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Archive 2008 · Exposure Compensation -- Why?

Mike Farren
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p.2 #1 · Exposure Compensation -- Why?

reggie747 wrote:
ALF,

I may be missing something here but your last post says "assume always shooting in Aperture Priority" yet you make 4 references to shutter speed adjusting ??
This line in particular has got me..

What gives here

Yeah this has got me bewildered too. As far as I know the only way to alter the shutter speed manually while in Av mode is to use exposure compensation.

Sep 05, 2008 at 05:07 AM

snurresprett
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p.2 #2 · Exposure Compensation -- Why?

Sep 05, 2008 at 05:12 AM

Fred Relaix
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p.2 #3 · Exposure Compensation -- Why?

Mike Farren wrote:
Yeah this has got me bewildered too. As far as I know the only way to alter the shutter speed manually while in Av mode is to use exposure compensation.

This is correct, if you adjust EC, your shutter speed will vary while your exposure should not.
EC is also very important if you use spot metering for exposure because (in my case at least) I would often spot in the shadow where I want to get details (then I would go EC between -1 and -2), or spot meter in the highlights (ie +1 to +2) to not blow them. Also for night pictures, if you want to take a picture that looks like what you see with your eyes, (again in Av mode) you absolutly need to get your EC down to -1 to -2. I aften wish my 5D would go to +/-3 or even +/-5.

Sep 05, 2008 at 05:19 AM

Steve Spencer
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p.2 #4 · Exposure Compensation -- Why?

Let me take a crack at this. Let's say I am shooting birds in AV mode to control aperture so that I have just the depth of field that I want in the shot. This means the camera will select the shutter speed and I will select the aperture and ISO. I know from either experience or perhaps from looking at the histogram from a previous shot, however, that the shutter speed that the camera will select will be too fast leaving the bird underexposed. To get a longer exposure I dial in positive exposure compensation which will cause the shutter speed to be longer and the bird to be better exposed. I could of course do exactly the same thing by flipping to manual mode and changing to a longer shutter speed. If the the light is pretty consistent then the two methods are six of one half a dozen of another. If the light is changing a fair bit, however, then AV mode can be nicer. Let's say the sun is going in and out from behind the clouds and changing the brightness of the scene a fair bit. With AV I only need to change the exposure compensation once and the setting will work whether the sun is at full brightness or behind a cloud. With manual I will have to reset the shutter speed each time the sun goes behind the cloud and each time it come back out. I hope this makes sense.

Best wishes,

Steve

Sep 05, 2008 at 05:37 AM

astrolucida
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p.2 #5 · Exposure Compensation -- Why?

alfarmer wrote:
Hmm. I'm not sure the point of my question is getting through to most responders, so let's assume always shooting in Aperture priority mode for a second.

When shooting snow or a white suite in daylight, I could certainly adjust exposre by +2 or -2. But it's MUCH easier and quicker to dial the big fat knob that adjusts shutter speed.

Well, this is exactly how exposure compensation is changed in Canon DSLRs! So you are using exposure compensation. When the aperture is fixed in the Av mode, the only way to change the effective exposure is to change the shutter speed. Thus, e.g. dialing in +1 for exposure compensation (you can see it in the viewfinder, too), results in doubling the exposure time.

alfarmer wrote:
And this leads to the one scenio not mentioned thus far, and the one case I can think of for my personal usage of Exposure Compensation -- low light settings where adjusting the shutter speed or ISO might result in unacceptable blur or noise.

No, exposure compensation does not help that because it changes those exact parameters, resulting in exactly the same results as if you had manually selected the same settings. There is no exposure compensation function like the one you seem to be thinking of.

Let's summarize:
- exposure settings consist of aperture, shutter speed and ISO,
- for ISO you can either pick the value or use Auto (in some cameras),
- you can set both the aperture and the shutter speed if you use M mode,
- you can set the aperture in Av mode and the camera sets the shutter speed,
- you can set the shutter speed in Tv mode and the camera sets the aperture,
- the camera will set both shutter speed and aperture in P mode.

When the camera decides some of the values, it is basing that decision on the built-in exposure meter. If you want to change that decision, by a relative value, you dial in exposure compensation (-2 to +2 with 1/3 stop steps). If you want to set the absolute values, you use M mode. In that case, you will see, in relative terms, how the camera would set the exposure values, in the viewfinder.

Edited on Sep 05, 2008 at 06:52 AM

Sep 05, 2008 at 06:50 AM

Dalantech
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p.2 #6 · Exposure Compensation -- Why?

The sensor in a digital camera reacts to under exposure in the same way as color positive slide film; colors saturate. So deliberately under exposing by 1/3 to 2/3 of a stop will give you saturated colors that look natural -something that can't be done by adjusting saturation in post...

