Shutter for 150mm Computar

Old-N-Feeble · Jun 11, 2017

I have a barrel-mount 150mm f/9 Computar. I know the cells are a direct fit into Copal/Compur/Prontor #1 shutters. I have a few shutters that will work fine; 1. Older Synchro-Compur, 2. Polaroid Copal Press, 3. Late all black Copal.

The first two will need f/scales made. The Copal has a 210mm f/6.8 Caltar II-E lens with the appropriate f/scale for the Caltar cells.

I know the 150mm Computar never changed throughout it's manufacture. The Computar, Apo-Kyvytar and Graphic-Kowa 150's were all identical. I also know those labeled Apo-Kyvytar were f/6.8 when in shutters.

So... is the f/6.8 scale on this Copal shutter with 210mm lens appropriate for the 150mm Computar? There's nothing wrong with the Caltar II-E cells and I can sell them.

shutterfinger · Jun 11, 2017

Aperture scales are engraved for each lens as the actual focal length can be 2% to 3% of the marked focal length and a nominal focal length scale will be close but not accurate.
150-2%=147mm/8 (f8) = 18.375 mm entrance pupil.
150+2%=153mm/8 (f8) = 19.125 mm entrance pupil.
The difference is .75mm which is between 1/3 and 1/2 stop at -2% of marked focal length and close to 1/3 stop at +2% of marked focal length for a nominal 150mm lens.
210/8 (f8) = 26.25mm entrance pupil.
The entrance pupil is the diameter of the aperture opening as viewed through the lens front group.
Its highly unlikely the actual aperture diameter will be anywhere close for the two lens.

removed account4 · Jun 12, 2017

ONF
i have computar symetrigon that is a thing of beauty
in a c/1 shutter i think, but it isn't a f9, i thinkit is 6.3
and it has mythic coverage /

Old-N-Feeble · Jun 12, 2017

Years ago, I thought focal length affected aperture scales. Recently, I've read that as long as the fully open mark is correct, focal length doesn't matter.

Since the Computar/Apo-Kyvytar are f/6.8 in shutters, AND IF the latter statement above is correct then the scale should be correct. If the former statement above is correct then the scale is wrong.

SHUTTERFINGER, are you stating that the aperture scale will be wrong if I swap 150/6.8 cells with 210/6.8 cells... because of the different focal lengths of the cells?

JOHN, thank you. The Symmetrigon is a 4/4 double-Gauss design, right? The original Computars were mostly 6/4 plasmats, though the design changed as the manufacturers changed. The 150mm never changed though... always remained the same plasmat design.

removed account4 · Jun 12, 2017

you're welcome !
i don't know the designs &c
i know the lens works great and is less filling !

shutterfinger · Jun 12, 2017

An f stop is the lens optical focal length divided by the diameter of the aperture opening as viewed through the entrance pupil ie the front lens. Different focal lengths will have different front lens magnifications.

John Koehrer · Jun 12, 2017

ONF whaddya got to lose? Try a sheet or two.
The shutter won't care what the fl is. and the aperture plate won't either.

Try a light source in front of the camera & compare readings through
both lenses with a meter.

Dan Fromm · Jun 16, 2017

Y'know, you could just put the cells in the shutter, set the pointer to 6.8 and look to see whether the aperture blades are just disappearing into the shutter body. If they are, the scale is fine. If not, time for a new scale.

Old-N-Feeble · Jun 16, 2017

Thank you, Dan. The aperture blades completely disappear into the shutter body when opened further than f/6.8 and barely show as they're closed. I observed this while the cells were installed and viewed through the lens cells from a sharply acute angle... so only a tiny sliver of light was visible through the edges of the glass in the front/rear cells. It was the same as viewed straight through the cells.

So you're indicating that lens focal length does NOT affect aperture (in this case)?

Dan Fromm · Jun 16, 2017

Think pupillary magnification.

Old-N-Feeble · Jun 16, 2017

Dammit, Dan. Must you always convolute simple questions?

Pleez 'splain...

Dan Fromm · Jun 17, 2017

The f/ number is the focal length divided by the lens' entrance pupil's diameter. The entrance pupil is the image of the diaphragm as seen from the front of the lens. The lens' front cell can make an image of the diaphragm that isn't the same size as the diaphragm.

Open the diaphragm fully, mount the front cell in the shutter, hold the shutter at arm's length with a ruler laid across its diameter. Read the entrance pupil's diameter. And so on.

Vaughn · Jun 17, 2017

jnanian said:
ONF
i have computar symetrigon that is a thing of beauty
in a c/1 shutter i think, but it isn't a f9, i thinkit is 6.3
and it has mythic coverage /

I have a 210mm/f6.3 Computar Symmetrigon -- but without the mythical coverage...just the usual coverage for a 210mm wonderfully sharp lens...covers 5x7 easily, but not 8x10. Comes with a nifty metal lens shade, though.

Shutter for 150mm Computar

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