aperture, etc.), or the camera is
malfunctioning. Can you post some images (preferably RAW) for us to poke
around at?

If the lighting across the rink is fairly constant, he might want to use
manual mode, find the correct exposure, and leave it there.

steve

Walter Dnes (delete the 'z' to get my real address) wrote:
In the "ISO setting vs Noise" thread, I mentioned a co-worker's
dilemma when taking pictures of his son playing hockey indoors. He
wanted fast shutter, smallest aperture (most DOF), and lowest ISO, all
at a reasonable price.

The lady at Henry's suggested that he first try increasing the
"exposure compensation" setting a bit to move the histogram to the
right. The situation was so badly underexposed that pushing
compensation plus 2 full stops took it from badly underexposed to
merely slightly underexposed. The result was a vast improvement,
i.e. a lot less noise after the raw file has been brightened to look
reasonable.

While he's not complaining at all, he's mystified, and so am I. The
camera's manual says that upping exposure compensation a full stop is
equivalant to opening up the aperture a full stop, or doubling the
length of the exposure. This is done by "increasing the sensitivity"
of the sensor. Huh??? Looking at photos take with +2 stops exposure
compensation...
- the players were not blurry, so the shutter wasn't slowed down
- DOF was unaffected, so aperture was not reduced
- the pictures (ISO 400 like previous shots) were *LESS GRAINY* than
before, and definitely nowhere near as grainy as he has seen with
ISO 1600 in the past

So where does this mysterious improvement come from? He expected
that upping exposure compensation would "borrow from Peter to pay
Paul", but that's not the case. If so, shouldn't he run around with
exposure compensation cranked up all the time, and lower ISO or cut
down aperture or faster shutter, in order to get optimal pictures
(except when he wants certain special effects)?

Post some shots, preferably with EXIF data on each (pbase.com will include
this automatically).

Your friends camera was likely metering off of the white ice, which fools a
camera into thinking the scene is way too bright...leading it to severely
decrease the exposure value. Without adjustment, people get similarly dark
results shooting snow scenes because camera meters are designed to render
middle tones. This is why images of guys in black suits can render light
faces as far too bright...and girls in white wedding dresses as too dark.

You may need to establish what a reasonable exposure is under the skating
rink lights using a grey card, or similarly middle-tones subject, and then
shoot in manual mode with those settings. Hockey ice is notorious for
throwing meters off because you're constantly reframing over white ice
and/or darker background uniforms or stands. Since the lighting in the rink
is usually fairly consistent, you should be able to shoot in manual mode
once you've established a basic exposure setting. You can easily check this
with the histogram after a few test shots if you don't have a grey card,
etc. If none of this makes any sense at all, then I'd suggest reading a
basic book on exposure, since this is pretty basic stuff...though that's not
to say it doesn't fool a lot of folks...


--
Images (Plus Snaps & Grabs) by Mark² at:
www.pbase.com/markuson

On Tue, 10 Oct 2006 20:58:17 -0700, Mark², mjmorgan@cox wrote:

You may need to establish what a reasonable exposure is under the
skating rink lights using a grey card, or similarly middle-tones
subject, and then shoot in manual mode with those settings.
Hockey ice is notorious for throwing meters off because you're
constantly reframing over white ice and/or darker background
uniforms or stands. Since the lighting in the rink is usually
fairly consistent, you should be able to shoot in manual mode once
you've established a basic exposure setting. You can easily check
this with the histogram after a few test shots if you don't have a
grey card, etc. If none of this makes any sense at all, then I'd
suggest reading a basic book on exposure, since this is pretty basic
stuff...though that's not to say it doesn't fool a lot of folks...

Thanks Mark and Steve. This actually does make sense. The ice fools
the automatic mode, and you have to over-ride it manually. Of course,
this may result in portions of the ice blowing out.

I ran into a similar issue photographing early-morning frostscapes.
The ground frost, by definition, was around only in shadowed areas.
Exposing to make shadowed areas reasonable blew out sky and areas in
sunlight. The fact that my FZ5 only does TIFF and JPEG doesn't help.
See http://www.pbase.com/waltdnes/fall_colours_2006 and go to the "Early
morning ground frost" sub-gallery. The p1010933.jpg has an interesting
sur-realistic look. The ground frost wasn't quite as bad as the photos
indicate. I didn't do any trick processing, just cropping or binning
down from 2048x1536 TIFFs to something more suitable for the web; the
photos just came out that way.

--
Walter Dnes; my email address is *ALMOST* like
Delete the "z" to get my real address. If that gets blocked, follow
the instructions at the end of the 550 message.

Exposure compensation is an awesome feature. It can keep you out of jail.
Here is how it works. When you approach the ladies and pull your trench coat
open to show them your equipment, it pulls the trench coat closed in like
1/8th of a second, before there was exposure.

R


"Walter Dnes (delete the 'z' to get my real address)"
wrote in message
...
In the "ISO setting vs Noise" thread, I mentioned a co-worker's
dilemma when taking pictures of his son playing hockey indoors. He
wanted fast shutter, smallest aperture (most DOF), and lowest ISO, all
at a reasonable price.

The lady at Henry's suggested that he first try increasing the
"exposure compensation" setting a bit to move the histogram to the right.
The situation was so badly underexposed that pushing compensation plus 2
full stops took it from badly underexposed to merely slightly
underexposed. The result was a vast improvement, i.e. a lot less noise
after the raw file has been brightened to look reasonable.

While he's not complaining at all, he's mystified, and so am I. The
camera's manual says that upping exposure compensation a full stop is
equivalant to opening up the aperture a full stop, or doubling the
length of the exposure. This is done by "increasing the sensitivity" of
the sensor. Huh??? Looking at photos take with +2 stops exposure
compensation...
- the players were not blurry, so the shutter wasn't slowed down
- DOF was unaffected, so aperture was not reduced
- the pictures (ISO 400 like previous shots) were *LESS GRAINY* than
before, and definitely nowhere near as grainy as he has seen with
ISO 1600 in the past

So where does this mysterious improvement come from? He expected that
upping exposure compensation would "borrow from Peter to pay Paul", but
that's not the case. If so, shouldn't he run around with exposure
compensation cranked up all the time, and lower ISO or cut down aperture
or faster shutter, in order to get optimal pictures (except when he
wants certain special effects)?

--
Walter Dnes; my email address is *ALMOST* like
Delete the "z" to get my real address. If that gets blocked, follow
the instructions at the end of the 550 message.

Hebee Jeebes wrote:
Exposure compensation is an awesome feature. It can keep you out of jail.
Here is how it works. When you approach the ladies and pull your trench coat
open to show them your equipment, it pulls the trench coat closed in like
1/8th of a second, before there was exposure.

R


That's not correct. It's when the ladies pay to look.
Dave Cohen

They pay? Really? 8^)

R


"Dave Cohen" wrote in message
news:R9xXg.5454$WD1.3725@trndny04...
Hebee Jeebes wrote:
Exposure compensation is an awesome feature. It can keep you out of jail.
Here is how it works. When you approach the ladies and pull your trench
coat open to show them your equipment, it pulls the trench coat closed in
like 1/8th of a second, before there was exposure.

R


That's not correct. It's when the ladies pay to look.
Dave Cohen