How can I measure different diffusion material in a consistent way?

This might seem like an odd question, but how can I measure the loss in f/stops of a bunch of different diffusion material (different plastic, paper etc) that can later be compared with each other in a somewhat consistent way?

I'm thinking I should use my Vivitar 285 for this one because of the easy and consistent placement of the diffusion material. I don't have an external light meter.

Any ideas?

set up a consistent light source, put up the diffuser, then take a photo of the diffused light at a fixed aperture for all tests. then look at the histogram of the photos to compare the differences in the light transmitted.

If I wanted to know the loss in actual f/stop increments?

Is there a way of knowing that other than to shoot at different apertures with and without the diffusion and then comparing the shots until you find a match?

Instead of using a flash, use a constant light source with a target set up at a constant distance. Use your camera meter to take a reading of the target. Place the diffusion material between the light source and the target and take another reading. Do this with each type of diffusion material you have. The light source, the target, and the diffusion material need to be in the same locations each time. The ideal target would be a large medium gray card, but you could use anything including a piece of white foam core or poster board as long as it was the same target each time. The bigger it is, the easier it will be to measure.

The second alternative would be to simply aim the camera at the light with different diffusion materials between the camera and the light source. You would not be able to determine the light lost for each of the materials, but you could compare the materials to each other.

The third alternative would be to find someone with an incident light meter and do it the second plan with it. You could measure the bare light that way.

I was thinking of your first alternative but was unsure to if the loss of light would be exponential to the amount of light? As I'm using this on flashes maybe a ambient reading would be misleading?

I'll do what you suggested and see how it turns out. I don't need this to be very accurate, but at least consistent between the materials. Knowing the actual f/stop with an accuracy of half a stop or so would be very interesting though. Maybe I'll just have to wait until I can afford a proper light meter.

If I was going to do this, with what equipment I have. I'd setup my flash as per the guild numbers. Set the camera to Program mode. Subject would be a gray card. Make sure the gray card is properly exposed, check the histogram and camera settings (exif data).

Then set everything manual, flash & camera. Begin by using automatic settings from the properly exposed photo. Put the diffuser in front of my flash. Take a number of test shots, changing the f stop and check the histogram and picture as I go to see when it appears exactly as in the properly exposed picture from the program mode. Reference the properly exposed picture from the program mode with the one you took in manual mode. Calculate the f stop difference.

Reproduce the same setup at different distances. If the f stop changes by equal value then you can say that the diffuser will drop a normally exposed picture by "X" f stop(s).

Technically this should work.

If you are concerned about ambient light, then set up in a dark room. Modify the procedure by putting the light in a cardboard box with an open end. Put the diffusion material over the end of the box. That should control all the stray light that needs to be controlled.

In the process of measuring light loss you should also take a look at falloff and spread. In other words, don't just measure at the center but take some additional readings at the edge and corners to see where you're losing light and by how much.