Incident light measurement

Incident light metering is a very effective way of determining the exposure of a shot. It is useful for both digital and analog photography. With negative film, in both black & white and color, it requires a few precautions.

A short documentary film shows American photographer Irving Penn (1917-2009) taking a portrait in Morocco (Irving Penn on Location in Morocco, 1971). At 2 minutes and 25 seconds, a Spectra light meter, held in the hand of an assistant, enters the field to measure incident light. Irving Penn favored this type of measurement for his famous Rolleiflex portraits. But what is incident light?

The light source

When a ray of light encounters a surface, it can be reflected, absorbed or transmitted. Incident light is the light that reaches the surface before it is transformed by reflection, absorption or transmission. Incident light can come from a natural source, such as the sun or a cloudy sky. It is artificial when it comes from a continuous-light projector, light bulb, flash, etc. In photography, incident light is measured using a light meter equipped with a white dome made of translucent material, known as a hemispherical integrator or diffuser dome. The white dome faces the camera.

Subject luminance

Luminance is the luminous intensity emitted or reflected by a surface in a given direction. Let’s imagine a perfectly flat, uniformly lit surface or object. Between its brightest and darkest parts, the difference in extreme luminances is generally 5 EV (Exposure Value). Or 5 f-stops if we refer to camera lens apertures (f/2, f2.8, f/4, etc.). The mid-point of this 5 EV difference between the darkest and lightest areas is 2.5 EV. If we convert this 5 EV into an optical density value, we get 1.5. Half of 1.5 is 0.75. And 0.75 is equal to the reflection density of 18% mid-gray. This 18% gray is regularly used to measure light in reflected light mode, as it corresponds to the average reflectance of most scenes or subjects photographed. In incident light, the light meter dome acts as if it were integrating an 18% gray card.

Shadows and highlights

If the subject is not flat, dark and light areas appear in recesses and protrusions. The 5 EV range of extreme luminances is then exceeded, reaching 6 to 10 EV, or even more. The contrast of a scene is measured as follows with a light meter in incident light. The light meter is set to its EV mode (Exposure Value). A measurement is taken in the subject’s highlights, with the white dome facing the camera. Let’s imagine we obtain 12 EV. A second measurement is taken in the shadows, with the white dome still facing the camera. Let’s imagine we obtain 10 EV. The difference between the two measurements is 2 EV. Let’s add this value to the 5 EV corresponding to the difference between the extreme luminances of a perfectly flat, evenly lit surface. We get 5+2 EV = 7 EV.

The normal contrast subject

A difference of 7 EV is considered to be a normal contrast scene, for example a portrait or still life in diffused frontal light. 7 EV represents a densitometric range of 2.1. Its average value is 2.1 / 2, i.e. 1.05, which corresponds to around 9% reflectance, i.e. a gray twice as dark as the 18% gray.

If the light measurement is carried out in the part of the scene that receives the most light, and the subject is photographed with negative film (black & white or color), the f-stop-speed setting indicated by the light meter will result in underexposure of 1 EV. This will reduce the reproduction of details in the shadows.

Compensating measurement

To avoid a loss of substance in the shadows with negative film, in the case of a normal subject of 7 EV, you need to compensate by opening one f-stop in relation to the incident light measurement taken in the highlights. Or close by one stop in respect of the incident light metering in the shadows. Negative film can withstand overexposure, which presents little risk of loss of detail in the reproduction of highlights. With reversal film (Kodak Ektachrome or Fujifilm Fujichrome slides) or digital photography, this compensation should be limited to a value of ½ f-stop, as it may result in a permanent loss of information in the highlights. In digital shooting, a quick check of the histogram on the camera’s rear screen will reveal any clipping if compensation is applied: if the histogram stumbles on its right-hand side, there is a loss of information. When shooting with reversal film, as with digital, we must be careful not to “burn” the highlights.

High contrast

When using negative film in a scene with high contrast, it’s wise to take one incident light reading in the shadows and another in the highlights, with the light meter dome pointing towards the camera. If the difference between the two measurements is 3 EV or more, apply the f-stop setting corresponding to the measurement taken in the shadows, to preserve maximum shadow detail. To avoid over-contrasting the negative, reduce film development time by 10 to 15% for a difference of 3 EV, 20 to 25% for 4 EV, 40 to 50% for 5 EV.

Discover the photography courses at Spéos

Spéos offers various training courses ranging from simple one-week photography courses (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.

Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.

Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos