One of the potentially unpleasant effects of this linear way in which sensors collect light, is that at the brightest end, there is a point at which suddenly the tones go pure, featureless white. With film it is different, because its more ‘Eyesight-like’ behaviour means that the brightest highlights shade more gently and smoothly towards white. This is sometimes called ‘Roll-Off’. With a digital photograph, if you over expose part of it, you will instead notice a sudden break.
Below is an example from one on my photographs. It shows clear over exposure on some of the petals which therefore has led to pure, featureless white breaks.
There are technical definitions to over exposure, however, there is also a simple practical meaning. A ‘Correct Exposure’ for a subject, is one that produces the image you wish to create. When an image is ‘Over exposed’ it simply means that the image has been given more exposure than you actually need, with the result being that the image is lighter than you desired.
You must relate the term ‘Correct exposure’ to the ‘Type’ of image you wish you create in order to ensure that exposure is both technical and subjective. Having a ‘Technically Correct Exposure’ for an image, means that your image will have averaged luminance distribution-the mean between lights and darks. However, in some images, you may feel that a technically correct exposure will not be what you desire. This is where the ‘Subjective’ comes in. You may therefore decide to have an underexposed image in order to produce for example, heavy (Black) shadows, or deeper, intense colours. Visually, people are likely to accept an image with dark, heavy shadows that are the consequence of under exposure, than large areas of overly bright, pure featureless white, which is the result of over exposure.
A valuable aid to let you know when you reach the ‘Danger point’ in an exposure, is the highlight clipping warning. This usually takes form of a flashing area on the camera’s LCD screen once you have taken a picture. If the highlight is a naked light such as the sun or a bare light bulb, this will be unavoidable. If you see the highlight clipping warning appear on your image, if possible, take a second shot with a darker exposure. You can also find highlight and shadow clipping on your histogram, on your image processing software.
Digital Photography Master Class, Tom Ang, Dorling Kindersley Limited, London 2008. Page Pages 162-163.