Snow reveals a problem with auto-modes.
The brightness of a snow scene throws off exposure. Cameras in auto-modes work to a “normal” type of scene. When unusually bright scenes are encountered the camera cannot compensate. It leaves the scene grey. This is because the camera is calibrated to work with a mix of colours, light and darks that represent the sort of scene we encounter every day. It senses the luminance levels in the scene and sets exposure to the middle of the range between light and dark.
If the camera senses an unusually bright scene, like the snow scene above, it is unable to compensate properly. The brightness is outside its normal range of compensation. The camera responds by underexposing the shot, and the snow turns grey. The photograph above shows the scene as shot in the right hand panel. It is grey. The left hand panel shows the correct exposure after the underexposure is compensated by an exposure compensation of 1.66 stops (one and two thirds stops). (See: Definition: f-stops)
Photographers are more accurate than camera auto settings
In these bright (or dark situations) we can dial in exposure compensation to manually set the scene to be brighter. Using the exposure compensation we can increase the exposure to brighten the scene (+ settings) or even darken the scene (- settings) as appropriate. Then, we take another picture. By looking at the screen on the camera (“Chimping”) we can get the exposure correct by adjusting the exposure compensation to correctly whiten the scene.
This exposure compensation principle applies to the camera modes which are automatic or semi-automatic. In full auto, the camera calculates the exposure by balancing the ISO, shutter speed and aperture settings. However, in the semi-automatic modes the photographer changes one of the settings while the camera changes the other. In these modes, as in full-auto, the camera can still make the mistake of under-exposing (or over-exposing). Only the photographer can judge the exposure correctly.
When using aperture priority mode, you the photographer, makes the decision on the aperture size (f-stop setting). The camera calculates the appropriate shutter speed. If your camera cannot correctly interpret the scene, as with snow for example, then you will need to dial the exposure compensation to correct for the under-exposure. If you are using shutter priority, same again, the camera calculates the aperture (f-stop) setting. If the camera makes a wrong exposure decision, you can manually set the exposure compensation to over-ride the f-stop setting the camera would set.
Full manual mode, where the photographer sets all the settings directly, cannot use exposure compensation. In this mode the photographer makes all the decisions. The camera does not interfere with the exposure settings. In this case the photographer is free to make decisions to fix the exposure setting for all three factors of exposure – ISO, aperture and shutter speed. There can be no element of compensation for a failed camera decision. The photographer stands by their own settings and has full discretion to control the exposure as they wish. Deliberate over or under-exposure as well as accurate exposure are all possible. It is for this reason that I urge you to work in full manual where you can. You have far more control over the outcome of the shot. You also as a consequence have more artistic control over your image.
If you are working in RAW, the native data mode of image files in camera, it is possible to correct for camera exposure errors in post-processing. As I have argued elsewhere it is better to try and get all the settings correct in camera than to spend time processing afterwards for a number of reasons. So, no matter what file type you are using, try to set the shot up correctly from the start.
There you have it. Exposure compensation is a camera setting which is wholly dedicated to compensating for the mess-ups that the camera makes with its exposure calculations in auto or semi-auto modes. But when using auto-modes, and/or using *.jpg files, it is essential to be able to compensate for the cameras’ failings by using exposure compensation – otherwise your snow will be grey!
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