Film-Tech Forum ARCHIVE: Lens Size Question

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Posted by Manny Knowles (Member # 1171) on 09-17-2002, 10:36 PM:

Will a 50mm lens by one brand produce the same image size as a 50mm by another manufacturer?

I'm about to replace some lenses here and it's just occured to me that there may be some funky detail that I never knew about here.

They ought to be equal, right?

Posted by Christopher Seo (Member # 7) on 09-17-2002, 10:47 PM:

I second that question.

Moreover, if replacing an old lens specified in inches and fractional inches, can one simply convert to millimeters and get the closest millimeter size?

Posted by Aaron Sisemore (Member # 145) on 09-17-2002, 11:08 PM:

Yes, a 50mm primary lens will throw the exact same size image, whether it was made by Isco, Schneider, Sankor, Kowa, etc. Picture quality may differ however.

And yes, convert inch lenses to millimeters and go with the nearest millimeter lens size UNDER what you come up with.

Example: you convert inches to mm and it comes up 62mm: Since nobody makes a 62mm lens (most are in 5mm increments, though there are always one-off exceptions and 'custom grinds' as well) you would use a 60mm lens, as a 65mm may throw too small of an image. As always, your mileage may vary.

-Aaron

Posted by Steve Guttag (Member # 268) on 09-18-2002, 06:19 AM:

Actually, not all lenses marked the same will put out the same size image. The focal lengths are nominal. It is easier to catalog in nice 5mm or 2.5mm increments (.25" increments in yesteryear) but the design of the lens may have the focal length off by a millimeter either way. Also, some lens manufacturers are better than others, within a brand if you line up 10 units of the same model and focal length lens, you will get different magnifications just due to manufacturing tolerences. Most people don't notice or care. The guy setting the masking and the guy building the theatre is normally the weak point.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Paul G. Thompson (Member # 655) on 09-18-2002, 07:36 AM:

I agree with Steve. If you fiddle around with the Schneider Lens calculator, you might notice that just a little with a different style - even though it is the same manufacturer. Either that, or I just plain goofed up.

But, it is "close enough for government work"

Posted by Manny Knowles (Member # 1171) on 09-18-2002, 08:02 AM:

Paul...that's hysterical. I thought that expression was unique to The Bahamas. I never heard it in all the time I lived in the U.S.

Over here, the expression is..."good enough for the government."

Posted by John Pytlak (Member # 331) on 09-18-2002, 08:27 AM:

Standard SMPTE 243M specifies: "The actual focal length shall not differ from the marked value on the lens by more than +/- 1%."

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Martin Brooks (Member # 1269) on 09-18-2002, 10:45 AM:

And the formula is FW=DA
where
F=Focal length of the lens in inches
W=Width of the image in feet
D=Distance from the screen in feet
A=Aperture in inches.

Posted by John Pytlak (Member # 331) on 09-18-2002, 12:03 PM:

Here are links to the lens calculator programs from IscoOptic and Schneider:
http://www.iscooptic.de/english/downloads/indexdown.php
http://www.schneideroptics.com/software/theatre_design_pro/

Or for "do it yourself" formulae:
http://www.edmundoptics.com/techsupport/DisplayArticle.cfm?articleid=251
http://www.mellesgriot.com/products/optics/ch1-00.asp

Posted by Steve Kraus (Member # 476) on 09-18-2002, 03:28 PM:

I just remember a few basic aperture parameters and re-derive the formulae as needed, keeping in mind it's a fraction and to keep FL and aperture dimensions in like units, and throw and screen dimensions in the same units. I know aperture specs in inches so I'll convert the FL's from mm by dividing by 25.4. Throw and aperture dimensions make the picture bigger as they get larger so they go on top of the fraction. FL makes the picture smaller as it gets bigger so that goes on the bottom. That and a "2" multiplier when dealing with Scope widths and it's all you need.

Posted by Paul G. Thompson (Member # 655) on 09-18-2002, 04:23 PM:

An easy to figure the lens size is:

Throw divided by the height of the image, then multiply that by the aperature size.

This formula does not take the vertical or horizontal projection angles into consideration. It is just for a "Head on" shot. If there are goofy angles, it would be best to use the lens calculator program. Those variables are built in to the programs in most cases.

Manny, that expression I used is common in the military service where we were always expected to do so much with so little for so long, we could practically do anything with nothing.