Film-Tech Forum ARCHIVE: Lens sizes

This is topic Lens sizes in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010491

Posted by Tom Wienholt (Member # 1510) on 06-06-2010, 11:57 AM:

I am installing a new and larger screen. I cannot get the lens calculators on the site to download properly. Here are the numbers:

scope image: 12'x29'
flat image: 12'x22'
throw: 60.5'

If someone could look up the lens sizes for me I would greatly appreciate it. Thanks!

Posted by Mike Croaro (Member # 3123) on 06-06-2010, 12:36 PM:

Hi Tom:

12 X 22/29 seems like a nice size screen for a 60' throw. Not too big, not too small.

I am curious, what was the size of the old screen?

Mike

Posted by Steve Guttag (Member # 268) on 06-06-2010, 12:42 PM:

Oh stop...this isn't hard math.

You have constant height so that will be easier since you only need to know the height of the screen...in your case, 12'

So you need the ratio of the height of the aperture and the height of the screen...in your case .446"/12' (keep your units straight too).

The equation sets up like this:

.446"/12' *60.5' = 2.249" (note the throw feet divided out the screen height feet leaving us in inches.

Multiply that by 25.4mm/" to get 57.11mm

You will need to use a 55mm. The actual image size will be 12.46' x 23'. This isn't too bad as you will only have about 3" of spill on your masking to hide the shadow of the plate...the side masking can be adjusted as necessary too.

For scope...same equation but now use .690" for the height of the aperture...you get 88.36mm. Thus you must use an 85mm (I don't think they have 87.5mm, off the top of my head.

Your final scope size will be 12.47' x 29.81'. Which again is a pretty decent fit.

Please note, not all lenses are exactly their marked focal lengths so you should allow for variances. Scope attachments interact with their prime lenses to make them ABOUT 2X in magnification Schnieder anamorphs are notorious for over expanding. ISCOs are a bit more uniform but the 4-element anamorphic lenses from both tend to have more interaction.

Steve

Posted by Tom Wienholt (Member # 1510) on 06-06-2010, 01:37 PM:

The current screen is about 11x21 and we mask down for scope. It doesn't make much sense though since the auditorium is about 32 feet wide. Bad design. Thanks for the help.

Posted by Ken Lackner (Member # 1002) on 06-06-2010, 03:17 PM:

Steve,

Can you explain how this equation works? I've never seen it before. Where does 25.4mm per inch come from?

Posted by Robert Minichino (Member # 3495) on 06-06-2010, 03:22 PM:

It's just a unit conversion; there are 25.4 mm in one inch.

Posted by Steve Guttag (Member # 268) on 06-06-2010, 04:58 PM:

(actual size/desired size) * Throw = Focal Length

As Robert said, 25.4mm/inch is just a conversion factor.

That is why I said...keep your units and they will divide out as needed.

So, regardless of the numbers it is (inch/foot)*foot*(mm/inch).

The inches and feet divide out leaving you with millimeters.

Calculating lens sizes on flat screens is one of the easier things to do by hand. When you have a curved screen it is a bit more tricky because it is often shorter to the sides than the center so you have to allow for that. Somewhere, I wrote a program (DOS) that allowed one to enter the screen's radius and it would tell you the limiting dimensions (how tall the image was on the edges of the screen).

The Schneider lens program does a decent enough job nowadays that one can get a decent visualization as to what is happening on a flat or curved screen. Another good thing about the Schneider program is that it knows about the expansion factor of the anamorphic lenses and how they relate to their lenses.

It can be downloaded here

Steve

Posted by Mike Croaro (Member # 3123) on 06-06-2010, 09:25 PM:

Hi Tom:

Thanks for the old screen size. The 12 X 29 scope wil be a much better fit for the auditiroum.

Mike

Posted by Martin Brooks (Member # 1269) on 06-06-2010, 10:37 PM:

FW=DA
F=Focal Length of the lens in inches
W=Width of the screen in feet
D=Distance from the screen in feet
A=Aperture in inches.
So F=DA/W

(I'm pretty sure that's it anyway.)

Posted by Gordon McLeod (Member # 33) on 06-07-2010, 12:03 AM:

The formula is as old as the industry
one only needs to look in any cameron's or richichardson's book

Posted by Monte L Fullmer (Member # 2797) on 06-07-2010, 12:32 PM:

quote: Gordon McLeod
one only needs to look in any cameron's or richichardson's book

(...and I've got a 1922 and a 1943 copy of James R. Cameron's, "Motion Picture Projection".)