With my collection of modifiers growing I thought I'd take inventory and also take stock in the various exposure factors of the modifiers. Specifically with in-line diffusers like diffuser screens and soft boxes.
Because the source materials for many diffusers come from a wide range of readily available products, knowing how much light a particular piece of material blocks can be a handy bit of information. So with no particular scientific procedure in mind I set out to determine light loss from the various types of materials I was using in my modifiers.
For a quick rundown of what I had to deal with here is a short sample of some of the materials I have in my collection; bed sheets, window sheers, shower curtains, wedding dress material, metal and nylon screens and a few other miscellaneous resources. You can see my use of window sheers and a piece of a hacked up shower curtain in the photo above.
This particular endeavor required my trusty incident meter and an open bulb light. I placed the meter at a typical working distance from my light of about two to three feet. I adjusted the distance until I got a reading of a full stop (in my case it was a reading of 5.6) from my meter. I then placed the various materials at the light source and took a second reading. Some simple math and I was able to determine the amount of light loss each modifier gave me. I then gave it an exposure value based on those results.
The reason I say this is a non-scientific method is that the light loss value actually changes depending on how close or far I place the modifier from the light (remember, Newton's inverse square law plays a major role here). In an actual working environment I usually just grab a diffuser and hold it up somewhere in between the light and the product. However, I did learn that the average difference of light loss under those situations was about +/- 1/3 of a stop. Not bad in my opinion.
As an example, if I take a reading on a piece of cloth and it reads f/4.0 I know I lost a full stop of light (f/5.6 minus f/4.0 is one full stop) or minus one exposure value. I also metered my shoot through umbrellas. At the light source the material metered at -.6 or two thirds of a stop loss of light. Since an umbrella is seldom held next to the light I figured the actual loss is closer to one full stop (give or take 1/3 of a stop).
While all this is not scientific in the least, it does give me a better handle on the diffusion capabilities of the different materials in use, specially when stacking materials to get a desired effect. A quick look at my EV rating I have assigned each piece of cloth gives me a rudimentary understanding of how it will behave when I hold it up to the light or double up on the material.
Do you use, or know of, a different method of classifying diffusion materials? If so, share your thoughts in the comments section below.