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Lens Testing for sweet spot


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#1 Tim Carroll

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Posted 12 July 2008 - 02:51 PM

I have a set of older lenses that I want to use for an upcoming project. I want to determine the sweet spot for each lens by shooting a test with each lens, first wide open, then stopped down one stop, then two stops, then three, etc, and then study the results on film.

What I am trying to determine is how to do this most effectively. Are there specific lens charts I should use, or should I light a relatively flat scene, or how would any of you more experienced folks set this up? Or is there another way, say with using a collimator or something, that I should use to determine where the lens has it's sharpest and best characteristics.

Thanks in advance for any and all assistance.

Best,
-Tim
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#2 Andreas Wessberg FSF

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Posted 12 July 2008 - 04:02 PM

Using a collimator is probebly the best way, but I have done the
same with just a siemens star with nice result.
Not shure about it but i belive it´s a good idea to do the test at different distace aswell,
like 1, 5, and 15meter.

good luck
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#3 Chris Keth

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Posted 12 July 2008 - 05:46 PM

I would do it with a proper chart like this one you can get from Panavision Australia. Most of the change you will see by stopping down will be sharpness in the corners, so you need a way to observe that.
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#4 Dan Goulder

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Posted 12 July 2008 - 06:30 PM

Unless you're making a movie about a chart, I wouldn't rely on that alone (if at all). Shoot your potential actors and actresses in the kind of lighting that gives you the look you really want for the particular project. Many older lenses may stand out at T4-T5.6 but, depending on the project, you may (or may not) want to light things to that level.
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#5 John Brawley

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Posted 13 July 2008 - 02:34 AM

Or is there another way, say with using a collimator or something, that I should use to determine where the lens has it's sharpest and best characteristics.



Tim, the best way to this by far is t project the lenses on a lens projector. This will show up many of the common issues and is far easier to pick than using a lens collimator, which is really only designed to check a target set to infinity. The small scale of the collimator will make it harder to judge compared with a lens projector, not to mention what's happening at the edges compared with the centre of the lens.

You will see all of this and more when the lens is projected. It's a revelation....

jb
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#6 Tim Carroll

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Posted 14 July 2008 - 03:54 PM

Thanks guys, your responses are a big help.

I would love to have access to a lens projector. I have a collimator, but not a lens projector. In fact, I have never seen a lens projector and everyone I have asked about them has been pretty fuzzy. I can't even find info online about them.

I am going to start a new thread specifically about lens projectors. If I can find one to use, that is the way I will test the lenses, if not, the Panavision chart from Australia will be a good start, and then shooting some screen tests with the actors.

Thanks again to everyone,
-Tim
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