Understanding ERS Nomenclature

[NateTheRiddler]

Good morning, CB, and welcome to Newbie Nate’s Question of the Day!!!

I’ve been reading through my textbook (y’all probably know why by now, but ask if you don’t), Stage Lighting: The Fundamentals, and in it is referenced a nomenclature relating ERS diameter to focal length. For example: 6 x 9, 6 x 12. I’d like to think ERSs are one of my few strong points of understanding in lighting; this nomenclature makes me question myself, though, because I’ve genuinely never heard of it.

Whenever I pull an ERS, it’s always in the form of “Get me a 26º” or “How about a 10º with a top hat?” I don’t think I’ve ever referenced the focal length, and diameter only in reference to pulls for gel cuts and frames.

I decided to pull up every ETC manual I could find on the S4, but I couldn’t for the life of me find the focal lengths. Am I blind, and it’s written in there, or is there an “understanding” that LDs just know that sort of information?

Regardless, can someone relate some focal lengths to standard lens diameters for me, if it’s not too much trouble? Also, is this information important to have committed to memory vs looked up in a manual?

I’m guessing, but I assume the widest lenses yield the shortest focal lengths, so I suppose 6 x 12 might be leaning towards, oh, 26º or 14º, while a 8 x 22 might be 10º or 5º?

Thanks everyone (again)!

 

[sk8rsdad]

The "6x" nomenclature existed long before ETC came into existence.

There are a few threads about this, some from more than a decade ago: https://www.controlbooth.com/threads/6-x-9-etc.5729/#post-62567

 

[NateTheRiddler]

Here's a good thread for you:
https://www.controlbooth.com/threads/6x9-or-degree.8536/

The "6x" nomenclature existed long before ETC came into existence.

There are a few threads about this, some from more than a decade ago: https://www.controlbooth.com/threads/6-x-9-etc.5729/#post-62567

Going to read through these now. At-a-glance, looks super informative, will look into it more. Thanks for the links, gents.

 

[derekleffew]

I’m guessing, but I assume the widest lenses yield the shortest focal lengths, so I suppose 6 x 12 might be leaning towards, oh, 26º or 14º, while a 8 x 22 might be 10º or 5º?

Good guessing on the first part. On the second part, a 10° is approx. 8x13 and a 5° a 10x23.

Some history: in 1992: 19, 26, 36, and 50° were chosen specifically because they were equivalent to the field angle s of existing prior fixtures 6x16, 6x12, 6x9, and 4 1/2 x 6 1/2. It might be noted that pre-SourceFour ERSs almost all had TWO lenses: a 360Q-6x9 has two six inch diameter and nine inch focal length lenses. Per the formula
For a two-lens system: EFL= (f1*f2)/(f1+f2-d), where EFL=Effective Focal Length, f1=Focal Length of Lens1, f2=Focal Length of Lens2, d=Distance between Lenses.
The EFL is actually 4.5.

More important for the lighting technician than focal length or EFL is the MF. The multiplication factor of a 6x12 (26°) is about 0.5, so I know that at a 20' throw, I'm going to get about a 10' circle of light. If I know the field angle in degrees, I can calculate the field MF from the formula
Field diameter = distance * (2 * tan (field angle in degrees / 2)) .

A good bookmark: https://www.controlbooth.com/threads/mathematical-formulas-for-lighting.7664

 

[derekleffew]

And, FYI it's distance between lenses and the curvature of the lenses that makes the difference. 6" is the diameter of the lens. 12" is the distance between the lenses.

Um, no.

To make the formula
For a two-lens system: EFL= (f1*f2)/(f1+f2-d), where EFL=Effective Focal Length, f1=Focal Length of Lens1, f2=Focal Length of Lens2, d=Distance between Lenses.
work, d can be considered to be zero, in fixed focal length ERSs having two plano-convex lenses. Except zooms, as a changeable d is exactly what makes them work.

 

[NateTheRiddler]

For a two-lens system: EFL= (f1*f2)/(f1+f2-d), where EFL=Effective Focal Length, f1=Focal Length of Lens1, f2=Focal Length of Lens2, d=Distance between Lenses.
The EFL is actually 4.5.

More important for the lighting technician than focal length or EFL is the MF. The multiplication factor of a 6x12 (26°) is about 0.5, so I know that at a 20' throw, I'm going to get about a 10' circle of light. If I know the field angle in degrees, I can calculate the field MF from the formula
Field diameter = distance * (2 * tan (field angle in degrees / 2)) .

Embarrassed to admit I didn’t know any of this; it’s foundational knowledge that my lack of formal theatre education has certainly deprived me of. Good news though, I learn when taught, so this is now bookmarked and memorized. It’s good to understand the background through the history, too, so thanks for that. Appreciate the details @derekleffew!!!

 

[NateTheRiddler]

Oh, @derekleffew, quick knowledge check/accountability for my understanding:

ETC’s 36º manual lists an MF of .63 for the lens; I presume, then, that with a 6” diameter lens, I can expect a 9” focal length? So a 36º equates to a 6 x 9? (EFL * MF = Diam, right? So EFL = Diam / MF, EFL = 6” / .63, which = ~9”?)

