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Asymmetric aperture?
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PostPosted: Thu Jul 26, 2007 8:56 pm    Post subject: Asymmetric aperture? Reply with quote
I guess this is a problem with many older lenses. But how does it affect the image quality?

This is the worst case of my lenses:



PostPosted: Thu Jul 26, 2007 9:03 pm    Post subject: Reply with quote
It's not really the problem of old lenses, more like the problem of cheap lenses.
For the little I know of optic physics, this shouldn't have a big impact on the image quality. Surely much less impact than the quality of the glass or of the lens design.


PostPosted: Thu Jul 26, 2007 9:28 pm    Post subject: Reply with quote
I agree to Orio's comment.
It's a problem of cheap lenses. My Greens London shows a very good performance but the blades started to behave funny some weeks ago.

And it might effect bokeh but not really the image quality, unless some of the blades do not close at all, in this case the exposure will be wrong.


PostPosted: Thu Jul 26, 2007 9:40 pm    Post subject: Reply with quote
This a common problem with low quality lenses like some of Pentacon or Steinheil Culminar etc they made inside the blade fix ring from plastic.


PostPosted: Thu Jul 26, 2007 10:33 pm    Post subject: Reply with quote
My Zeiss planar has that problem starting at f/8 or so Sad but it's not really noticeable.


PostPosted: Thu Jul 26, 2007 10:50 pm    Post subject: Reply with quote
ooh, I am sure it has also plastic aperture blade ring inside... nice Zeiss Plannar


PostPosted: Thu Jul 26, 2007 11:06 pm    Post subject: Reply with quote
naplam wrote:
My Zeiss planar has that problem starting at f/8 or so Sad but it's not really noticeable.


What Planar?
I have several Planars and they are all perfect with the aperture blades.


PostPosted: Fri Jul 27, 2007 5:35 pm    Post subject: Reply with quote
Orio wrote:
naplam wrote:
My Zeiss planar has that problem starting at f/8 or so Sad but it's not really noticeable.


What Planar?
I have several Planars and they are all perfect with the aperture blades.

Carl Zeiss Planar T* 50mm f/1.7
It's in really good condition, little wear and perfect optics. Maybe it was serviced at some point in time and they screwed the diaphragm, who knows...


PostPosted: Fri Jul 27, 2007 6:17 pm    Post subject: Reply with quote
That's easily possible.
Some weeks ago, I reassembled a strange M42 lens, a Converto-Tamron 4.5/135 from bits.
This lens has a 12 blade aperture and putting these blades together in a way that they work was the hardest part of all.
But now it shows a nice circle:



PostPosted: Fri Jul 27, 2007 6:19 pm    Post subject: Reply with quote
If anybody made a little damage on any blades that is not possible any more, I bought a Steinheil Culminar 85mm f2.8 lens with broken aperture. Some guy tried to fix it already and made damage on blades I couldn't make any more working lens.


PostPosted: Fri Jul 27, 2007 6:44 pm    Post subject: Reply with quote
I don't know. I have two 1.7/50 (besides, I will be selling one soon), and they are both perfect to my eyes.

Of course it's a budget lens so it has only 6 blades, not 8 like better Planars.

By the way, don't underestimate the optical effect of the lens when looking at the aperture. Apertures with a small number of blades are more likely to seem imperfect even when they're not. The reason is that the blades are wider and so teh curvature of the glass can make one or more blades to look irregular. Try rotating the lens in your hand and inclining it, and look fmo different angles. If the impression changes, then it means it was a optical illusion.


PostPosted: Fri Aug 03, 2007 11:33 pm    Post subject: Reply with quote
Orio wrote:
I don't know. I have two 1.7/50 (besides, I will be selling one soon), and they are both perfect to my eyes.

Of course it's a budget lens so it has only 6 blades, not 8 like better Planars.

By the way, don't underestimate the optical effect of the lens when looking at the aperture. Apertures with a small number of blades are more likely to seem imperfect even when they're not. The reason is that the blades are wider and so teh curvature of the glass can make one or more blades to look irregular. Try rotating the lens in your hand and inclining it, and look fmo different angles. If the impression changes, then it means it was a optical illusion.

Yes, I tried rotating it and so on, it's a bit "imperfect", but it's a bit different every time I close it so I guess the problem is some parts are a bit worn out. It's noticeable at f/11 and f/16 but anyway it's not a big deformation. By the way, does the 400D have a tendency to overexpose at f/8, 11 and 16 on the planar 1,7/50? In Av mode, my camera overexposes about 1 stop at f/8, more so at f/11 or 16. Between f/5.6 and 1.7 the exposure is correct.


PostPosted: Fri Aug 03, 2007 11:56 pm    Post subject: Reply with quote
naplam wrote:
By the way, does the 400D have a tendency to overexpose at f/8, 11 and 16 on the planar 1,7/50? In Av mode, my camera overexposes about 1 stop at f/8, more so at f/11 or 16. Between f/5.6 and 1.7 the exposure is correct.


Yes this happens with 400D and manual lenses (not only Planars). This had to do with the way the Canon digital cameras measure the light, they are set to work with EF lenses which meter wide open, and the other stops are calculated by the computer inside, they are not really read through the lens.
Of course when using manual lenses, for which these cameras are not made, what happens? If you meter wide open, or close to, it's all right, the values are close to the preset ones. But if you meter with a lens stopped down to f/8 or further, the computer inside the camera is fooled, because it assumes that you are metering wide open, but you are not, so it applies to the stopped down metering that you make, the parameters that are preset for the wide open aperture of the EF lenses. And the farther you get from wide open, the more the computer is fooled.

Having that said, I have to say that with my 400D the effect is not as strong as you describe it. I only get values off when shooting at extreme narrow aperture. In normal situations even when using f/8 or f/11 I get acceptable results. It used to be much worse with the 5D! Even worse than what you describe! But after I have installed the EE-S screen, and changed the metering preset in the custom functions, the 5D meters light perfectly.
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