Assessing the degree of compensation

Photographers calculate the amount of compensation from MIE in different ways. The method chosen is one of personal choice. As accuracy is the primary consideration the method chosen is usually the one with which the photographer is confident and has proved from past experience to be the most reliable.

18% gray card  Photographers can use a mid-tone of known value from which to take a reflected light meter reading. A mid-tone of 18% reflectance is known as a ‘gray card’. The gray card must be at the same distance from the light source as the subject. Care must be taken to ensure the shadow of neither the photographer nor the light meter is cast on the gray card when taking the reading. The indicated exposure is suitable for an SBR not exceeding 32:1. If highlight or shadow detail is required the exposure must be adjusted accordingly. If saving as Raw files the indicated exposure is suitable for an SBR not exceeding 128:1. If the SBR exceeds 128:1 the exposure can be increased and subsequently optimised in post production.

Caucasian skin  A commonly used mid-tone is Caucasian skin. A reflected reading of Caucasian skin placed in the main light source (key light) is approximately one stop lighter than a mid-tone of 18% reflectance. Using this knowledge a photographer can take a reflected reading from their hand and increase the exposure (opening the aperture one stop) to give an exposure equivalent to a reflected reading from an 18% gray card. Further adjustment would be necessary for an SBR exceeding the sensor’s latitude.

Bracketing To ensure correct exposure a photographer can bracket the exposures. An incident or reflected reading of a mid-tone is taken. Either side of MIE a photographer can vary the exposure, either by time or aperture, in half stop or one stop increments. The degree of variance does not need to cover the entire SBR.

Digital  Correct exposure with digital capture is achieved by interpretation of the histogram, visual assessment of the monitor image, subsequent compensation of exposure and/or lighting ratios where necessary and the use of appropriate post-production software for final output.

Polaroid   Although no longer the primary medium by which to assess exposure and composition Polaroid has a similar ISO and comparative contrast range to most films available. To best understand the relationship between Polaroid and film, testing of both is recommended. This will give you the best correlation between how the correct exposure for film would appear on an equivalent Polaroid. Polaroid film holders (backs) fit most medium and large format cameras. Polaroid backs suitable for small format cameras are limited.

Judgement  The best technique for exposure compensation is judgement, gained from experience and knowledge. This requires previsualisation of the image and the degree of compensation required to produce the desired effect. This comes with practice and time.  Digital exposure A 24-bit RGB digital image file is separated into three color ‘channels’ (8 bits per channel). Each channel can store 256 levels of brightness between black (level 0) and white (level 255). When viewing channels simultaneously each pixel is rendered in any one of 16.7 million colors (256 x 256 x 256). These brightness levels can be displayed as a graph or histogram. The horizontal axis displays the brightness values from left (darkest) to right (lightest). The vertical axis shows how much of the image is found at any particular brightness level. If the subject brightness level exceeds either the latitude of the image sensors or there is either under- or overexposure shadow or highlight detail will be lost.