Sep 05, 2008 at 07:07 AM

Stripper
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p.2 #7 · Exposure Compensation -- Why?

alfarmer wrote:
What's the point to having an Exposure Compensation setting?

ALF,

This is a great question and a lot of people have posted great answers. I guess I will take a shot at telling you why I use exposure compensation, at least in one situation.

First, let's take a look at what exposure compensation really does with a little help from Ansel Adams. Knowing that his camera could only capture a portion of the dynamic range in a scene he was trying to photograph, Adams developed his "Zone System". To simplify this hugely complex idea down to its simplest, what Adams was saying was that with exposure comp he could slide his camera's potential/capturable dynamic range around on the scale of the scene's actual dynamic range.
http://www.johncotephotography.com/Tutorials/ExposureComp/UnComp.gif
http://www.johncotephotography.com/Tutorials/ExposureComp/Plus1Comp.gif
http://www.johncotephotography.com/Tutorials/ExposureComp/Minus1Comp.gif
Adams knew that an exposure comp would put his camera's dynamic range closer to the shadow or highlight end of the scene's dynamic range.

I implement Adams' system by using the "Exposure Compensation" feature on my modern DSLR in certain situations. Take for instance when I am shooting a race car at noon on a partly cloudy day. The exposure is changing as the thin clouds move acorss the sky and block a bit of sunlight. The action is fast and I can not set a manual exposure for every shot. If I simply set the camera on the "Shutter Priority" of auto exposure the camera will compensate for the intermittently blocked and unblocked sun. However, if I don't dial in some negative exposure compensation, the sun coming from directly above will burn out the tops of the cars and my editor will not use any of my pictures.
http://www.johncotephotography.com/Tutorials/ExposureComp/Helio0.jpg
http://www.johncotephotography.com/Tutorials/ExposureComp/Helio-7.jpg
As Adams did in his chemical darkroom, I may have to make adjustments to the "Tonal Curves" of my exposure compensated image in my digital darkroom to bring the in-between tones back to "normal". But by using exposure compensation, I can come closer to capturing what I saw.

My camera is my tool. I use it to build an image. Exposure comp is a part of my tool. I have learned to use it to help me build the image I and my clients want.

BTW, you can not achieve the same thing by shooting in RAW mode and compensating later. It is far better to capture at the exposure you want...even in RAW.

I hope that makes sense. Happy photography!

JohnCote

Edited by Stripper on Sep 05, 2008 at 08:35 AM GMT

Edited on Sep 05, 2008 at 08:35 AM

Sep 05, 2008 at 07:34 AM

Photon
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p.2 #8 · Exposure Compensation -- Why?

astrolucida wrote:
Well, this is exactly how exposure compensation is changed in Canon DSLRs! So you are using exposure compensation. When the aperture is fixed in the Av mode, the only way to change the effective exposure is to change the shutter speed. Thus, e.g. dialing in +1 for exposure compensation (you can see it in the viewfinder, too), results in doubling the exposure time.

No, exposure compensation does not help that because it changes those exact parameters, resulting in exactly the same results as if you had manually selected the same settings. There is no exposure compensation function like the one you...Show more →
Excellent, you've justified the "lucid" portion of your name!
I want to stress one more point:
- Photographers have always used exposure compensation in manual mode!
Reading an exposure meter, selecting a combination of shutter speed and aperture that yields that exposure value, then altering one or both of them in the same direction (toward over or under exposure) is exposure compensation.

Hand held exposure meters often have a setting for exposure compensation. It could be accomplished by changing the ISO on the meter (not on the camera), but it can be simpler to set the dial to, say, -2 EV and have that bias apply to all readings in a particular circumstance regardless of what ISO, shutter speed, and aperture are chosen.

Sep 05, 2008 at 07:37 AM

nathanlake
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p.2 #9 · Exposure Compensation -- Why?

There seem to be a lot of posts here from people that don't understand the basics of photography. It is hard to expain the uses for EV when they don't even understand the basics of exposure and do not seem to have any experience with backlit or high contrast scenes.

Edited on Sep 05, 2008 at 09:34 AM

Sep 05, 2008 at 09:33 AM

dhphoto
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p.2 #10 · Exposure Compensation -- Why?

nathanlake wrote:
There seem to be a lot of posts here from people that don't understand the basics of photography. It is hard to expain the uses for EV when they don't even understand the basics of exposure and do not seem to have any experience with backlit or high contrast scenes.

And lets face it none of this has anything to do with Canon gear, it should be in the lighting forum or whatever

David

Sep 05, 2008 at 09:35 AM

wilt
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p.2 #11 · Exposure Compensation -- Why?

All reflected light meters (what is in your camera) are calibrated on the 'average scene' content, which averages to 18% gray. If you have an extra bright or extra dark area predominating the scene, you are now ready to be victimized by 'subject failure'. So rather than use the exposure indicated in the camera, you dial in a 'fudge factor' via EC change. Plain and simple.

