Canon 28mm f2.8 diaphragm

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Toni Nikkanen

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Hi,

Is it normal that on a Canon 28mm f2.8 LTM lens, when the aperture is set to f2.8, the diaphragm still appears to be a hexagon and not a round circle? The diaphragm does appear to open and close consistently as I turn the aperture ring.

Attached 2 images shot from the backside of the lens, first at f2.8 and second at f4.
 

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ZorkiKat--I also have this lens serial number 139XX, and it looks exactly like yours. The aperture is hex shaped at f2.8. Of course I noticed this soon after I got it, but I didn't think too much about it because I've owned two 35mm lenses for the Kodak Signet 80 and they were both this way. I just checked under magnification and cannot see any evidence anywhere on the back of the lens that it has been taken apart. I can only comment on my one lens. Hopefully, someone that has worked on or owned several of these lenses will comment. Now I'd like to know the answer too.
 
Hmm, so it might actually be possible this hexagonal shape at full aperture is actually normal for this lens? If that is so then there is nothing to complain about on that regard.

I guess next thing to try is get some really even light source and measure both this and another f2.8 lens and see if the camera meter is pretty closely showing the same measurements. ..and develop my test shots of course! 🙂
 
I'm used to see opening round with no aperture blades at max aperture, though technically it's possible to use blades to make opening at widest aperture.

If this is same what other owners see then today I have learned something new.
 
..Now that I am paying attention to this detail, I notice that the earlier f3.5 version also exhibits this phenomenon, though much less obviously. So it really seems like a feature of the lens.
 
Toni Nikkanen said:
Hmm, so it might actually be possible this hexagonal shape at full aperture is actually normal for this lens? If that is so then there is nothing to complain about on that regard.

I guess next thing to try is get some really even light source and measure both this and another f2.8 lens and see if the camera meter is pretty closely showing the same measurements. ..and develop my test shots of course! 🙂
Click to expand...


I thought you were asking about the shape of opening. At the full wide end, the opening for my Canon 28/2.8 is a circle. Although, I do not think the blades are forming the circle. Sorry for any confusion.



/
 
Eeps.. now I would like to get confirmation from Spavinaw to make sure I didn't misunderstand...

so, how does your lens look at when fully open?
 
Toni--This is to confirm that when fully open you can still see the edges of the six aperture blades forming a hex shaped opening. To be a bit more specific, when you look in the front of the lens the six corners of the hex exactly touch the circle formed by the lens itself. It is a hex circumscribed with a circle.
 
My Canon 28/2.8, 28/3.5 (late black/chrome) and 25/3.5 all show the hexagonal edges of the diaphragm opening at full aperture. Also, my current 25/3.5 is early; a late one that I sold also showed the same aperture wide-open. So I'm guessing it's unusual or even incorrect for at least the 3/4 lenses/types I've seen to have perfectly round openings at max aperture.
Ari
 
Toni, I have a Canon 28/2.8 ser#127xx in very good condition which I've used without any problems, and the 2.8 opening is hexagonal, as you described, not round.
 
mine looks similar

mine looks similar

Type II s/n 187xx

Best thing to do is measure a white evenly lit wall and compare with other lenses at 2.8, pref. same or close to same fl.

Biggest issue I've seen with these is decentering or unevenly replaced elements causing edge softness or curvature of field wide open on ff.


Toni Nikkanen said:
Thanks, oh well, if there indeed is a problem with the diaphragm as it seems, I may have to return the lens.
Click to expand...
 
I bought a canon 35/2.8 a few weeks ago and saw the blades when fully open. I discovered that been twisted past 2.8 so that when one might expect. 2.0 tick mark was where it was fully open. Opening up to 22 effectively reset it so that 2.8 showed no blades. I think it's a quality/design issue. There's no hard stop in either direction.

Mark
 
My Canon 28mm/2,8 (SN 159XX) shows a round full opening at f/2.8.

28-4ew.jpg
 
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