Double aperture on 3.5/28mm Skopar

[_lou_]

Hello,

When looking into my skopar 28mm, I noticed that the lens has in fact two
diaphragms, on being slightly more closed than the other, but they both close simultaneously. Is there any reason for this design ?

 

[Graham Line]

Odd

Odd

Mine only has one. Can you post a photo of it?

 

[kbg32]

Two sets of diaphragm blades????

 

[ssmc]

My 35/2.8 SC Skopar only has one set... but I know what you're seeing 🙂

Both my Canon 50mm LTM lenses look like this - but only when viewed through the back; from the front they look like any other lens. Here's a pic I just took - I imagine that your lens looks something like this?

As for what it represents... I'm not entirely sure, but I think it's an illusion caused by the blades having a (sort of) boomerang shape. The inner part of the "elbow" is overlapped by the next blade in sequence, so it appears that there are two distinct sets of blades, but it's actually the edge of the same blade seen twice, all the way around. That's a really bad description... the Nikkor Millenium 50mm almost has this "double-aperture" effect but the blades don't completely cover the bend. I'll get a shot and post it.

Scott

 

[ssmc]

Here we go!

It's harder to see on this lens because it's a lot newer and the blades are very black, but imagine if the "elbow" was at a more acute angle, so that it was hidden under the following blade, you'd have the appearance of two sets of blades - but only from one side. I'm having some difficulty visualizing why it would be done this way, though, so any more info on this would be appreciated!

Scott

 

[_lou_]

Thanks for the pics. It looks a bit like your lens, but the blade sets create two ring that are closer in diameter. I think you have the right explanation too. I need to take some pictures on my return home end of this week.

 

[_lou_]

Here is a close-up shot of the Skopar's aperture :

 

[lcpr]

My 50mm Summicron and 35/2 UC Hexanon have a similar arrangement of aperture blades to that. The 35/1.2 II also had it.