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View Full Version : 240mm & 355mm lens cells in same '3' shutter - aperture scale ?



Dave Tolcher
26-Aug-2014, 06:40
My googling skills cant be up to much as I thought I would find the answer easily. I want to move a 355 G Claron out of a barrel into a spare prontor 3 shutter that came from a 240mm symmar. The prontor has the scales for the 240mm lens.

F8 for the 355mm lens is 44.375mm and F5.6 for the 240mm is 42.85mm. The diameters stay within 5% as you stop down. For non critical work can I just assume that the shutter scale is one stop different or do I have to apply some additional factor to get the true value ?

If I need to adjust the aperture scale is it a simple matter of getting F values from the calculation that match the aperture size and reading it from the scale - so for e.g. F45 for the G Claron would be where F30.5 (silly, I know !! - theory I am testing here please humour....)

Thanks

Dave

Paul Ewins
27-Aug-2014, 05:20
Yes, if you measure the physical aperture in the barrel and set the shutter to the same measurement then the F stop will be the same. Get an adhesive address label (avery or the like) and stick it over the scale on the new shutter, trim around the edges with a blade and then mark the appropriate apertures on the label with a fine marker as you measure them.

Note: Things are entirely different if you are trying to calculate an F-stop without access to the original shutter/barrel. Then the physical properties of the lens (not just the focal length) come into play and you end up measuring apparent apertures as viewed from the front of the lens

Dave Tolcher
27-Aug-2014, 06:33
Thanks Paul

Bob Salomon
27-Aug-2014, 07:23
Why not just contact Schneider and see if they still have the scales for the lens you want to mount?

Jim Noel
27-Aug-2014, 07:55
If the apertures are as you say staying within 5% I believe you are safe to sue them. Very few shutters are within 5% of marked values.

Jim Noel
27-Aug-2014, 07:58
On second thought, is the 5% actual diameter or is that the measured light transmission? If the diameter, then my 1st answer s totally invalid.
I have to blame such poor analysis on the vertigo and associated medication.

DrTang
27-Aug-2014, 08:09
I've done this... I am not sure if it 'actually' works ..but it seems to:

I have quite a few old lenses front mounted into old shutters - compounds and ilexes mostly.. most of the lenses have apertures with them so I set the aperture to wide open on the shutter and use the one on the lens. However..I have a couple of lenses that do not have aperture blades for one reason or another so what I did was mount lens on camera and lock shutter open with shutter aperture set to wide open.. then I aim camera at a uniformly lit card (I do this at night in a darkend room usually) and measure the f stop off the ground glass

then I keep closing the aperture down until the reading is a stop less than wide open then I check to see if it lines up pretty close to a marked f stop on the shutter - it usually does

then I make a note in notebook with various lens notes that say: marked f8 = actual f5.6...or whatever

then I know when using that lens in that shutter.. actual f-stop is one f-stop more open from marked f-stop






(more open?? hahahaha..oy)

Dave Tolcher
27-Aug-2014, 14:16
Thanks all, I think I need to follow DrTang advice and actually measure the light transmission. I can do this quite easily as I have a 360mm APO Artar in a copal shutter that I can preset apertures and measure. I cant copy the scale because the shutters are different types and I cant measure the apertures due to the completely different number of blades.

Half naively I thought that the theoretical physical aperture size would be good enough, actually measuring the aperture with a copal and prontor shutters is difficult as they are such a different shape and would have to calculate area fairly accurately.

Bob, thought about that but the prontor shutter seems to not have removable scales (its engraved on the casing I think).

I am back with lens and shutters in 2 weeks so will have to wait until then to try it.

Best regards

Dave