essential skills: digital photography in available light
Assessing the degree of compensation
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
refl ected light meter reading. A midtone of 18% refl ectance 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 fi les the indicated exposure is suitable for an SBR of
approximately 128:1 or greater.
Caucasian skin - A commonly used midtone is Caucasian skin. A refl ected reading of Caucasian
skin placed in the main light source (key light) is approximately one stop lighter than a midtone
of 18% refl ectance. Using this knowledge a photographer can take a refl ected reading from their
hand and increase the exposure by one stop to give an exposure equivalent to a refl ected 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 refl ectance
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 fi nd 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 fi nal image and estimate the
degree of compensation required to produce the desired effect.