Any owners of the SOM Berthiot Paris Angulor 1:3.3 F=28mm L39?

[nitroplait]

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I got this sample expecting that it was defective as the aperture blades don't fully open when the lens is set to max aperture F/3.3.

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So, I looked around on the web for more information. No one mentioned this phenomenon but I found  this video on Youtube which seem to show the lens in the exact same state at f/3.3:

That confused me.

I put my lens on a digital camera set to 200 ISO, set the lens to f/3.3 and got a shutter speed reading of 1/30 when pointing to an evenly illuminated surface.
I got same result with a modern 28mm set to f/3.3 pointing to the same surface.

The logical conclusion is that my sample is working correctly. But I can't help thinking that almost half closed aperture is very weird. 

Any owners of this lens who can confirm it displays same behaviour?

 

 

Edited 5 hours ago by nitroplait

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[jerzy]

formula is A(perture)= f(ocal length)/D(iameter of opening). For your lens, when aperture set to 3,3 opening shall be roughly 8,5mm in diameter. There could be optical performance reasons why lens is throttled

[luigi bertolotti]

I have not that lens (no many owners, I think... a fine collectible, compliments !) but an iris that looks not fully open with lens wide open, is not rare in short focals with rather wide front elements... I'd bet that if you remove the diaphragm, you get a 28mm f 2 or 2,5/2,8 with strong vignetting and/or distortion at edges.

Edited 3 hours ago by luigi bertolotti

[UliWer]

Yes, it is not rare that you can seen the aperture blades when you use max. opening of a wideangle lens.

Here are two examples from Canon LTM lenses with wide angles and (for the time) big openings - both examples fully opened:

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The aperture blades of the 2.8/28mm are even better visible if you look straight into the lens - which I didn't manage in the photo.

 

 

[nitroplait]

1 hour ago, jerzy said:

when aperture set to 3,3 opening shall be roughly 8,5mm in diameter.

Thanks Jerzy. Just checked. 8,5mm is very close to what I can measure.

1 hour ago, luigi bertolotti said:

I'd bet that if you remove the diaphragm, you get a 28mm f 2 or 2,5/2,8 with strong vignetting and/or distortion at edges.

Funny! In my research, I fell over this Korean teardown of the lens. The Google translation mentions poor performance at f2, but it didn't make sense to me - I wrote it off as a translation error. 
I just re-translated with Apple's translator, and it turns out the repair tech's customer wanted the lens to open fully which equalled f2.

[nitroplait]

44 minutes ago, UliWer said:

Yes, it is not rare that you can seen the aperture blades when you use max. opening of a wideangle lens

I have to take a look at my wides again. Never noticed.

[luigi bertolotti]

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33 minutes ago, nitroplait said:

I have to take a look at my wides again. Never noticed.

A quick look at my old Leitz 28s and 21s apparently doesn't show this feature; but I made an hipotesis (not supported by any real evidence) : SOM Berthiot made also cine lenses... maybe this 28mm design was used for some cine lens, for which a wider aperture could be acceptable given the "neg crop". (for 16mm a 25mm f1,8, named Lytar, is recorded among Bolex lenses... and also cine 35mm is not a 24x36 frame...); as a partial support to this hipotesis, f 3,3 is such an odd aperture for 35mm cameras, that it could someway be a "post design" adaptation...

Edited 1 hour ago by luigi bertolotti

[Maarten]

Could it be that the larger lenses reduce natural vignette?

[UliWer]

20 minutes ago, Maarten said:

Could it be that the larger lenses reduce natural vignette?

Yes, certainly. 

The 25mm Canon Lens I showed above was a copy of the  1:4/25mm Topogon by Zeiss (Canon always confessed the origin). Though they tweaked it to f/3.5 which of course resulted in (even) stronger vignetting. A larger lens was a way to reduce this vignetting, but only if the opening wasn't increased even further.  So they used the aperture to reduce the opening in order to keep the vignetting in an acceptable range (for its time).

The 1:2.8/28mm Canon lens was introduced in 1956 - at the same time when Leitz offered the Summaron with just f/5.6 and it took them several years to achieve f/2.8 for 28mm with the first Elmarit, which has a much more complicated design with 9 lenses than the Canon with just 6.  So again a rather large front lens was used to reduce vignetting and with the aperture they avoided an opening which certainly would have been too much.

The Som Berthiot with f/3.3 is not far from the Canon and it looks as if they went the same way.