Focal Length of 360Q?

[Charc]

Sorry to bring this question up again, just that I'm at school, so I don't have anytime to do the research myself. I'm having difficulties determining the focal length of my 360Qs. I was just up in the catwalks with a measure tape, with a unit that I could have sworn was a 6x9, but measuring the length of the tube, from the gelframe to where it opens, was about 6 inches. That's where I thought measurements were supposed to be on these instruments, I'm obviously wrong though. Can anyone do the footwork for me on this one?

[icewolf08]

I know that we have talked about this before on the boards, a little searching may turn up some good answers. But if you measure from the front of the lens to the gate you should com close to focal length of the unit. If your unit measured 6" from lens to gate you are probably holding a 6x9. Or, if you have the equipment, compare it's beam angle to a 36˚ source four (or similar unit) as the 6x9 and 36˚ will be very similar.

[gafftaper]

Here you go from Bill in an old thread...

Originally Posted by BillESC

Using the measuring tape you have in the picture here are the minimum lengths of the altman fixtures with lens completely retracted.
6 X 9 - 16 1/2"
6 X 12 - 20 1/8"
6 X 16 - 21"
6 X 22 - 29"

[thirdoctive]

Charcoaldabs, you might want to check your post on July 8, 6x? .

[derekleffew]

Good answer, but let's go one step further.

To find the focal length of any lens: take it outside in the sun, preferably at noon on the vernal equinox at the equator, so you have near parallel light rays coming into it. "Focus" the lens at a point on the ground, as if trying to burn an ant (Please no comments from members of PETTY, People for the Ethical Treatment of Insects). Measure the distance from the lens to the ground. Since the optical train uses two lenses, apply the following formula, updated for modern spreadsheet notation:

EFL=(f1*f2)/(f1+f2-d)

where:
EFL=effective focal length
f1=focal length of first lens
f2=focal length of second lens
d=distance between the two lenses

For 360Q's and most other NON-Zoom ERSs, you can use zero for d, even though it isn't. But when comparing the beamspread of a 360Q-6x16 (field angle=19°) to a Kliegl 1355 (field angle=18°) which has one 6x8 step lens, you will see that their beamspread is similar. Of course the d in the formula is what allows zoom fixtures to work in the first place. Not to restart that debate again.

To find the f-stop of a lens, a measure if its efficiency/how much light it lets through:
f-stop=(focal length)/(effective diameter).
"Effective diameter" not "actual diameter" because an iris against a camera lens effectively reduces its diameter.

Perhaps someone can give the focal lengths of the "red dot" "blue dot" "yellow dot" and "not-dot" S4 lenses. Then we can have fun doing all sorts of esoteric algebraic equations. And you thought the math portion of the SAT wasn't important!

[gafftaper]

Originally Posted by derekleffew

And you thought the math portion of the SAT wasn't important!

Crap I only took one math class in college (I got a 2.1 too!). I didn't know it was going to be required for this!

[derekleffew]

Originally Posted by gafftaper

Crap I only took one math class in college (I got a 2.1 too!). I didn't know it was going to be required for this!

Was that an American 0R metric 2.1? Does 2.1 sound better to you than "C-"?

If I write an article called "Algebra and Trigonometry for Lighting Designers," do you think that would be useful or just scare most people?

[gafftaper]

Originally Posted by derekleffew

Was that an American 0R metric 2.1? Does 2.1 sound better to you than "C-"?
If I write an article called "Algebra and Trigonometry for Lighting Designers," do you think that would be useful or just scare most people?

We are too liberal in this state to believe in letter grades. Heck the big school district I used to work for even took up the policy of no "F" grades. Instead everyone gets an "N". Which means no credit... and it doesn't get calculated into your GPA as a "0". The following year district wide we had massive improvement in GPA... even a math idiot like me could figure that one out.

Oh, I'd read love to read your math article but then my favorite part of the backstage handbook is "Shop Math" because I'm too dumb to remember.

[derekleffew]

Hey, gafftaper, I forgot to tell you I thought of you the other day while watching The Colbert Report. Steven was going off a rant that included "There are only four countries today that don't use the metric system: US, [something] [something] and Alaska!"

I had forgotten about the Shop Math section in Backstage Handbook. I just looked for my copy and of course can't find it. I've bought at least 4 friggin' copies of that book, but someone always "borrows" it or I give a copy to someone more needy than myself. I'm now going to order two copies. Again!

I'll admit that I had to look up for verification the EFL formula. There it was on the first page of Lighting the Stage, Art & Practise by Willard F. Bellman, where I had written it in 1980. Also written was the f-stop formula, which I've never used.

And speaking of looking up formulas that are never used. There is a typo in Backstage Handbook (brown cover). Expressing the quadratic formula, there's only a "-" sign after the "2ab". It should be a "±" (plus and/or minus) before the radical. At the time, I believe I called for a global recall, but never received a corrected copy. If I remember right, it IS correct in the blue cover edition.

A free white china marker to the first respondent who can demonstrate a real-world purpose for the quadratic equation! Call now, operators are standing by.

I swear it's a conspiracy by educators who make these things up just so they have something to teach and grade students. Unlike deriving a lighting fixture's multiplication factor from its beam angle using trigonometry: [Excel format] Multiplying Factor(MF) =2*TAN((x/2)*PI()/180), where x is the beam angle in degrees. BTW, the "*PI()/180" is just to force Excel to express the answer in degrees rather than radians, and therefore isn't needed if calculating by hand.

[derekleffew]

Originally Posted by charcoaldabs

Perhaps put your name address and phone number across the whole front and back covers? Include the line "Return to:" for good measure. Then if someone borrows it, they might get a few questions on it.

Smart***. You know I mean that with all respect intended.

So we're dying to know. Were they 6x9's or 6x12's in your catwalk? And does it really matter if the fixture performs as desired? It's been so long since I've seen a 360 I suspect I couldn't tell a x9 from a x12 unless they were next to each other. I can't remember anyone bringing a 360Q into my current building in the last 8 years. I almost said non-ForceSore, but that's not true. We did have 8 2Kw Robert Juliat ERSs on a show in June. I was not impressed. The 750w 410s on the same truss were brighter, and a larger beam.