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01-04-2014, 05:04 AM   #1
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Does TC increase projection circle of lens ?

Disclaimer : i think it's the appropriate section of the forum for this, if it's not, then I apologize, and feel free to move this topic.

Hello

I was wondering : Does TC increase the projection circle of lens ?

01-04-2014, 05:52 AM   #2
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In a word, yes.

"For example, a 2 teleconverter for a 35 mm camera enlarges the central 1218 mm part of an image to the size of 2436 mm in the standard 35 mm film format."

From Teleconverter - Wikipedia, the free encyclopedia
01-04-2014, 06:51 AM   #3
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If the teleconverter was designed for a full frame camera then yes, it will expand the image circle of a lens designed for APS-C to fully cover a 35mm full frame film or sensor.

If however the TC was designed specifically for APS-C then this will not necessarily be the case.
01-04-2014, 08:29 AM   #4
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will it make a FF lens to have a bigger projection circle than 24x36 ?

01-04-2014, 08:43 AM   #5
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QuoteOriginally posted by aurele Quote
will it make a FF lens to have a bigger projection circle than 24x36 ?
Perhaps, but there is no real way of knowing without testing, since any extra coverage of light/area would also lead to loss of contrast due to the internal reflections inside the mirror box.

If it were me, I would have baffles installed to prevent this, but I did not design a TC so the only real way to answer is maybe
01-04-2014, 09:26 AM   #6
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QuoteOriginally posted by aurele Quote
will it make a FF lens to have a bigger projection circle than 24x36 ?
Most FF lenses have larger projection than 24x36 even without TC.
01-04-2014, 02:55 PM   #7
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i was wondering if i could use a Lens + TC for building a custom Medium Format for some film testing.
01-04-2014, 04:12 PM   #8
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Yes it does increase the size if the image circle. However image quality may be an issue around the edges and cormers. When a TC is designed the manufacturers only design it with the central image area sharpness in mind as this is the only area of TC projected image that gets used in normal use. Light that is spread to the outer limits of the image circle normaly 'gets lost' and is given no consideration when designing lens distortions / corrections. Assuming you are producing a 6x7 negative your center 36x24mm of image will be the same image quality produced with the TC on a standard negative and will degrade radialy out from there. Your focal plane distance will remain the standard 45.46mm as it is for Pentax 35mm cameras.


Last edited by Tool_Horder; 01-04-2014 at 04:26 PM.
01-04-2014, 04:26 PM   #9
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ok, i have not problem with the way it design though.

i just need to grab a cheapo TC then :-)
01-04-2014, 04:35 PM   #10
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QuoteOriginally posted by aurele Quote
ok, i have not problem with the way it design though.

i just need to grab a cheapo TC then :-)
The standard design furmula for teleconverters is really very simple maths - 'cheapo = poor image quality'. However for your initial experiments they will proove the concept.
01-04-2014, 04:49 PM   #11
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Something else I just thought of is that the light rays will be reaching the film at very acute angle relative to the film plane on the outer limits due to the shorter focal plane distance of 35mm compared to the longer focal plane distance of medium foumat. This will no doubt also degrade image quality on the outer limits and you will probably see a loss of contrast in outer limits
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