Assigning exposure values to your DIY diffusers

As a product photographer I am always tweaking or building light modifiers. Since lighting products is different than lighting people, I find that making my own modifiers is easier than trying to find commercially bought equipment. Plus, it's cheaper and, since I'm fairly handy, appeals to my artistic creativity.

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.