Paul C. Buff, Inc. Technical Forum

64" silver parabolic umbrella just received (used with White-Lightning units) - I was expecting to find a focal point guide mark along the shaft, the position of where to adjust the umbrella in relation to the flash tube. Objective: get the parabolic's full value/output plus most uniform distribution of light.
In combination with the "light-spill dish" does that location point shift?
If the answer is already available I'd rather not spend the time experimenting and "reinventing the wheel." A search on the forum site didn't locate an answer.
Thanks.

With or without the reflector, you should place the flash tube just inside the outer edge of the PLM. (i.e., just inside where the front diffusion fabric would be/is.)

Optimal focal point depends on the distance from light to subject, so it's difficult to put calibration marks on the shaft. Generally, placing the flashtube approximately parallel to the out edge is the best starting point. Putting the tube too far into the PLM causes blacked out center of pattern.

Version 2 (due in about a month) has improvements in this regard, and provides a speedring mounting mechanism to place the light exactly in the center of the PLM, with no rod protrusion out the back of the flash.

Thanks for the heads up, Paul.

By the way have you folks considered an off-set light source placement for the parabolic umbrella? It's used on dish antennas sometimes:

http://www.dsscentral.info/index.php?c=offsetdish

Makes for a more uniform light / no central spot shadow from the body of the monolight.

Thks

Alex

Great idea - but really hard to implement with an umbrella structure - but you got my wheels turning.

Thanks Paul.

Now I remembered something and can't take the credit :-) You see, many years ago I used to use a Minolta Bounce Reflector III flash attachment, and it actually implemented this idea. Flimsy looking thing, in principle it was a very small umbrella with 4 plastic spokes. One of the clearer online pix I found is this Amazon listing: http://www.amazon.com/Minolta-Bounce-Reflector-IV-Set/dp/B00004TQ0P.

What that picture does not show is that at the hub this device had an additional stub at just the optimal angle, where the removable shaft could latch (besides the conventional "straight" stub used when the thing is folded or used "symmetrically").

I don't know what patent coverage Minolta had for the thing - they were notoriously bad on the patent front. SONY has a few years ago acquired Minolta's photography business. This thing was never super-popular in its hey-day (it produced good light but was flimsy and looked stupid), and now SONY neither offers it nor are its present flashes compatible. But of course to be on the safe side one could at least google for any patents that haven't lapsed.

You may find this document on offset parabolic dish design to be of some use. It's got lots of good diagrams and equations in it.
http://www.w1ghz.org/antbook/chap5.pdf

I had this idea myself a year or two ago, and I came to the conclusion that it would be rather pointless. In addition to being really hard to implement with an umbrella structure, it would also be very inefficient unless you found a way to pre-focus all of the light from your strobe onto the reflector fabric.

Even in the worst case scenario (the 42" PLM), the central spot shadow takes up a mere 2.5% of the reflector area. By the time you get to the 86" PLM, you're looking at 0.7%. If the objects you're shooting are specular enough to show a clear reflection of the PLM, you'd be better off using the diffusion fabric on it anyway.

Great link and observations. I think it would be easier to direct a receiver at the dish properly than to light it with a flash . . . getting 100% of the light to perfectly hit the dish, without spill, using an umbrella-like structure would be a huge undertaking. As you say, PLM can direct and focus nearly 100% of the light, with no spill. Conventional back loaded reflectors can only focus around 60% of the light and always have spill, unless gridded.

PLM V2 (about 6 weeks away) actually uses a speedring to put the flash dead-on-axis and the focus is more controllable. An Elinchrom speedring is also being made, and it has an adjustable 7mm shaft that doesn't protrude behind the light (if used without the speed ring.) I will be filing a patent on this design.

For ultimate long distance, we have a 22" polished aluminum front-loaded dish reflector in production right now that is focusable down to about 6° and produces incredible narrow beam output, or nearly incredible output beams up to around 30°. Info coming soon. Uses the same speedring assembly as PLM V2.

As an Elinchrom user, I find this surprising and very interesting. I've been waiting for the 7mm shaft version of the PLMs to become available expecting to mount them in the Elinchrom's oddball 7mm umbrella tube. Based on the quote cited above, am I correct in interpreting this to mean that the 7mm shaft of the V2 PLM is not long enough to mount using the 7mm tube and will require a speedring type of adapter? The adjustable shaft part also has me a bit confused. The near-centrally located umbrella mount on Elinchrom lights would seem to be well enough positioned that a speedring adapter offering exact central placement of the umbrella shaft wouldn't improve matters much.

If you can spare any additional details, I'm all ears, errr, eyes.

Dave F.

The shaft is only being shortened to avoid having more than a foot of shaft sticking out the back of the light (don't want to poke out your shooting eye :o ). There will be sufficient shaft length to hold it in place. The speed ring adapter is optional to exactly center the PLM on the light, maximizing output. As for how much it will improve things, I personally cannot say one way or the other right now, as i have not tested them myself, yet.