Conflicting information about Exposure Compensation

[Neiby]

I'm confused about how to use exposure compensation since I've read conflicting information about it, and by conflicting I mean exactly opposite information. Let's take a bright scene as an example, like perhaps something with a lot of snow. As a total noob, the "intuitive" thing is to use negative numbers because the scene is too bright. I'm not saying that's correct, but I have seen that exact interpretation many times.

However, at least twice I have read the exact opposite, that in a scene with a lot of white you need to go positive with exposure compensation or your whites will come out gray.

To make matters more confusing to me as a beginner, this seems more like a function of getting the white balance correct and not just the exposure, although I'm sure each has an effect on the other.

I'm doing a bit of reading on exposure compensation now to see if I can get this straight, but if anyone has a simple way to clear this up, feel free to share.

Thanks, as always!
John

 

[Neiby]

I just found this really good article that is helping to explain this:


Here's how I understand it now. Let's say there is a lot of white snow in an image. There are two immediate problems that could arise. First, if your subject is also white, the automatic exposure system on the camera will make everything gray since it presumes 18% gray when doing its calculations. Second, if your subject is darker than the background, the white snow will end up being exposed at close to the right brightness (although too gray) and the subject will be too dark, hence the reason to lighten up the exposure with positive exposure compensation.

Is that getting closer to being correct?

 

[analog.universe]

The 0 mark on the meter, meaning no exposure compensation, represents a scene that averages out to 18% grey. If most of your scene is brighter than that, say a frame full of snow, you need to tell the camera that you're shooting a bright scene instead of the 18% average scene. So you increase the exposure compensation, that way the meter reads "whiter than grey" for your white snow.

White balance has to do with the color of white, as opposed to how bright white is. If you search around the forums you can find a more in depth explanation of white balance than I can type for you here. Basically though, except in extreme circumstances, white balance doesn't affect exposure. Exposure determines the total amount of light, white balance determines of the proportions of each color within that total.

 

[Neiby]

The 0 mark on the meter, meaning no exposure compensation, represents a scene that averages out to 18% grey. If most of your scene is brighter than that, say a frame full of snow, you need to tell the camera that you're shooting a bright scene instead of the 18% average scene. So you increase the exposure compensation, that way the meter reads "whiter than grey" for your white snow.

White balance has to do with the color of white, as opposed to how bright white is. If you search around the forums you can find a more in depth explanation of white balance than I can type for you here. Basically though, except in extreme circumstances, white balance doesn't affect exposure. Exposure determines the total amount of light, white balance determines of the proportions of each color within that total.

Ah, that makes so much sense. I hadn't really thought it through enough. It makes sense now that WB is trying to get the color of whites to show up correctly, or at least in the way that you desire, whereas the exposure compensation is about getting the important areas of the image to the right brightness.

It seems obvious in hindsight, but there is so much to photography that it's difficult to keep all the factors in mind sometimes.

 

[Gaerek]

whereas the exposure compensation is about getting the important areas of the image to the right brightness.

You probably meant this, but exposure compensation doesn't affect the brightness of just important areas of the image. It affects the entire image. So in your snow example, everything in the frame is going to be brighter when you dial in your compensation, not just the snow.

 

[Neiby]

whereas the exposure compensation is about getting the important areas of the image to the right brightness.

You probably meant this, but exposure compensation doesn't affect the brightness of just important areas of the image. It affects the entire image. So in your snow example, everything in the frame is going to be brighter when you dial in your compensation, not just the snow.

Yes, you're right! I phrased it very poorly. What I meant was that you have to decide what is important to you and then set the exposure compensation so the important parts are at the correct exposure.

Thanks for everyone's help! This is all starting to make a lot more sense.

 

[Gaerek]

whereas the exposure compensation is about getting the important areas of the image to the right brightness.

You probably meant this, but exposure compensation doesn't affect the brightness of just important areas of the image. It affects the entire image. So in your snow example, everything in the frame is going to be brighter when you dial in your compensation, not just the snow.

Yes, you're right! I phrased it very poorly. What I meant was that you have to decide what is important to you and then set the exposure compensation so the important parts are at the correct exposure.

Thanks for everyone's help! This is all starting to make a lot more sense.

:thumbup:

Awesome, just wanted to make sure you fully understood.

 

[Compaq]

It all boils down to how your camera meter the light. As mentioned, your camera "expects" that any given scene will reflect the same intensity of light as "18% grey" does (that number may vary from manufacturer to manufacturer). So, let's say you're shooting something pitch black: your entire frame is supposed to be black. Take the shot (make sure there are no harsh lights coming on to the scene) and look at your histogram. What you'll see is that the graph is peaking around "middle grey" in the middle of the histogram.

Now shoot something white, as white as it gets. Fill the frame with it, don't worry about focusing. Now you'll see that the histogram averages around "middle grey" as well. Your white is grey, and your black is grey. What does this mean?

It's quite simple, really. To achieve white as white, you must over exposure your shots a little => pushing the histogram peak toward the highlight part (to the right). To get black as black, you need to under expose a little: push the peak back toward the shadow part of the histogram. To apply this to real life:

Scenes with a lot of white will: your camera will most likely render the white as grey. It "thinks" that all of the light values hitting the sensor should average to middle grey, and so all the white is reduced to grey. If there's a lot of black in your scene, for example a portrait with a person with black clothes, your camera still thinks that everything should average to middle grey, and you must under expose to get the black as black.

Lots of white: over expose
Lots of black: under expose

Do the test yourself, and look at the histograms, it really helps!

Exposure compensation: + values means over exposing and - values means under exposing. You're just adjusting the aperture (if in shutter priority) or shutter speed (if in aperture priority) to over/under expose by x stops.

 

[analog.universe]

Yeah, histograms are an important point. Looking at the histogram is the easiest way to see how good your exposure is.

 

[KmH]

The 0 mark on the meter, meaning no exposure compensation, represents a scene that averages out to 18% grey.

Only if Evaluative (Canon) or Matrix (Nikon) metering is being used.

Spot metering only meters a small fraction of a scene (about 2%), though it is also calibrated to 18% gray for that 2%.of the scene . Nikon cameras allow the size of Center-Weighted metering to be changed. Center-wighted give 70% of the exposure weight to the center portion of what is metered.

I know Canon has both Center-Weighted and what they call Partial metering.

It should also be understood that with Nikon DSLR cameras the focus point used is also the place in the scene spot metering is done, but that does not apply to all Canon DSLR cameras, some of which can only spot meter at the center focus point. Also, not all Canon DSLR cameras have a spot metering mode.

 

[FstopRay]

Hi, another test that might help. Take three bits of plain card. White, grey and black. With the camera on auto, and in the same light, take an image of each. You will find the three images will be the same, just about. This verifies that the goal of the camera is to set what it sees to a sort of mid grey.