 

[derekleffew]

No. Don't try to force a calculable relationship between focal length and MF. Other than "longer focal lengths make longer lights which are used for longer throws."

Note also that a S4-36° uses two lenses of differing diameters, neither are 6" diameter, and ETC won't tell us what the focal lengths are. Much of the information on the cut sheet comes from laboratory testing of a typical fixture, not by calculation. Just accept it.

Embarrassed to admit I didn’t know any of this;

Many, many lighting designers and technicians don't know any of this. Or maybe don't know that they don't know. Experience plays as much if not more than knowing the formulas. We've all met the designer who walks onstage, looks up at the box boom, and says "hang four 14° at the top, then four 19°, then four 26° at the bottom." And she's right, 99.44% of the time.

 

[NateTheRiddler]

No. Don't try to force a calculable relationship between focal length and MF. Other than "longer focal lengths make longer lights which are used for longer throws."

Note also that a S4-36° uses two lenses of differing diameters, neither are 6" diameter, and ETC won't tell us what the focal lengths are. Much of the information on the cut sheet comes from laboratory testing of a typical fixture, not by calculation. Just accept it.

Noted, will comply. Guess I was a bit too eager to apply math. I’ll keep the other equations and details you mentioned memorized, though. Thanks a bunch!

 

[Les]

Embarrassed to admit I didn’t know any of this; it’s foundational knowledge that my lack of formal theatre education has certainly deprived me of.

I doubt a formal education would've helped much; they probably don't teach a whole lot about beam projectors or x-ray's either. You just haven't worked in old enough theatres!

 

[BillConnerFASTC]

and ETC won't tell us what the focal lengths are

Well, remove the damn lense and measure it. Not hard.

And IIRC a 6 X 24 is a 6 x 12 with one lens. The system appealed to some (like me) that also think the imperial ft and inches is somehow better than metric - which it obviously is not but I do like ft and inches.

It's sad to think knowing this bit of tech history is no longer taught, but then I don't know how to care for horses for my travel.

 

[SteveB]

Well, remove the damn lense and measure it. Not hard.

And IIRC a 6 X 24 is a 6 x 12 with one lens. The system appealed to some (like me) that also think the imperial ft and inches is somehow better than metric - which it obviously is not but I do like ft and inches.

It's sad to think knowing this bit of tech history is no longer taught, but then I don't know how to care for horses for my travel.

My memory is the only single lens unit as described was the Altman 360Q 6x22. I remember it as about the equiv. of a an S4 10 deg. No clue why x22 vs. x24.

 

[BillConnerFASTC]

Maybe 6x22 but pretty sure it was a single lens same as used in 6x12.

 

[RonHebbard]

Maybe 6x22 but pretty sure it was a single lens same as used in 6x12.

Like the Strand 2200 series: 2205 = 4.5 x 6.5 50 degree, 2209 = 6x9 36 degree, 2212 = 6 x 12 26 degree, 2216 = 6 x 16 19 degree, 2213 = Single lens 6 x 12 which was extremely close to an Altman 6 x 22.
Toodleoo!
Ron Hebbard

 

[SteveB]

Maybe 6x22 but pretty sure it was a single lens same as used in 6x12.

Yes, that’s correct Bill. Why 6X22 and not 6X24 escapes me. I guess if you measured the focal length it was 22”. I have some 360Q’s in storage somewhere, I should check someday.

 

[derekleffew]

Like the Strand 2200 series: 2205 = 4.5 x 6.5 50 degree, 2209 = 6x9 36 degree, 2212 = 6 x 12 26 degree, 2216 = 6 x 16 19 degree, 2213 = Single lens 6 x 12 which was extremely close to an Altman 6 x 22.

Not quite.
Strand 2213 was a dbl 6x12 with iris.
2111 was a sgl 6x12. BA 9°, FA 14°
2112 as above plus iris.
2113 was a single 8x13. BA 7°, FA 12°
2114 as above plus iris.
2123 was a single 10x23. BA 6°, FA 9°
2124 as above plus iris.

Strand-Century's numbering scheme made good sense: Two--# of lens--Focal length, add one for iris. Except for the b*stard single-lens 6x12 where we reverse iris and regular.

Thanks to the Photometrics Handbook for enhancing my memory

 

[JonCarter]

All very interesting for one who had the choice between an 8" 2kW Klieg or a 6" 750W Klieg. Each of which came with a lens.

 

[JChenault]

Once again you are all making me feel old

 

[BillConnerFASTC]

All very interesting for one who had the choice between an 8" 2kW Klieg or a 6" 750W Klieg. Each of which came with a lens.

Some stepped lenses iirc. Maybe those were later. The early ones were also greenish ?

 

[JonCarter]

Some stepped lenses iirc. Maybe those were later. The early ones were also greenish ?

Stepped lenses - 1950s-'60s. Not particularly greenish, but the PCs were! The stepped and Fresnel lenses always did pretty well outdoors, but the 10" PCs cracked when the rain hit 'em. We putt the pieces together, wrapped 'em in chicken wire & put 'em back in the instruments.