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Witness Marks?


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#1 Stuart Brereton

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Posted 30 May 2008 - 01:38 PM

I've just purchased a Kern Vario-Switar 12.5-100. It appears to be a beautiful lens, in great condition. Lens tests will done soon.

It has two witness marks on the aperture ring, one white, one red. I'm assuming that one of these is for F stops, and the other T stops. One thing bothers me, though. As the aperture scale is not linear, it would seem that the t-stop compensation increases as the stop gets smaller.

Can anyone verify for me that these marks are indeed f & t stops?
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#2 Jonathan Bowerbank

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Posted 30 May 2008 - 02:21 PM

Red = T Stops :)
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#3 Nick Mulder

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Posted 30 May 2008 - 07:11 PM

oooh, yeh cool - they are nice huh -

but which one ?

the preset iris version - the PTL/power zoom - or the standard with Aspheron option ?
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#4 Stuart Brereton

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Posted 31 May 2008 - 08:17 AM

It's the standard with Aspheron. Lovely bit bit of glass, I'm looking forward to shooting tests.

Thanks for the confirmation on the witness marks.

Actually, as a newcomer to the world of bolex, what exactly is a 'preset iris'? Also, I read an article recently somewhere online that stated that RX lenses have a built in compensation for the prism, and therefore you need only open up an extra 1/3rd stop for the shutter.

Anyone care to comment?
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#5 Nick Mulder

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Posted 31 May 2008 - 06:38 PM

It's the standard with Aspheron. Lovely bit bit of glass, I'm looking forward to shooting tests.

Thanks for the confirmation on the witness marks.

Actually, as a newcomer to the world of bolex, what exactly is a 'preset iris'? Also, I read an article recently somewhere online that stated that RX lenses have a built in compensation for the prism, and therefore you need only open up an extra 1/3rd stop for the shutter.

Anyone care to comment?



cool, you have the best 12.5-100 - you can use the aspheron knob without the aspheron lens as a extended focus range thingy, meaning you in essence have a macro lens there also - it achieves this by moving the backfocus so any zooms you make in this mode will move the plane of focus also, which is not what usually happens with zooms but can with proper (fiddly) planning make some ordinarily tricky shots very easy (pulling focus on macro zooms, ouch!)

And of course you can fork out a trillion dollars on ebay for the aspheron to get a 6.5mm - woo!

The preset iris version which you dont have anyhoo is a kind of spring loaded iris function like you find on many stills SLR lenses, you compose and focus with the iris open, so you get full brightness on your ground glass and can get a better idea of exactly where the plane of focus is. When you take the shot it springs down shut to the setting - f16, f4 whatever ...

It is different from the preset macro-switar primes however - they aint as fancy in terms of that functionality, they dont spring into place, but you can blindly (as in not take your eye away from the finder) move them to a preset aperture, which still makes things much easier - they sure are nice glass also

RX issue, have a read here:

http://www.apecity.c...6mm_cameras.pdf

Its a proper description from Bolex themselves on the issue...

about %40~50 of the other info you will find is incorrect, but is instead what most peoples intuition tells them to expect is the case - it does however mean that any footage shot with this intuition will be overexposed ...

So that explains what the 'RX' is all about - for proper exposure have a search here in the forums, it been discussed quite a bit.

Oh, and if you want to swap your lens for the power zoom PTL version (with aspheron knob, auto-iris (yip, auto iris) and perfect glass also) - I'm keen ;)

Edited by Nick Mulder, 31 May 2008 - 06:39 PM.

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