Sep 05, 2008 at 09:56 AM

jerrykur
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p.2 #12 · Exposure Compensation -- Why?

alfarmer wrote:
Ok, I just don't get it and nobody I know can explain it. So hopefully some FM expertise will help.

What's the point to having an Exposure Compensation setting? Why not simply adjust the ACTUAL exposure? Bracketing has a clear and obvious value, but dialing in some constant offset does nothing more than complicate the mental process during shooting. Why would anyone EVER want/need to do that?

Thanks,
ALF

I use exposure compensation when the camera's algorithms do not provide the exposure I want for a scene. For example, if you are in a dim room and want to take a picture in which you capture the room's dimness. If you set you camera to AV or TV and take the shot the shot will come out much brighter than reality. You have to dial in a -1 or more exposure compensation. Likewise on a bright sunny day with snow in the picture, the camera will tend to under expose. Unless you like gray snow you need to compensate by using a +1.5 or +2 exposure compensation.

jerry

Sep 05, 2008 at 10:31 AM

DavidP
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p.2 #13 · Exposure Compensation -- Why?

If you're in Av mode, Exposure Compensation simply adjusts the metered/calculated shutter speed by the number of stops (partial stops, plus or minus) that you've selected.

If you're in Tv mode, Exposure Compensation simply adjusts the metered/calculated aperture by the number of stops (partial stops, plus or minus) that you've selected.

In M mode, there's no point in Exposure Compensation (thus, there's no way to set it).

Sep 05, 2008 at 10:49 AM

alfarmer
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p.2 #14 · Exposure Compensation -- Why?

Thanks everyone for the answers. I can see now why some people use exposure compensation.

Sep 05, 2008 at 11:11 AM

AJSJones
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p.2 #15 · Exposure Compensation -- Why?

Yup, You either shoot in manual and give no responsibility to the camera's intelligence or you give it some responsibility such as Av or Tv etc. Those modes can have the camera do things for you faster or more easily than you can and have their uses. However, you know it's not always as smart as you are and cannot ever read your mind, so you sometimes have to compensate for its inadequacies. Once you get to know its limitations, you'll know when and how to override them - EC is one of those ways.

Sep 05, 2008 at 11:32 AM

Photon
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p.2 #16 · Exposure Compensation -- Why?

DavidP wrote:
...

In M mode, there's no point in Exposure Compensation (thus, there's no way to set it).

Except that as I pointed out above, we set the compensation "manually". It was called exposure compensation even when the only in-camera meters were "match needle", and AE hadn't been introduced.

Sep 05, 2008 at 11:41 AM

Lord Fluff
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p.2 #17 · Exposure Compensation -- Why?

DavidP wrote:
In M mode, there's no point in Exposure Compensation (thus, there's no way to set it).

Hmmm..except I wish there was. I wish I could take a reading and then 'lean on it' and go back to it using EC - then I would always know where I stood without having to remember what my base setting was.

Sep 05, 2008 at 01:03 PM

freaklikeme
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p.2 #18 · Exposure Compensation -- Why?

dhphoto wrote:
Yes, of course.

Exposure compensation alters the camera's attempt at correct exposure (i.e. to render 18% grey) by an amount decided by the user.

The example given is not a good one as frankly there is no free lunch in this situation and the contrast range is just going to be beyond what the camera can handle, you need to lower the contrast, but here's another: If I want to photograph caucasian skin in daylight I know from experience I'm going to want +2/3 of a stop to render the skin the tone I want. If the overhead conditions change and the...Show more →

That makes sense. Thank you, David.

Sep 05, 2008 at 01:12 PM

Luke Mislinski
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p.2 #19 · Exposure Compensation -- Why?

Dalantech wrote:
The sensor in a digital camera reacts to under exposure in the same way as color positive slide film; colors saturate. So deliberately under exposing by 1/3 to 2/3 of a stop will give you saturated colors that look natural -something that can't be done by adjusting saturation in post...

+1

I usually underexpose my sunrise/sunset pictures by anywhere from 1 to 2 stops to do just this.

Sep 05, 2008 at 01:22 PM

mabidally
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p.2 #20 · Exposure Compensation -- Why?

alfarmer wrote:
dialing in some constant offset does nothing more than complicate the mental process during shooting

Lets say your shooting in aperture priority say f16 you shoot a frame and its too bright and you need to re shoot the frame two stops slower... you have two options...

1. Flip the wheel anticlockwide with you thumb 1 or 2 stops negative EC and shoot again - presto to you get a less bright exposure.

2. Set the camera to manual, now recall from memory what was that exposure setting and reduce two stops, then dial in the same less two stops...it does take a bit more mental effort and time too me thinks.

Edited on Sep 05, 2008 at 01:24 PM

Sep 05, 2008 at 01:23 PM

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