Question about the aperture of Kowa Six 110mm lens

[Oblidor]

Hi

I discovered that my 110mm lens, which has a max aperture of 5.6, will show the aperture blades at 5.6. In the attached images one can see the lens when the shutter is cocked. The second image is when activating DOF leaver at 5.6. None of my other Kowa Six lenses show the aperture blades at max aperture. The only that does is my Kowa SW (28mm f3.2) 135 camera.

I just wondering if this lens is supposed to be like this or something is wrong with the shutter. Find it odd that it can be something wrong and when I comparing to my 150mm and 85mm lenses the aperture size seems to correspond with 5.6.

A bit strange why this lens could not have a stop or two larger aperture, but perhaps it has to do with the image quality at larger apertures so that is why it was limited to 5.6?

What do you think?

 

[loccdor]

I don't have experience with this specific lens, but I've seen lenses that slightly showed aperture blades at wide open - less than this example.

You could try measuring the light coming through the lens at both the "open 5.6" and "closed 5.6" in comparison with a known accurate lens at 5.6.

In most cases, a manufacturer would want to advertise a wider aperture even at the expense of some image quality so it does seem odd to me. Perhaps the aperture ring mechanism is misadjusted or damaged. Maybe the springs lost their original tension.

 

[Kino]

I do not have the 110mm lens for my Kowa 6, but here is the description in the Kowa Super 66 Technical Bulletin:

Since the lens appears to be specifically designed for flat field copying, it might be that f5.6 is the minimum (maximum?) acceptable aperture to meet the resolution requirements for this sort of work.

https://www.cameramanuals.org/kowa/kowa_super_66_technical.pdf

 

[OAPOli]

I have a Topcor lens that also shows the diaphragm at the minimum aperture. Inspect it from the front of the lens to get a better idea of the effect on f/number.

 

[Alex Varas]

I just got my copy of the 110mm and yes, aperture behaves like yours so it has to be this way hopefully

 

[reddesert]

I don't have any experience with this lens. For some lenses in leaf shutters, especially slow lenses, they are mounted in a shutter whose clear aperture is larger than the lens design needs. (In other words, the aperture limit is set by the optical design, not by how wide the shutter opening is.) Typically the maker will put a stop so that you can only open the aperture to the max of the lens, not the max of the shutter. That means aperture blades will still be visible.

Since this is a 110mm/5.6 lens, the apparent aperture diameter viewed from the front should be 110/5.6, roughly 19.6 mm. You can lay a ruler across the front element and look at the apparent aperture diameter (you have to measure this with the lens glass in place, not the mechanical diameter of the aperture). If it's about 19mm, then the lens is open as far as it should go and you can stop worrying about it.