Film-Tech Forum ARCHIVE: Help with lens calculations please

This is topic Help with lens calculations please in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Mark J. Marshall (Member # 1409) on 11-26-2011, 10:40 AM:

Hey, folks. We're planning on showing two films next month for Christmas. One is 1.66 and the other is 1.37. We don't have lenses or plates for either format. I don't have the means to get measurements on the screen and throw between now and then, but I wanted to see if there was a way someone could help me back into the calculations for the lenses that we need based on the other lenses that we have. I'm not sure if the math works that way or not, but figured it was worth a try. This is a two projector setup. What we currently have is listed below.

1.85 Lense:
Kollmorgen Snaplite Series II BX-163, 3.75 IN F:2

Scope Lense
Kollmorgen Snaplite Series II BX-163, 6 IN
Bausch & Lombe Cinemascope Projector Attachment I

On projector 1, the scope lens has "RR4755" printed on it, and on Projector 2 it has "RR568" instead. I'm not sure if those are model numbers or what. Maybe someone has some insight on that?

We have some extra plates also. Projector one has the following plates:
.471 x .825 - 1.75:1 (They have this instead of a 1.85 plate)
.715 x .839 - 2.35:1
.715 x .755 - 2:1 S-R (Not sure what this is.)
And another plate that simply has "2.33-1-U" - Not sure what this is either. It has the word "SILENT" scratched on it. Maybe it's for the 1.33 silent aspect ratio. It looks pretty square, but the dimensions are not marked on it like the others.

Projector 2 has the same except they have a ".446 x .825 - 1.85:1" plate instead of the 1.75:1 plate. Obviously we also need another 1.85 plate so we can stop using the 1.75 plate, but that's not as critical right now.

So, I need to figure out what lenses I need for 1.66 and 1.37, and then I need to locate them along with the proper plates for a pair of Simplex E7s.

Any info would be appreciated. Thanks!

Posted by Tony Bandiera Jr (Member # 2365) on 11-26-2011, 01:10 PM:

Mark:

Candidly, there is NO safe way to give you the info you want based on the lens size and plates. Why?

Because the same combination of a given plate and lens focal length can be used on literally millions of possible throw/screen size combinations. The probability of anyone getting the required sizes for the other lenses from that one variable is too high to count on.

Now I have guesstimated some sizes in the past, but ONLY when the size of the scope lens is also known. But my guess was far off enough from what was actually needed that again it is not worth the risk.

You MUST get the throw and at least one of the screen dimensions in the flat format for us to be able to help.

You need to find the means to get those measurements..if you don't have the tape measure go to Home Depot and buy one of the 300' cloth tape measures. There is still plenty of time.

Oh, and the plate specs you gave are also "worthless" as they have most likely been cut to size them to the screen. (So if the image is correct on screen #1 with the "1.75" plate, as verified with RP-40, then leave it alone.) Don't waste time recutting plates just because the stamped size isn't what you want it to say. Just get a label maker and mark it with the correct size.

Again, for us to be able to give you the accurate lens selections, you MUST, I repeat, MUST get the throw and screen sizes for the calculations.

Good luck and get back with the info.

Posted by Brian Guckian (Member # 1678) on 11-26-2011, 01:32 PM:

If I'm reading your data right Mark, your lens focal length for the 1.85 ratio appears to be 3.75 ins = 95.25mm, close to 95mm.

If your screen is common height one can derive focal lengths for 1.66 and 1.37 as follows (to nearest lens size):

- 105mm for 1.66 (95 x 1.85 / 1.66)
- 130mm for 1.37 (95 x 1.85 / 1.37)

The long focal lengths are noted.

[EDIT - Tony posted while I was typing, per his comments this can only be a guide]

Posted by Mark J. Marshall (Member # 1409) on 11-26-2011, 02:22 PM:

Thanks, guys. I will see what I can do to get the throw and dimensions. I have a couple of ideas.

Posted by Paul Mayer (Member # 355) on 11-26-2011, 03:00 PM:

When I do site surveys I use a relatively cheap laser range finder to get one or both screen dimensions for a given format and the throw. Got mine (a Bosch) at Lowes for under $100.00. Will measure distances from 2" out to about 130' or so - good enough for most throws. You can get your distances and dimensions in a couple of minutes!

Posted by Louis Bornwasser (Member # 3063) on 11-26-2011, 04:16 PM:

Really good lense are on the street cheap. You should avail yourself now; better than old snaplites/B&L. Louis