shatterkiss wrote:
Since you've got a much better understanding of the physics and performance of these things than I do, could you explain how your new PLM line differs from standard umbrellas (other than in dimensions)? In what ways will the quality of light that they produce be appreciably different?
And you noted that the shaft is twice as thick as on other umbrellas...does that mean it's closer to 16mm in diameter than the "standard" 8mm?
I'm sure he means that the wall thickness is twice as thick. An umbrella's surface is arched to help gather light and soften it. The PLM is a parabolic shape. It will actually focus the light instead of just gather it. A parabolic reflector doesn't have to be deep like the elinchrom deep throat. In fact it's shape and depth is relative to the placement of the light. The Elinchrom deep throat is deep because the light source is placed at the base of it, like a headlamp in your car. The farther out that the light source is, the flatter it will be. Moving it in and out slightly will have a fairly large effect on the output direction.
If the PLM is indeed a parabolic reflector the difference between it and a regular umbrella design is that standard photographic umbrellas s are designed to produce a defocused light while a parabolic design (like the PLm, like the Bron PAra reflector are designed so that when the light source is in the correct position, the light produced will be a collimated large directional source -- meaning most of the light produced will be coming out in one direction .
My question is : Is there an optimized distance from the rim of the PLM for the effect to be most notable? . If the diameter across the rim of the bowl is 75 inches for the large version is that also the "sweet distance" for the subject from the edge of the PLM?
E-Vener wrote:
My question is : Is there an optimized distance from the rim of the PLM for the effect to be most notable? . If the diameter across the rim of the bowl is 75 inches for the large version is that also the "sweet distance" for the subject from the edge of the PLM?
In principle, the more narrow the shape of the parabola the shorter the focal length, i.e. the light source has to be set closer to the parabola's inner "backside". From the drawing I saw (if it is quite exact) I suppose that the distance as shown is pretty much the correct focal point.
Btw, the "sonic" test works very well. When you move your head inside a Briese / Bron Para on the axis while making a (e.g.humming) noise, you get a perfect stereo effect when you've reached the focal point
E-Vener wrote:
If the PLM is indeed a parabolic reflector the difference between it and a regular umbrella design is that standard photographic umbrellas s are designed to produce a defocused light while a parabolic design (like the PLm, like the Bron PAra reflector are designed so that when the light source is in the correct position, the light produced will be a collimated large directional source -- meaning most of the light produced will be coming out in one direction .
My question is : Is there an optimized distance from the rim of the PLM for the effect to be most notable? . If the diameter across the rim of the bowl is 75 inches for the large version is that also the "sweet distance" for the subject from the edge of the PLM?...Show more →
Yes, the lamp focal point is just inside the outer edges of the umbrella. This yields about a 40° beam and about f22 at 10' ISO100 at 320WS. Moving the lamp in about six inches defocuses to about 80° pattern and f16. Beyond that the pattern evenness suffers.
I have this all detailed in instructions that come with the PLM system. It will be online soon. Pardon me but my mother in law is dying and I am 500 miles from the office taking care of things.
PeterBerressem wrote:
In principle, the more narrow the shape of the parabola the shorter the focal length, i.e. the light source has to be set closer to the parabola's inner "backside". From the drawing I saw (if it is quite exact) I suppose that the distance as shown is pretty much the correct focal point.
Btw, the "sonic" test works very well. When you move your head inside a Briese / Bron Para on the axis while making a (e.g.humming) noise, you get a perfect stereo effect when you've reached the focal point
You can always depend on Peter for correct information. Thanks Peter.
Unless you're out in the sun you can easily see the focusing with the modeling lamp.
Daniel Heineck wrote:
Shatterkiss--not to steal words from Paul, but I think he meant that the wall thickness of the tube is twice as thick.
Otherwise, I'm out of my league in this discussion.
Correct. 8mm thick wall steel tube - so it will fit standard umbrella mounts instead of requiring a bunch of extra hardware.
I'm sorry to hear about your Mother in Law. Hopefully her passing will be painless for her. My condolences to you and your wife.
Peter (And Paul as well but Paul you're busy so don't sweat this one unless you need the distraction): I think I was imprecise in asking my question. I meant is there an ideal distance for the light to subject distance for each size PLM for the effect of the PLM to be most pronounced. For instance if the diameter of the largest one is 75 inches will the effect be best seen when the subject is 75 inches from the edge ofthe PLM?
Obviously there will be some widening of the beam as you get further back (Paul didn't select an ultra hash mirror-like silver, and mathematically we’re not talking about a mirror), but wouldn’t the optimum distance depend more on artistic considerations?
E-Vener wrote:
I meant is there an ideal distance for the light to subject distance for each size PLM for the effect of the PLM to be most pronounced. For instance if the diameter of the largest one is 75 inches will the effect be best seen when the subject is 75 inches from the edge ofthe PLM?
Like with other true parabolics the effect of the parallel rays works best at some distance from it where remainders of stray light are neglectable.
Not comparable with beauty dishes. By design (imperfect parabola, light source at the base = focal length close to zero = wrong) they emerge rays converging and crossing at a point often referred to as the "sweet spot".
It's basically a matter of the incidence/reflection angle. Here's a link to a little applet that will let you alter the P's shape and shows the corresponding focal point.
www.ies.co.jp/math/java/conics/focus/focus.html
id say the build is definitely sufficient... its not something i'd fear to move onto location... and the pole bending shouldnt be a concern... if you mount properly, theres a SLIGHT sag but nothing inconvenient... sorry no light meter at work :/
Wow, those specs indicate a 2.5 stop difference (at 10') when adding the front diffuser. I didn't realise that much light would be lost to the wider angle.
if you are waiting for the 'spill kill reflector" and really can't wait, I suspect a speed ring insert will work in the meantime , or if you can find one (they sometimes shot up on eBay) Balcar used to make a handy flat disk umbrella reflector that also works quite nicely for White Lighting/Alien Bees /Zeus heads.
It can do double duty as an angle reducing ring when used withany brand round 7 inch grid spot and grid spot reflector . When used this way put the balcar ring i nthe reflector first and then slip the grid into place. I used the rings this way with Balcar, Buff, Dynalight, Elinchrom, Profoto and Speedotron grid reflectors. It reduces the area lit by the grid by about 50%, depending on the flashtube placement and the reflector design.
Seems to offer linear light withvery little spill, and that with the 7" reflector on and the strobe moved a little outward from the sides. I think if you were to go bare bulb and move the light in more to the face of the umbrella it would be even more focused
