How much exposure to compensate
A compensation setting is required when the photographer wishes to continue working with an automatic metering system instead of manual controls. Using an automatic metering mode the photographer cannot simply adjust the exposure, from that indicated by the meter, using the aperture or shutter speed. The automatic mode will simply re-compensate for the adjustment in an attempt to record the overall tone of the framed subject matter as a midtone equal to the 18% gray card.
Photographers calculate the degree of compensation from MIE in a variety of different ways. The method chosen is often dictated by whether speed or accuracy is required.
Digital histogram - Most DSLR cameras and high-end prosumer digicams allow the user to view a ‘histogram’ of the exposure immediately after capture and/or indication of highlight clipping (overexposure). In the case of many of the prosumer cameras the histogram can be viewed live with the preview before capture takes place. This is now the most popular method for assessing whether exposure compensation is required when capturing images with digital cameras. It is worth noting however that DSLR cameras shooting in the RAW format are capable of capturing a broader dynamic range than the histogram may indicate (see the chapters ‘Exposure’ and ‘Camera RAW’). Although this is a reliable method for assessing appropriate exposure compensation it does not replace some of the traditional methods where compensation must be immediate and reasonably accurate.
Bracketing - The photographer can estimate the necessary compensation by bracketing the exposures. To bracket the exposure the photographer must expose several frames, varying the exposure in 1/3 or 2/3 stop increments either side of the MIE.
18% Gray card - Photographers can use a midtone of known value from which to take a reflected light meter reading. A midtone 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. When capturing in JPEG make sure 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. When capturing RAW files the indicated exposure is suitable for an SBR of approximately 128:1 or greater.
Caucasian skin - A commonly used midtone is Caucasian skin. A reflected reading of Caucasian skin placed in the main light source (key light) is approximately one stop lighter than a midtone of 18% reflectance. Using this knowledge a photographer can take a reflected reading from their hand and increase the exposure by one stop to give an exposure equivalent to a reflected reading from an 18% gray card. Adjustments would be necessary for an SBR exceeding the latitude of the image sensor.
Re-framing - If the photographer is working quickly to record an unfolding event or activity the photographer may have little or no time to bracket or take an average midtone reading. In these circumstances the photographer may take a reading quickly from a scene of average reflectance close to the intended subject. This technique of re-framing may also include moving closer to the primary subject matter in order to remove the light source and the dominant light or dark tones from the framed area. Many modern cameras feature an exposure lock to enable the photographer to find a suitable exposure from the environment and lock off the metering system from new information as the camera is repositioned.
Judgement - The fastest technique for exposure compensation is that of judgement, gained from experience and knowledge. The photographer must previsualize the final image and estimate the degree of compensation required to produce the desired effect.