240mm & 355mm lens cells in same '3' shutter - aperture scale ?

[Dave Tolcher]

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]

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]

Thanks Paul

[Bob Salomon]

Why not just contact Schneider and see if they still have the scales for the lens you want to mount?

[Jim Noel]

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]

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]

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]

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