Octabox from an sun parasol
Okay, I asked a few weeks ago regarding octaboxes. So, I figured I could try making one myself from a largish parasol I had lying around. I am in the middle of it right now. Materials are bought, the skeleton assembled in a new apex (for fitting my portable flash) etc. So I started thinking about the shape of the reflective fabric sections. Turns out it was more difficult than I had planned. Octagonal paraboloids, or polygonal paraboiloids in general isn't really a matter that should be taken lightly. But there are a few solar cooking enthusiast who have written a fair amount on the subject.
The problem is that their calculations are more focused around focal points, then even light. The biggest problem was that you always have the arc length given in an umbrella based octabox, soft box, and that you are more interested in base area or depth of the thing. So I sat around and made a small octave/matlab script. It isn't really optimal in any sense, and it doesn't yield the nice straight end edges you see in commercial products. But hey, if it helps anyone, I'm happy. Use it as you please. It renders a few data files which can be plotted with gnuplot. Both gnuplot and octave are open source and quite nice if you are used to terminal work.
You need the arc_length function for the octabox script to work. Rename them to just *.m to get them to work in matlab or octave.
I couldn't attach them, but if you find yourself lusting for building an polygon softbox, pm me. EDIT: Oh I could!
Hello again, so this was the weirdest thread ever huh? I forgot to update. Let me expand.
I needed an octabox for location shooting. Cheapest way, not counting shallow inefficient chinese boxes, seemed to be making one yourself. I used the above program to make the pattern for the sections.
I started with the typical low cost OEM beach sun parasol, I bet you have the same model everywhere. I dissassembled it, it was fairly easy, the legs of the umbrella are held to a flange with steel wire. I then made two rings from 1/4 birch ply, that was that I had at home. The rings had such a large diameter that I could stick my press flash thru the hole. One ring was for the apex and the other was for the inner legs of the umbrella. I used all thread and locking bolts as a distance, so the umbrella would stay fixed in position. I attached the legs of the umbrella with fence staples. All interior hardware was painted white. I use wingnuts for assembling the thing.
I then ran the little scripts above with the appropriate spine length and depth (I guesstimated 50 cm as a good depth), that gave me the following figures:
Half of the parabola, or the curve each spine/leg of the umbrella would make in the finished octabox
The size of the octagonal base area
And finally, the pattern for each of the eight sections.
I couldn't find a suitable fabric so I contact-cemented silver mylar to ripstop nylon. I put bias tape on all seams and double stitched it. It was easy. I also put the umbrella leg holders from the original umbrella to the end of the seams. I then added some fastening hardware and made the diffusor to go on the outside of the box. That diffusor didnt get so good, cause the elastics were hard to get even, so some edges became shorter. It will sort itself out over time I guess.
Anyways, that's the short version of the long version, but in the end I had an octabox for roughly $50, it folds down to a cylinder roughly 12"x40" and gives very nice even light.
Here's the thing in action at f/22 and software EV correction of -2.5. Flash set to lowest possible. Not really even, but I'm gonna add an internal baffle to, already have the hooks and elastic inside. The issues with the sagging top is fixed.
Here's a quick test shot in a rather dark room (taken with that forbidden technology...):
And here's a horrible phone snapshot of the thing from behind, as mentioned, the sagging thing is fixed now:
So, I hope I inspired someone to do their own lighting modifiers.