Summicron-M 50mm v5 aperture blades

[Harpomatic]

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Hi everyone,

I had a look around but I can’t  find this information: I have a Summicron 50mm v5 silver chrome and when I look at the diaphragm closing from the back of the lens I see what looks like a second diaphragm closing together with the main one. Is it an extension of the 8 normal blades? A second set of blades? If so, to what purpose?

It doesn’t seem to be an extension of the front blades, I don’t see how that could be given the profile, but I might be wrong.

What am I seeing?

 

Thanks!

[farnz]

I don't have the 50/2 Summicron v5 anymore but I don't remember it having what you're describing.  Does it look any different from the diaphragm in the final picture at this link?

The blades have an angle about halfway up and then finish in a curve so could it be an optical illusion created by the shape of the blades perhaps?  Or a reflection in the front group of elements even?  (Clutching at straws here.)

Pete.

[lct]

Déjà vu on several lenses. Optical illusion due to the shape of the blades i suspect but i'm not sure at all.

[adan]

It is an illusion. Although it has fooled me into thinking there were two sets of blades for years - until your question drove me to do some research.

It is due to the actual complete shape of the aperture blades in M-Mount lenses (among some others).

Which is....... L-shaped, roughly.

The overlapping twin legs of the L give the impression of a "second aperture" with a slightly larger diameter than the actual diameter, closing in on almost identical at the smallest f/stop, as the "Ls" swing in from the corner pivot point.

Here is a very rough schematic of how the illusion is formed - with each aperture blade in a different color to make the "Ls" more distinct.

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I can confirm the visual effect with virtually all my M-mount lenses: CV 75 f/1.5 and 35 f/1.4 II, 28 Elmarit v.3, 75 APO-Summicron, 90 Summicron v.3, 21 Elmarit-M, 135 Tele-Elmar. My 90mm TE blades are too scuffed to be sure.

I suspect the interesting "L" shape used by Leitz/Leica and Cosina has something to do with fitting the blades into as compact a lens barrel as possible (for the given max. aperture speed). They can be seen in real life here.

https://sites.google.com/site/kameraque/home/bokeh-king-is-naked-repairing-leica-summicron-35mm-pre-asph

Edited November 22, 2020 by adan

[lct]

Always learning something on this good old forum. Thank you Andy .

[Harpomatic]

12 hours ago, adan said:

It is due to the actual complete shape of the aperture blades in M-Mount lenses (among some others).

Which is....... L-shaped, roughly.

Andy, learning from you as usual. Thank you.

You got me looking in the back of many lenses and indeed you are right, I have no idea why it didn't occur to me to check the other ones! I guess I took it for granted that it wouldn't be the case with those. But most of them are indeed the same, CV and Leica.

It is quite striking though how much it looks like a second set, it seems impossible that the blades would create four layers by being superimposed this way.

[Harpomatic]

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On 11/21/2020 at 1:18 AM, farnz said:

optical illusion created by the shape of the blades perhaps?

 

On 11/21/2020 at 1:49 AM, lct said:

Optical illusion due to the shape of the blades i suspect

Optical illusion indeed. Thanks for chipping in!