Exposure Compensation/bracketing - How does it work

PaulieC

Hi all,

This is something I've been wondering for a while and could never get a satisfactory answer. How does Exposure Compensation work? I know that it darkens or brightens your exposure by 1-stop or whatever you've set, but how does the camera do this?
The only ways I know of to alter exposure is shutter speed, aperture size and ISO. Of these ways, the only way I can think of that a camera could create different exposures without affecting the picture (i.e. DOF, grain etc) would be by using different shutter speeds.

can anyone satisfy my curiousity? Obviously it's not critically important, just nice to know. I did a photo course last year and asked the instructor how it worked and her answer was 'it just does' :rolleyes:

elven

I had to look it up - but it basically does change the settings, without telling you on the main display! There is no other way to let more/less light in apart from changing the shutter speed/aperture. Here's a bit of the thing I read:

Depending on how your digital camera deals with exposure compensation (and the shooting mode used), it may adjust the aperture while maintaining the shutter speed constant, it may adjust the shutter speed while maintaining the aperture constant, or it may adjust both the aperture and shutter speed. On the digital camera I am using for this tutorial, the camera first adjusts the aperture while maintaining the shutter speed constant; when it can't adjust the aperture anymore, it then adjusts the shutter speed.

Camera Metered Exposure
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

So, let's say that the light meter measures a shutter speed of 1/125 sec. and aperture of F4.0.

+1EV Exposure Compensation
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

When I dial in a +1EV exposure compensation, the meter opens up the aperture by 1 f-stop to F2.8, while still maintaining a shutter speed of 1/125 sec. In effect I have purposefully dialed in an overexposure.

-1EV Exposure Compensation
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

If I dial in a -1EV exposure compensation, the meter closes down the aperture by 1 f-stop to F5.6, while maintaining the same 1/125 sec. shutter speed. In effect I have purposefully dialed in an underexposure.

PaulieC

Thanks Elven
that's interesting...I would have thought that changing the aperture would alter the picture, slightly changing the depth of focus?
So how would bracketing work? If the camera has to make three different exposures of the same pic in one go?

Morlar

elven wrote:

I had to look it up - but it basically does change the settings, without telling you on the main display! There is no other way to let more/less light in apart from changing the shutter speed/aperture. Here's a bit of the thing I read:

Depending on how your digital camera deals with exposure compensation (and the shooting mode used), it may adjust the aperture while maintaining the shutter speed constant, it may adjust the shutter speed while maintaining the aperture constant, or it may adjust both the aperture and shutter speed. On the digital camera I am using for this tutorial, the camera first adjusts the aperture while maintaining the shutter speed constant; when it can't adjust the aperture anymore, it then adjusts the shutter speed.

Camera Metered Exposure
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

So, let's say that the light meter measures a shutter speed of 1/125 sec. and aperture of F4.0.

+1EV Exposure Compensation
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

When I dial in a +1EV exposure compensation, the meter opens up the aperture by 1 f-stop to F2.8, while still maintaining a shutter speed of 1/125 sec. In effect I have purposefully dialed in an overexposure.

-1EV Exposure Compensation
F1.8 F2.8 F4 F5.6 F8
1/500 1/250 1/125 1/60 1/30

If I dial in a -1EV exposure compensation, the meter closes down the aperture by 1 f-stop to F5.6, while maintaining the same 1/125 sec. shutter speed. In effect I have purposefully dialed in an underexposure.

/*takes notes*

elven

Well, it will affect the DOF if it's messing with your aperture. But unless you're shooting macro I don't see 1/2 - 2 stops of a difference having a perceptible (is that a word?) effect on the DOF.

Bracketing is basically getting the camera to take 3 shots at different exposures, at however many stops of a difference either side of what you have it set at - it's basically an automatic way of you setting it to manual and taking one picture underexposed, one exposed properly (I use the term loosely) and one overexposed. These days it's a handy thing to do if you want to produce HDR images because depending on the tonal range of the scene you should end up with one exposure with detail in the shadows, and one with detail in the highlights which you can merge together.

