Paul C. Buff, Inc. Technical Forum

Is there a formula for figuring out the coverage of the various reflectors? For example, the 11" reflectors coverage at 65 ft would be...?

If you need the actual formula, I can get that for you. If you need a close ball park, you can visit this link: http://www.paulcbuff.com/popups/grids/7_grid_chart.php

This graph shows the beam spread of various grids when installed on a 7" reflector. The 11LTR is 28 degrees, so I would use the 30 degree zone for reference. As the distance doubles, so does the diameter.

Yes, an 11" reflector would have a slightly larger area than the 7", but at 65 feet it is irrelevant (it would be a 4" difference at any distance).

So at 65 feet, you are looking at a 40' spread.

incidentally, at ISO100 and 640Ws, you are looking at f/5.6.