Not everyone has a continuing need for a light meter, particularly if you’re going to have to pay upwards of $4000.00 to buy a good one. But, if you occasionally need a light meter and don’t have time to rent or borrow one, you can use your digital camera and a gray card with reasonably good results.
The following discussion is based on a paper published in 1999 by Kodak entitled Estimating Luminance and Illuminance With Reflection-Type Exposure Meters and an18% Neutral Test Card. A PDF copy of that paper can be downloaded by clicking the icon to the right.
A quick review of the basic operational characteristics of cameras is first in order. There are two parameters that determine the amount of light falling onto a camera’s image sensor, be it film or a digital solid state. They are exposure time and lens aperture.
Exposure time is the time interval during which the camera’s shutter is open allowing the image sensor to receive light. It is measured in seconds. The inverse of exposure time is called shutter speed. Doubling the exposure time doubles the amount of light that reaches the image sensor. Cutting the exposure time in half reduces the amount of light by a factor of two. Cameras usually provide a range of fixed exposure times that vary one from another by a factor of two. For example, 1 second, ½ second, ¼ second, 1/8 second and so on.
The focal length of a lens is the distance from its rear nodal point to its focal plane. Lens aperture is the physical diameter of the central opening (iris) through which light passes on its way from the outside world to the image sensor. As lens dimensions are scaled up, image illuminance remains the same if the relative dimensions all remain the same. Thus, a lens with a focal length of 5000 mm and an iris diameter of 2500 mm will produce the same image illuminance as a 50 mm focal length lens with a 25 mm iris diameter. Because of this property, photographers prefer the term relative aperture, by which is meant the ratio of focal length of the lens being used to the physical diameter of its iris opening. Photographers usually denote the relative aperture as f/no, spoken "f-number". For example, if a 50 mm focal length lens has an iris opening of 25 mm, the relative aperture would be 50 mm/25 mm or 2.0. If the physical aperture for that lens were reduced to 12.5 mm, the relative aperture would now be f/4.0.
The amount of light passing through the iris is proportional to the area of the iris diameter. Recall that the equation that defines the area of a circle is A = π∙r^2. That is, the area is proportional to the square of the radius. Because the diameter of the iris is twice the radius of the iris, the area of the iris is also proportional to the square of it diameter. So, if the iris diameter was decreased by a factor of two, the area of the iris and therefore the amount of light passing through the iris would also be reduced by a factor of 4. Camera designers long ago realized that creating positions (or stops) on the aperture control such that the area changed between each stop was a factor of two would mean that the diameter needed to change by the square root of two. Thus, the relative aperture values for a typical lens follow the series: f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/16, f/22/, f/32 and so on with each succeeding stop reducing the amount of light reaching the sensor plane by a factor of 2. Notice that smaller f-numbers mean bigger apertures and more light passing through the lens.
If a certain image sensor (or type of film) requires a given amount of light to produce a well-exposed image, a photographer can achieve that by selecting the correct values for shutter exposure time and lens relative aperture. But, if the photographer notices that because the subject is moving there is too much motion blur, a shorter exposure time may be necessary, perhaps one half of the initially chosen setting. But making that reduction in exposure time will mean that the lens opening area will have to be increased by a factor of two to maintain the correct exposure. So, shortening the exposure time by one stop will mean increasing lens area by one stop.
In the days of photographic film, the photographer could choose from variety of films with some being relatively insensitive and some being very sensitive. Until 1974 in the United States the film sensitivity rating (also called film speed) was called ASA for the American Standards Association. Since that date, the notation ISO (for International Standards Organization) has been used. With the arrival of solid-state digital sensors, the same notation scheme has been used meaning that if a film camera produced a well-exposed image of a scene on ISO 200 film with a shutter exposure time of 1/100th second and f/8, a digital camera with ISO set to 200 will also produce a well-exposed image of that same scene with the same shutter speed and relative aperture settings.
What this all means is that there is a fixed relationship between scene illuminance, ISO, shutter speed and relative aperture. And, with that relationship, if we know the ISO, shutter speed and relative aperture when the camera recorded the well-exposed image, the illuminance onto the scene can be determined .
The above Kodak paper demonstrates a technique for using an automatic camera as a light meter to determine the illuminance arriving on a standard 18% neutral test card, usually referred to as a gray card. Set the camera to shutter priority mode, the ISO to 400, and the shutter speed 1/30th second.
Place the gray card in the illumination field in question. Set the camera to Shutter Priority. Set the metering to Spot Metering mode. In this mode, the camera is only “seeing” the gray card and excludes the background. You will want to be sure that you are not blocking the illumination pattern that you are trying to measure. That is, neither your body nor your camera are casting their shadows on the card.
The camera should now display the relative aperture that it will be using to properly expose the gray card. In the table shown on the second page of the Kodak paper, you will see that if the camera calls for a relative aperture of 4.0, the illuminance falling on the card will be 23 foot-candles (250 lux).
Now, here’s where the cool part comes in. By back solving the table, you can discover an equation that can be used to determine the illuminance falling onto a well-exposed 18% neutral gray object within a police scene photo (such as a Kodak gray card or a gray road) that appears in the photograph.
The equation is:
Illuminance = (K∙ f-no^2)/(ISO ∙ExposureTime)
Where:Illuminance is in foot-candles f-no is the camera’s relative aperture number ISO is the sensor sensitivity setting ExposureTime is in seconds K is 19.1666… for measurements in the Imperial system
And, here’s how you use that equation:
Below is a night scene photograph where the photographer chose to not use the camera’s flash. The metadata shows that the camera’s ISO setting was 1600, the relative aperture was 2.99 and the exposure time was three seconds. Alongside is a marketing image for Kodak’s 18% gray cards. The distant side of the road is about that same shade of neutral gray.
Using the above equation, we calculated that the illuminance level on the road at the time that the photograph was taken was about 0.0357 foot-candles. So, as we were preparing for our site visit, we knew that it was going to be very dark right at this location.
On another case, plaintiff (our client) was alleging that it was extremley dark at a railyard where a collision had occurred. When the defendant finally produced photographs taken at the scene, we were able to determine that lighting levels averaged about five foot-candles throughout the yard, far more than necessary to stimulate full color fine detail vision. It was not dark. But, pur client had already arranged for a site inspection. When we got there, we did indeed find five foot-candles nearly everywhere within the yard.
So, look through your scene photos for ones that show the ambient conditions. Do the above calculations using the ISO, ExposureTime and relative aperture shown in the metadata. You'll be able to tell your client what the lighting levels were even before you get to the site.