Am I rambling yet? *snore*

311

Was reading up on it aswell ,seemingly it changes depending on what way you use the camera ,AV mode EC changes shutter speed ,TV mode it changes the aperture.

elven

It's a bit annoying then, considering that mostly I use Av and rely on the meter reading to know whether the shutter speed is going to be too slow for me to hand hold without shake. I suppose you just have to get into the habit of remembering to add on a stop (or whatever) if you're using exp comp. Or go back to the old fashioned way and do it manually... even though I had it on my 35mm EOS I never used it until recently, with the new camera.

311

I always have my 350D set down a little ,otherwise the images on my camera get burnt. I find myself when I take night shots in manual that I constantly use the meter when taking shots ,I suppose the proper way is to know what to set the camera at before you take the photos.

Does anyone take photos without using the meter ??

JMcL

It will change the aperture if you're in shutter priority mode, ie you want the shutter speed to maintain constant.

If you're in aperture priority mode it will change the shutter speed,

elven wrote:

These days it's a handy thing to do if you want to produce HDR images because depending on the tonal range of the scene you should end up with one exposure with detail in the shadows, and one with detail in the highlights which you can merge together.

I'd stick to AP for multiple shots for HDRs (or process a raw image 2 or more times) If the aperture opens up, you run the risk of things close to you being out of focus

PaulieC

so I understand what bracketing does, but how does a camera do it? How does it produce 3 different exposures of the same scene with one shtter click?

I realise that I am being completely anal in wanting to know how this works, but I'd really love to know :-)

elven

It doesn't do it in one, it takes 3 separate pictures. From what I understand from the 350D manual you only have to hit the button once - I could be wrong of course, I've never used it myself.

311

Is this what you meant ,
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/995/EXPOSURE/EV-auto-bracketing.html

JMcL

cambo_gueno wrote:

so I understand what bracketing does, but how does a camera do it? How does it produce 3 different exposures of the same scene with one shtter click?

I realise that I am being completely anal in wanting to know how this works, but I'd really love to know :-)

As elven says, it's 3. On the 300d, so presumably the 350d as well, if you change to burst mode and hold the button down, it'll then take the three shots in succession.

elven

If you don't have it in burst mode, do you have to hit the button 3 times or does it fire off three shots if you just press it once? I can't figure it out from the manual...

joolsveer

For HDR shots I set the camera to aperture priority mode, use the selftimer or the remote to trip the shutter after setting the bracketting to +2, 0 and -2. I make sure the camera is on the tripod of course! I then end up with three RAW exposures which I mess around with. So the only variable here is shutter speed.

Alun

elven wrote:

If you don't have it in burst mode, do you have to hit the button 3 times or does it fire off three shots if you just press it once? I can't figure it out from the manual...

That's certainly the way the Nikon D70 works, if that's any help?

PaulieC

thanks all, i'm now satisfied :-)

JMcL

elven wrote:

If you don't have it in burst mode, do you have to hit the button 3 times or does it fire off three shots if you just press it once? I can't figure it out from the manual...

Yep. The AEB display will flash until you've taken all 3 shots.

joolsveer wrote:

For HDR shots I set the camera to aperture priority mode, use the selftimer or the remote to trip the shutter after setting the bracketting to +2, 0 and -2. I make sure the camera is on the tripod of course! I then end up with three RAW exposures which I mess around with. So the only variable here is shuuter speed.

Good point, I'd forgotten about that!

Dimy

I wouldn't use the exposure compensation setting on the camera itself, it's better (I think) to shoot in RAW and manually play with the exposure compensation slider in the RAW editor you're using.
I never tried bracketing and/or create HDR shots so can't really say anything about that, it's one of those things I still need to experiment with sometime in the future

elven

But sometimes, even if you're shooting in RAW, there still isn't enough latitude to deal with the tonal range of the scene in front of you... that's where you'd still need to bracket.

(Those woods in glendalough are a good example)

Alun

Dimy wrote:

I wouldn't use the exposure compensation setting on the camera itself, it's better (I think) to shoot in RAW and manually play with the exposure compensation slider in the RAW editor you're using.

