Diaphragm and aperture value issues

[Shotto]

Hello,

I shared this already on Photo.net but let's see what you think. Please have a look at the attached two photos for reference. I have a newly acquired Minolta Minoltina-s rangefinder camera and I am cleaning its lens elements. While at it, I keep looking at the diaphragm and the aperture opening vs aperture ring setting; I am not sure if the blades are correctly installed (by some previous owner). The smallest f16 setting looks really small in my experience and the widest f1.8 does not seem to open up as wide as it should. The aperture ring comes to a stop at both ends.

So my question is: if the measurement for the smallest aperture available is approximately 1,4mm in diameter, and the widest is 14,6mm, can I just check the corresponding f values by dividing the focal length 40mm by these values? It gives me roughly f28 and f2.8, telling me that the blades are not correctly installed or, as a fellow member at Photo.net kindly suggested, I should have a look at the linkage between the diaphragm and the aperture ring.

I am trying to avoid rebuilding the diaphragm if possible because it is clean and smoothly functioning and difficult to access but of course it would have to be correctly functioning as well - in particular I want the max aperture to be available for use.

Many thanks for your comments,

Regards

Otto

 

[Richard G]

The aperture diameter for f2 with a 40mm lens is 20mm. For f1.8 it will be larger than 20mm. For f16 it is 2.5mm. For each 2 conventional f stops there is a halving of the diameter.

Here is an online calculator for non root 2 f stops.

Aperture Area Calculator

The aperture area calculator helps you to compute the aperture area of a lens.

www.omnicalculator.com

This gives 22.22 mm for f1.8.

 

[Shotto]

The aperture diameter for f2 with a 40mm lens is 20mm. For f1.8 it will be larger than 20mm. For f16 it is 2.5mm. For each 2 conventional f stops there is a halving of the diameter.

Here is an online calculator for non root 2 f stops.

Aperture Area Calculator

The aperture area calculator helps you to compute the aperture area of a lens.

www.omnicalculator.com

This gives 22.22 mm for f1.8.

Thank you, Richard! I will try to figure out why the aperture diameters are not right with this camera.

 

[Mos6502]

IIRC the measurements of apertures are not made on the physical aperture itself, but rather how it appears through the glass.
However, you should not have to touch the actual diaphragm blades at all. Most of the time there is a degree of adjustability built in, either with set screws in the aperture ring, or in the linkage between the ring and the tab (or whatever it may be) that's actually connected to the diaphragm mechanism.

edit: it is, by the way, not even slightly unusual for the diaphragm of a lens to be capable of being stopped down smaller than the highest marked f/stop. Particularly among old cameras, being able to move past f/16 is not very uncommon, even if there are no markings past f/16.

edit 2: found this video explaining how the f/stop is derived from the entrance pupil, and not the physical dimension of the aperture.