Explanation of use

All measurements are in metric. It's much easier to do this kind of trigonometric manipulation, measurement and building in a base 10 system. Also, it's the 21st century. ;)

The diameter is across the widest point of the reflector at the front, between opposite parallel sides of the square or octagon. The depth is front to back at the centre. I say 47cm is recommended because that's what the Profoto softboxes all are. You don't have to use this value if you don't want to.

The number of sections dictates the smoothness of the curve. The more sections you have, the closer it will be to a true parabola, but it will also increase the amount of effort involved significantly without, I suspect, any kind of significant improvement in light quality. I think that around ten sections is probably sufficient. Alternatively, you could make it easy and just use two or three sections. It's entirely up to you; feel free to experiment.

Interpreting the results

The first column is the lengths between the current row's point and the previous point, that I've termed "dl." These are the actual lengths of the sections when laid out flat, as seen below.

The second columns, w, are the total width of each section. Draw a line this length perpendicular to dl, centered on the x-axis.

The number of decimal places is, as you can see, very high. Given that the results are in centimetres and the most accurate you can realistically measure with a ruler is about 1/3 to 1/2 a millimeter, the number of d.p. given here is unnecessary. However, rounding in Javascript isn't the easiest task in the world and this calculator was intended primarily to be functional, not pretty, so you can choose if you want to use one or two decimal places yourself.