Have to disagree there ... if the information isn't there because of, say, blown highlights, then it just aint there, and no amount of twiddling is going to bring it back. Think about it, if you've got a vast expanse of white sky that's over exposed, and each and every pixel in that part image has an RGB value of 255,255,255, then there's not a lot you can do about that, RAW or not.

I never tried bracketing and/or create HDR shots so can't really say anything about that, it's one of those things I still need to experiment with sometime in the future

The point of HDR is that you have several exposures where in each shot a part of the scene is correctly exposed, and other parts are either over or under-exposed. The software then effectively extracts and combines the correctly exposed parts to create the HDR image.

digitalbeginner

Don't forget that not everyone has an EOS 350D, or even an SLR.

On SLRs in Auto or Programme Mode the Camera will always try to give the correct exposure using Aperture and Shutter, but also use a little bit of ISO (not enough to introduce noise, but enough to gain maybe up to a stop).

On Compacts in Auto mode all 3 are utilised (remember Compacts generally have a very small aperture range).

Where you have the manual or Exposure compensation control you can deliberately over- or under-expose to suit. If your camera has a 'Burst' mode (usually 3 shots in a row to capture moving objects) it may also have an exposure braketing mode as well (not always on Compacts though)

Elven has it right, work backwords:
- Use Aperture Priority (or the Aperture in Manual) to control the depth of field and use the others to vary the exposure up or down
- Use Shutter Priority (or the Shutter in Manual) to freeze or blur movement and use the other 2 to vary the exposure
- Use the ISO to give you maximum flexibility but be aware of noise generally at ISO 400 and above (varies from camera to camera)

Hope this helps non-SLR users

JMcL

Dimy wrote:

I wouldn't use the exposure compensation setting on the camera itself, it's better (I think) to shoot in RAW and manually play with the exposure compensation slider in the RAW editor you're using.

Yes, it's best to shoot in raw, but it's also always best to get exposure as right as possible in the camera, or as I'll explain later, use the exposure to your best advantage.

There are a couple of issues.

Firstly and most importantly, the automatic exposure on the camera will try to expose for an "average" tone, or 18% grey. This is not infallible, and can get fooled if you're shooting either very bright or very dark scenes, and will thus underexpose or overexpose respectively. A very good example is snow. Shoot a snowy landscape, and you'll end up with grey snow (you need to compensate with between +1 and +2.5 to get a correct exposure depending on how bright the day is etc). Similarly, shoot something against a black background, the automatic metering will try to expose the scene to the average 18% grey, and will render the black as grey and probably blow the highlights on the actual subject, which as somebody else has said, no amount of post processing will bring back.

The second more technical reason to use exposure compensation is due to the way that digital sensors capture light. Half the values available to represent a given light level, 2048 in 12 bit sensors, are used for the brightest f-stop, then this falls off logarithmically for each subsequent f-stop, so the second brightest f-stop gets represented by 1024 possible values, and so on falling to a mere 128 representing the darkest tones in an image. This is described in an article Expose Right on the Luminous Landscape by Michael Reichman. The upshot is that to maximise the amount of detail in an image, you should expose with the histogram as far to the right as possible without blowing the highlights, and then adjust the exposure back down in the raw converter afterwards. All of which flies in the face of the conventional wisdom of keeping the histogram well away from the danger zone on the right!

elven

Don't expect anyone to take you seriously if you're going to use words like 'logarithmically' in your posts...

That's a handy tip though, and easy to put into use when I'm out and about, just keeping an eye on the histogram. It makes total sense - since I usually end up with most of the histogram bunched up on the left, and when I try to bring up the mid tones with the RAW converter, they are awfully noisy and it's difficult to bring out the colour and the detail. Cool.

JMcL

elven wrote:

Don't expect anyone to take you seriously if you're going to use words like 'logarithmically' in your posts...

Heee... It was either that or delve into counting in base 2, I decided on the lesser of two evils