How interchangeable are aperture scales?

[Tom Stanworth]

I have picked up a cheap 210mm Rodenstock Geronar 6.8 on ebay largely for the immaculate copal 1 shutter. I also have a 210 G-Claron f9 and have used other copals 1 shutters for this lens in the past.

Am I right in thinking that the 210 G claron cells will be entirely useable with the 210 6.8 Geronar aperture scale because both are 210mm lenses. I understand that one lens might be 208mm and the other 212 or something like that, but to all intents and purposes, for B&W film, such variances should be almost irrelevant, right?

With the G claron cells in the Geronar shutter, presumably the aperture will open up to F9 on the scale and then just not open up further as the lever moved towards 6.8?

Am I missing something?

I ask because I am overseas and it will be a long time before I get my hands on the lens/shutter to try for myself.

 

[Rick A]

No. The Aperture for a specific lens depends or the focal length devided by the diameter of the opening in either inches or millimeters, not interchangably. If the focal length of a given lens is 100 mm and wide open the aperture is 50 mm, then the f-stop is f2. You can tape a corrected scale over the existing one and use the shutter.

You will have to measure each f stop and do the math for the lens you want to use with that shutter, note the new f stop and make the new scale.

 

[Tom Stanworth]

The G-Claron seems to be a 208mm lens =/- 1% and the Geronar I haven't a clue, because Rodenstock don't seen to have any info online any more about this lens.

 

[Tom Stanworth]

Hi Rick, bear with me; maths is not my forte:

If the focal length of the two lenses is the same (or as near as dammit) and the opening is at the same physical size, then surely the effective aperture will be the same? If not, the please explain slowly !

 

[Tom Stanworth]

I just divided, for example, a physical aperture of 15mm into 208mm and 212mm and got f13.86 and f14.1 respectively. This is a tiny fraction of a stop (don't know how to calculate ho much of a stop mind you) and presumably could be ignored. Therefore, with the G claron being a 208.5mm lens and the Geronar likely being no more than a few mm more or less, the aperture scales should be almost the same, right?

Am I barking up the right tree or missing the point entirely?

 

[holmburgers]

Clearly the two apertures will be close enough (for jazz), but the real concern, I would think, is whether or not the two scales will match in terms of size & "travel".

Does that make sense?

If the aperture scales measure the same physical distance between stops and if the diameter of the scales are the same, then it should work.

Considering that you clearly state the focal length of your lenses, I'm not sure why Rick says no.

 

[Ian Grant]

The aperture scales will be the same for all intents and purposes in this case.

I've just placed a 150mm f9 G-Claron set of cells in an older Compur which came from another 150mm lens with an f4.5 max aperture and the restriction to f9 is the rear diameter of the front cell & inner edge of the rear cell, as you close the aperture at f9 it can't be seen but past f9 it stops the lens down. The cells are now in an Agi (Epsilon) shutter probably for a 75mm/80mm lens but the aperture scale's a stop different. I also compared the actual apertures of the G-Claron's originalmodern Scheider fitted Compur (broken) with the older Compur and they were identical

Now there's 150mm Geronar cells in the older Compur again correct across the scale.

Ian

 

[zach]

Just shoot wide open

 

[Tom Stanworth]

Great, thanks all.

As for shooting wide open, it is not such a good idea with a Geronar or G-Claron (for landscapes) :O

BTW I was seriously impressed with my 300 Geronar f9. Light as can be and super cheap, with fair coverage for 10x8. At f32 I have about 1.5 inches of rise, which is workable for landscapes. At F22 and beyond it is as sharp as any other lens. While I probably won't use the 210 Geronar much, I will give it a whirl and expect it to also be v good at F22.

 

[Ian Grant]

Clearly the two apertures will be close enough (for jazz), but the real concern, I would think, is whether or not the two scales will match in terms of size & "travel".

Does that make sense?

Provided there's an aperture scale fitted the ratio's between each stop remain the same regardless of the focal lenght of the lens fitted. So in the case of the G-Claron 150mm f9 I've fitted into an Agifold shutter at the shutters marked f4.5 the aperture is actually f9, at the shutters f16 the G-Claron's true aperture is f32. (I inadvertently said a stop difference before in fact it's a simple doubling of the apterure which is 2 stops.)

The F stop is calculated by FL/D where FL is the focal lenght of the lens and D the diameter of the aperture pupil. So in the case of the Agifold shutter which had a 75mm lens originally - the F stop -75mm/D will always be double - 150mm/D when a 150mm lens is used in the shutter regardless of what the actual chosen pupil diameter (D) is.

Just remembered that I put some 1950's T-coated 150mm f4.5 Tessar cells into a 1931 Compur #1 that had originally had 135mm f4.5 Tessar cells, this has been fine in use although I should in theory give 1.111 extra exposure but it's less than 1./3rd of a stop difference and so not noticable in B&W.

Ian