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Prime lens quality control data


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#1 Bill Munns

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Posted 12 July 2009 - 04:33 PM

Just curious if anybody has information or links on the quality control data for 16mm film camera prime lenses.

There is a debate that a 25mm lens, such as a Cine Ektar 25mm, may be off spec in effective focal length by 10%, resulting in an effective focal length of 22.5mm.

I find the idea unfathomable, that a lens could be so far off spec, but wonder if anyone has any data or links to data on quality control tests performed on lenses of the same make and model, and how close they are to the described specification, as well as how much variance their might be from one lens to another of the same make and model.

Thank you.

Bill
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#2 Tom Jensen

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Posted 12 July 2009 - 05:45 PM

I'm going to go out on a limb here and guess that it isn't true. It might be another hoax. But why not get a lens and test it. Shoot a frame chart with the lens and another 25mm lens and see what you get.
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#3 Bill Munns

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Posted 12 July 2009 - 06:32 PM

Tom:

I've tested one 25mm Cine Ektar and got a horizontal anlge of view (HAOV) of 22.65 degress, on a K-100 camera. As compared to ASC Manual listing of a 25mm lens having a 23 degree HAOV, the lens is maybe 25.4mm, which actually would be a true 1" lens.

But there have been claims made by others in another discussion that a lens could be off by 10% of specified focal length, and I personally find that beyond comprehension. SO I'm curious about lens data and quality control tolerances.

One frined found data on Kodak slide projector lenses, and their specs stated each lens is accurite to 0.1mm of rated focal length. I'd expect 16mm filming lenses to be at least this consistant, if not more so.

But i am hoping to get some data from other actual tests or product evaluations.

Bill
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#4 Patrick Neary

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Posted 12 July 2009 - 06:45 PM

Tom:

I've tested one 25mm Cine Ektar and got a horizontal anlge of view (HAOV) of 22.65 degress, on a K-100 camera.


Hi-

How did you test the Horizontal AoV on that camera/lens combo?
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#5 Bill Munns

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Posted 12 July 2009 - 08:16 PM

Patrick:

"How did you test the Horizontal AoV on that camera/lens combo? "

I set a measure bar at 72 inches from the lens focal center, at a true perpendicular to the camera centered line of sight, and photographed the ruler, and then scanned the processed film at 4K and measured how much measure bar was in the horizontal picture, and then replicated the distance and width in a digital software, and measured the angle from focal center of lens to edge of picture based on those dimensions.

Bill
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#6 Chris Keth

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Posted 12 July 2009 - 09:25 PM

Did you scan the projected frame size or the recorded frame size? Generally, film cameras are made to record a slightly larger area than is meant to be projected so there is a bit of room for error in placing the projector mask.
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#7 Bill Munns

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Posted 12 July 2009 - 11:17 PM

Chris:

'Did you scan the projected frame size or the recorded frame size? Generally, film cameras are made to record a slightly larger area than is meant to be projected so there is a bit of room for error in placing the projector mask."

I overscan, including bits of the frame above and below, and some sprocket area, and crop.

Any chance we could get back to the actual question, which is does anybody know about quality tolerences from other camera lens tests?

Thank you.

Bill
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#8 Stephen Williams

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Posted 13 July 2009 - 03:44 AM

Hi,

I remember setting up lenses on a moco that could auto follow focus based on perfect infinity focus & focal length. +/- 1 mm was not unusual from what the lens claimed.

Stephen
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#9 Tim Carroll

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Posted 13 July 2009 - 11:22 AM

Got a 9mm Cooke Kinetal and an 8mm Zeiss Distagon, and putting the lenses on the same camera, the horizontal field of view with the Cooke is wider than the horizontal field of view on the Zeiss. Go figure.

Best,
-Tim
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#10 K Borowski

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Posted 13 July 2009 - 11:52 AM

Well, just in general, +/-10% is the maximum allowable variance in terms of selling a product to someone. Knowing Kodak, they are probably near the 10%, whatever is in their favor.

Yes, 25mm (~0.984") lenses were often true 1" (25.4mm) lenses.

So it is based on percentages, not on allowable amounts in terms of a measurement. If you are thinking a lens is off more than 10%, Bill, forget it.

The risk of fines in the modern era would make Kodak stay well away from 22.5mm (0.885") on a 25mm lens. There's no way it'd go below 23mm or so.
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#11 K Borowski

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Posted 13 July 2009 - 12:12 PM

Double Post

Edited by Karl Borowski, 13 July 2009 - 12:12 PM.

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#12 Patrick Neary

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Posted 13 July 2009 - 03:28 PM

Patrick:

"How did you test the Horizontal AoV on that camera/lens combo? "

I set a measure bar at 72 inches from the lens focal center, at a true perpendicular to the camera centered line of sight, and photographed the ruler, and then scanned the processed film at 4K and measured how much measure bar was in the horizontal picture, and then replicated the distance and width in a digital software, and measured the angle from focal center of lens to edge of picture based on those dimensions.

Bill


Hi (again)-

WOW, thank God for the EDIT feature!

There is the formula right in front of my face, p164 in my tattered AC manual.....


EDIT:

But still, curiosity got the best of me and about 5 minutes of searching found this:

http://www.bobatkins...cal_length.html

along with a few "google books" results (on optics and photographic optics) that point out quite clearly that precise determination of the focal length of a compound lens is a bit of a complex issue, certainly more complex than the formula in the AC manual. I wonder why the difference in methods?

So again, unless I'm just misunderstanding your method of finding the focal length, respectfully, I wonder if that's why you're getting an odd reading.

I assume you're in or near LA, with some of the best camera houses and optical techs within driving distance, why not consult with someone who knows this stuff and can assist?

And with some egg on my face, I'll bow out now.....
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