Paul C. Buff, Inc. Technical Forum

IS there a chart or formula I can use to determine the amount of light spread at various distances from a 7" reflector?
Scout

http://www.paulcbuff.com/hgx4.php Click on the last image under the main image. There will be a graph with a series of magenta....triangles. The widest is 40 degrees. Our 7AB/R is 80, so you can double the width as you double the distance.

Note, this width is from center line out to where the light meters -1f relative to the center line. Beyond this point can vary greatly.Some times it is a sharp rapid fall off, other times there is great feathering. The grids will have a pretty sharp fall off, while the 7AB/R will feather quite a bit.

Hope I understand you correctly: the seven inch reflector light spread at five feet would be forty-eight inches, then at ten feet 96 inches, each side of the centerline. Would the spread on the 8.5 reflector be just slightly wider, 45 degrees, compared to the 40 degree grid on the 7 inch reflector?

I did a poor job in my description. The graph shown shows only 40 degrees as its widest. The 7" reflector is 80 degrees (without a grid), so you should double the width of the 40 degree spread at the same distance. Then, for an increase in distance, you can then double the width of the beam (at the subject).

The 8.5" reflector will be slightly wider and with a little more feathering than the 7" with a 40 degree grid, which will have a very defined falloff.

Is there simple calculation to get to the minimum throw distance

- based on a fixed angle and
- diameter of the light source
- beam spread required

that I can use to come up with the the right distance of an OMNI dish or PLM?

There is no simple automatic calculation for what you're asking. You also have to know what ISO you want to use, how mush flash power you have and what f stop you want to shoot at. This would really require a book full of info, drawings, etc.

Also, it's important to understand that beam angles are based, by physics, as coverage of an arc whose center is the light modifier. So if lighting a flat wall, say from 10', you will see more falloff at the edges than when lighting and arc wall with a 10' radius. This comes into play more with wide angle sources than with narrow angle sources.

Wish I could offer more help. A flash meter really helps in practice - but of course won't help when you're planning a purchase.

We have a good amount of info at http://www.paulcbuff.com/sfe.php

Thanks for your comment Paul. The question is not so much purchasing related (I already have about 10 light modifiers from PCB) but really just related to getting the setup right without having to measure the edges. I saw your chart for the small reflector which was helpful. Having a similar one for the PLM or OMNI would be very helpful.