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Master your camera and shooting routines for beautiful photos!
Exposure compensation is fine-tuning exposure to compensate for situations where your camera's metering system does a poor job.
You would use this in strong, contrasting light when highlight detail would otherwise be lost, or when photographing snowy landscapes or other tricky scenes.
If you haven't already done so, it's probably best to read Exposure Explained before reading further here.
Cameras expose for the middle/ average brightness value of the scene (middle grey, 12-18% reflectance or 50% luminance), and metering modes are just different ways of placing this mid value by weighting.
In some situations, your camera's estimate of this middle value needs some adjustment or "exposure compensation" for a better photograph.
You would need to consider using exposure compensation in the following circumstances:
Landscapes are usually shot at wider angles (zoomed out) and often include bright skies and dark shadows. Your camera's estimate of the mid value in such contrasty situations can often result in important highlight details being be lost. To remedy this, you would darken the image slightly by reducing exposure, usually -2/3 EV.
Snowy scenes are unusually white, and your camera will think this is supposed to be more towards mid grey. To remedy this, you would lighten the image by increasing exposure by 1 or 1 1/3rd EV.
You would usually apply positive exposure compensation to better expose a subject or compositional element in favour of a brightly lit background or secondary element. Examples of this include back-lit portraits and interior shots with bright windows in the frame.
Most cameras will have an EV display (in the viewfinder or on-screen). It may look something like this:
The zero in the centre is where no EV compensation is applied; to the left we have -EV and to the right we have +EV, with 1/3rd EV steps between.
Other cameras just display the setting like this:
Depending on your camera and display settings, the EV display may only show when in use, or when compensation has been applied.
Firstly, you have to find your camera's method for adjusting EV.
Cameras with one command wheel usually have a button marked +/- or EV which you hold down before scrolling the command wheel, either +right or -left depending whether you wish to increase or reduce exposure (lighten or darken the image). Other cameras have custom programmable buttons that you can assign the EV compensation adjustment to in the menu.
With cameras that have two command wheels, you just scroll the corresponding wheel - no need to hold down another button.
With either arrangement, as you scroll, the marker will move to the corresponding value on the scaled display or the value will simply update on the single type display.
Remember to set the EV back to zero once you've finished taking exposure compensated shots.
In fully manual, you set the shutter speed and aperture values, and the EV display tells you how much this may differ from what the camera's metering suggests. The EV button isn't used, but the effect on exposure is the same.
The advantage of manually applied exposure is that you don't have to remember to reset compensation. You do, however, have to set exposure yourself for each shot.
If you have live view (CSC, SLT, super zoom/ bridge, compact), check skies and other highlight areas for detail. If details are missing, reduce the exposure (scroll to left, -EVs) just up to the point where details appear. Similarly, when photographing a snowy scene, increase exposure (scroll right, +EV) so the snow looks white, but you can still see details in the snow.
In most circumstances, the remit is to ensure things look the way they should, and important highlight details are preserved. An exception would be interior shots with a window in the composition; here, you could choose to let details of the exterior seen through the window blow-out/ clip and favour good exposure of the interior.
Histograms are very useful if you understand the basics of what they're telling you.
A histogram is a graph of tonal values, and the shaded area under the graph is your image. The left area of the histogram represents the shadows and the right area represents highlights; between these are mid-tones. The height of the graph represents the proportion of pixels at each tone level in the image.
The histogram illustrated on the right shows that the image area has been cut off at the highlights before the graph has returned to the bottom. This means highlight details will be missing ("blown-out" or "clipped") from your photo.
The histogram on the left represents the same image with exposure compensation applied.
The idea is, by using EV compensation, to pull the histogram back so the graph returns to the bottom at the right-hand side. If a small horizontal line persists at the very bottom of the graph, don't try to pull that back in; it represents specular highlights that are best left clipped.
The shadows on the left will just have to luck after themselves; if contrast is great, shadow detail may suffer and perhaps be lost into silhouette (particularly with JPEG), but that would usually be preferable to lost highlights. Of course, if you shoot RAW, much more shadow detail will be available to lift in post processing. RAW or JPEG, mid tones can be suitably brightened, along with other finishing touches, on computer later if necessary.
Live view cameras, such as CSCs, SLTs, super-zoom / bridge and advanced compacts, can usually display a real-time mono histogram of the frame's exposure before you've taken the photo; this allows you to apply compensation based on the live histogram and can help capture a perfect exposure on the first shot. But if you use an optical viewfinder such as that of a DSLR, you can only check the exposure by reviewing the image after the shot has been taken; so it's best to shoot at the metered exposure, and if anything is amiss, adjust the EV accordingly and re-shoot.
Despite a live histogram being a great advantage in speeding up the photo taking process, due to the nature of how some of them are derived, it's still best to check colour histograms (see right) on review if your camera has them. This is especially the case when photographing brightly coloured objects, because one channel (usually red) can be blown-out (clipped) despite it not appearing to be so in the mono histogram. If any colour is blown, you have to re-shoot at a reduced exposure, as you would with a DSLR.
For this reason, I would recommend selecting a default review display setting with colour histograms aside of the preview image - if your camera has them.
The above routines are recommended whether you shoot JPEG or RAW. However, if you do shoot RAW, you can further optimise exposure by using the routines listed in my tutorial, Exposure Optimisation.
This tutorial applies to zoned metering modes (Multi/ Pattern/ Matrix/ Evaluative) and, in some circumstances, Centre-Weighted; it doesn't apply to spot metering. Spot metering (or Canon's Partial) is for specialised uses, such as when manually deriving exposure using a Zone System, or for evaluating a scene when using graduated neutral density filters, blended multiple exposures and HDR.