MadSci Network: Physics

Re: is it possible to show the strength of light on a blank roll of film?

Date: Fri Jan 28 10:38:32 2005
Posted By: Harry Adam, Retired/self-employed
Area of science: Physics
ID: 1106701575.Ph

Hello, Amber. What a good question! You have a very nice little project here, and I think you have already gone a long way in thinking of film as a way to measure the different strengths of a variety of light sources. In fact, this is a great choice for a number of reasons.

Firstly, a reproducible device to expose the film to your light source is readily available: a camera! What you must ensure, of course is that it is a manual camera, not an automatic. That is to say you need one for which you can set the aperture and shutter speeds to specific values and know that these do not change as you do your experiment.

Secondly, modern film has very long latitude. That means it has a long and linear density response to equal changes in the exposure (or strictly logarithm of the exposure). This means you have a good potential for being able to measure sensible differences between your light sources.

Despite these good properties you do face some other difficulties. To interpret the results you will need to be able to measure the densities of your exposures. In the photographic industry, we measure densities using specialised instruments for the purpose, called densitometers. Without something like this you will have a problem. Assuming you can overcome that, a camera is a great device to do some experiments to find what exposure setting (or settings) to use as the range of shutter speeds and apertures on a camera should allow you a great deal of scope, beyond which you also can experiment with different film speeds. Having found an ideal shutter and aperture setting combination for a given film, stick with that film, and ideally use a diffusing filter over the lens (something like the top of a Pringles tube would do) for both your setting up and the experiments, say with the lens focused on infinity, and with each source at the same distance from the front of the lens. If you can mount the camera on a tripod that would be good and it will help you set it all up to be the same for each exposure.

Now, again assuming you can measure the density of each processed exposure, you need to know how that density varies as a consequence of exposure. In other words, you need to calibrate your system. The best way to do this is to use a constant light source and to vary the exposure by means of the shutter and/or aperture setting. Here you need to be careful in understanding the relationships, especially with the aperture. What you want to do is make exactly changes of a factor of two, known as a "stop". For example, for a given aperture, changing the shutter setting from 100 to 200 (i.e. halving the shutter-open time from 1/100th of a second to 1/200th second) is reducing the exposure by exactly one stop. Do a range of exposures with a light source you think is in the middle of your set, process the film and plot a graph of the density of each exposure against the relative exposure, each point being separated by a doubling of exposure. Settle on a nice mid point of the graph as a place to aim for your average brightness light and chose a setting combination that will give you that and use that setting for your whole set of lights.

I think you will need the help of an adult with some of the equipment, a good camera (such as an SLR), and a means to measure the film densities. If you can get that help, you have a great project here from which you will learn a lot, as well as have great fun. If only I was near at hand, I'd love to help!

A brief explanation of aperture values can be found at
And a table of values at

Best of luck, and thanks again for the question.

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