# Focal Length of 360Q?

#### Charc

##### Well-Known Member
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

##### CBMod
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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

##### Senior Team
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Here you go from Bill in an old thread...

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

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

#### derekleffew

##### Resident Curmudgeon
Senior Team
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!

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#### gafftaper

##### Senior Team
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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

##### Resident Curmudgeon
Senior Team
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?

#### Charc

##### Well-Known Member
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?
Future textbooks will say: "For an unknown reason, after 2007, the number of sound designers exploded, outnumbering lighting designers 100:1."

#### gafftaper

##### Senior Team
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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

##### Resident Curmudgeon
Senior Team
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.

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#### derekleffew

##### Resident Curmudgeon
Senior Team
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.

#### Van

##### CBMod
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................If I write an article called "Algebra and Trigonometry for Lighting Designers," do you think that would be useful or just scare most people?
I think That's a fantastic idea !!

......................................
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.
I've always subscribed to Einsteins practice. He never remembered formulas for anything, " That's what books are for," he would say, "I leave my mind open to do the work." I love that man.

#### derekleffew

##### Resident Curmudgeon
Senior Team
Trajectories, stopping distances, forces on a wing.

FedEx overnight if you don't mind?
BUZZZZZ. Thank you for playing, sorry. I asked for A real-world purpose and you gave 3. "A" means one. Maybe you'll learn to follow directions more closely next time. Email me your address so I can you send A (one) box of Rice-A-Roni as a lovely parting gift. It was onsale last week and my idiot roommate bought like 20 boxes. I'll send you a box of my least favorite flavor. PopTarts and Rice-A-Roni are two of my major food groups.

Note: Prizes received shall be considered as taxable income by the I.R.S. so this offer may affect your tax bracket. Management is not responsible. Irresponsible even. Cash value 1/20 of one cent, CDN. That's .05¢ to you, Gafftaper. Goodness, I just found the new version of KeyCaps and I've used it four times today. Objects in mirror are closer than they appear. Can you believe I just went out to my truck to read the mirror to ensure I was quoting correctly. I need to get a life.

#### gafftapegreenia

##### CBMod
CB Mods
I think this thread has just pwn'd eveything we've been talking about over the past few months. Hell, if I ever LD a touring show I'll be sure to make it all ETC with ONE Altman P/C boxspot or an Olivette or just something so totally obscure that only ship will be able to track one down.

I would totally buy a math book for lighting designers, now thats called making something useful in the real world. Why not build a wrench into the book, everything has a wrench in it these days.

Let us also not forget the other basic foodgroups: Ramen and vending machine.

#### gafftaper

##### Senior Team
Senior Team
Fight Leukemia
Objects in mirror are closer than they appear. Can you believe I just went out to my truck to read the mirror to ensure I was quoting correctly. I need to get a life.
This is why I love Derek.

As for the quadratic equation... that one math class that I barely passed was a calculus class. The prof was insane and in love with numbers, his mustache, and clothing that would be popular at a New Mexico dude ranch. We spent an entire class period with him filling two 10 foot wide white boards full of equations demonstrating how the quadratic equation was created and worked. The next day with about 3 minutes left in class he asked if there were any questions. A girl in the back of class said, "I'm having a hard time understanding some of the calculations from yesterday." He said, "Ok, I'll do it again for you."... and proceeded to fill two white boards with formulas and calculations in less than 3 minutes. 18 years later and I still have nightmares about that class.

Yes boys and girls it's possible to barely pass your only college math class and still get a masters degree and become a certified teacher.

Wow. I've got Conan O'Brien on and Bjork is "performing". She's like Cirque du Soleil without the incredible athletic ability... cool flying effects... underwater tricks... amazing lighting... inspirational music... or talent... She does have the weird clowns though, they are playing trumpets. She also has the cool flowing drapery... she's wearing them. Weird stuff man.

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#### derekleffew

##### Resident Curmudgeon
Senior Team
...As for the quadratic equation... that one math class that I barely passed...
So did you catch the typo in the brown cover edition of Backstage Handbook? BTW $33+ with free shipping from tower.com. The white china marker contest is still open, but no duplicating any of Charc's answers. Also no using the internet, (I'll KNOW!), and the correct answer must first STATE the formula, and then SOLVE the equation, defining ALL the variables. ...Wow. I've got Conan O'Brien on and Bjork is "performing". ... Isn't she the one who came to an awards show in a swan suit? The lengths to which some will go for attention. I'll never forget the 1972 Grammy Awards when Bette Midler came onstage with a 45 RPM record in her hair as a hat! Is that on YouTube? ...Let us also not forget the other basic foodgroups: Ramen and vending machine. Roman Numerals=yes, vending machines=no. I'm too cheap/frugal to patronize vending machines. #### gafftaper ##### Senior Team Senior Team Fight Leukemia So did you catch the typo in the brown cover edition of Backstage Handbook? BTW$33+ with free shipping from tower.com.
I have the blue cover so I didn't have to.

#### Charc

##### Well-Known Member
Rather than creating a new thread, I thought I'd use the tail end of this thread.

I finally got around to determining the focal length of my x9s, x12s, and x16s. My solution was to use the Bucknell Photometrics calculator (best one out there!) to determine the expected cone diameter at 5', given the field angles. It involved me getting a friend to hold a light as I stepped back 5' and held a tape measure in the air. Hey, it works! (But I'm hot circuiting the lights! )

So, anyways, I'd love to know if there are any conventions concerning labeling instruments. I plan to label the yokes with a single wrap of color coded e-tape, and write, in silver paint pen, the focal length on-top of the tape.

I know labeling the yoke isn't an ideal solution, but I think it'll serve me the best over the next year.

(P.S. In other news, I've decided to "permanently" attach every safety to a light, by running one end through the yoke... I think I'll be short by a handful. I also hate those metal spurs, frayed metal wire rope sucks!)

Edit: BTW, sorry I didn't search, I have some work to do before heading to bed.

##### Active Member
I don't quite follow. You measure from the front of the lens to where?

#### icewolf08

##### CBMod
CB Mods
I don't quite follow. You measure from the front of the lens to where?
You measure the beam diameter at 5' from the lens and then based on trig (or a pre-compiled table) you can figure out the type of fixture you have.

As for labeling standards, there aren't any. It is whatever you want it to be. however, labeling the yoke with e-tape is not a good way. Paint the color frame holders, it is quicker to identify and it is less likely to come off. My color scheme is:
4.5x6 or 50˚ - White
6x9 or 36˚ - N/C
6x12 or 26˚ - Red
6x16 or 19˚ - Blue