# Thread: f-stop correspondence to wheel stop Dallmeyer?

1. ## f-stop correspondence to wheel stop Dallmeyer?

Acquired a No.1 Dallmeyer Rectilinear and am wondering if there is a place where I can convert the wheel stops to f stops? There are 5 stops in addition to the wide open.

To get the speed of the lens would one divide the focal length by the diameter of the front lens or by the diameter of the largest stop as seen through the lens or by the diameter of the largest stop viewed directly (not through lens)?

If the next stop diameter is 1/2 of the previous then the f-stop for that is 1/2 of the previous? Eg. IF 1cm hole works out to f/4, then 0.5 cm hole is f/8?

Aso, is this the wide angle version of the Rapid Rectilinear? And lastly, the wheel is loose (no click stops for the apertures) - any suggestions on how to stabilize it when photographing?

2. ## Re: f-stop correspondence to wheel stop Dallmeyer?

Need to know the serial number as early ones started at F.15 (oh!) and later ones at the more conventional F16. Are you sure there are 6 openings - not 5? I don't have the wide-angled Dallmeyer - always lost out! I have checked my (UK) wheel stops that have F values. The values run 18, 25, 36, 50, 71 and 11, 16, 22, 32, 44. I don't think that repair to the edge catch spring is feasible! Would suggest you carry thinned wooden toothpicks so that you can jam the wheel in the side. The toothpick will stick up so you will remember it is there.

3. ## Re: f-stop correspondence to wheel stop Dallmeyer?

To calculate f values, I've always heard you measure the amount of light you can see through the front lens. I.E. set the lens at the largest stop. Hold it up to window light. Measure the width. Divide Focal Length by Width = F.

I have a 1a I believe, and if there are numbers on the wheel, I just ignore them and use the "...big...smaller...about there..." method. But I imagine each smaller hole let's in half the light of the previous. So you might be ok guessing the lens is f16, f22, f32, f45, f64.

4. ## Re: f-stop correspondence to wheel stop Dallmeyer?

Originally Posted by Steven Tribe
Need to know the serial number as early ones started at F.15 (oh!) and later ones at the more conventional F16. Are you sure there are 6 openings - not 5? I don't have the wide-angled Dallmeyer - always lost out! I have checked my (UK) wheel stops that have F values. The values run 18, 25, 36, 50, 71 and 11, 16, 22, 32, 44. I don't think that repair to the edge catch spring is feasible! Would suggest you carry thinned wooden toothpicks so that you can jam the wheel in the side. The toothpick will stick up so you will remember it is there.
The serial number is 55441; that makes it an 1895? f/16? Checked, yup 6 openings..
Great idea re toothpick, thank you!

5. ## Re: f-stop correspondence to wheel stop Dallmeyer?

I have my 135mm 1886 one down as f/16. The stops are in normal progression from there. A really sharp lens corner to corner on 4x5. Wish I had a 5x7 Speed Graphic;-)

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