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Orion-15 diaphragm question
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PostPosted: Fri May 05, 2017 12:03 pm    Post subject: Orion-15 diaphragm question Reply with quote
I just took delivery of a lovely Orion-15. It needs re-lubricating, but other than that, very nice indeed.

However, I notice that the diaphragm, when wide open at f/6, is stopped down slightly (see pic below showing lens from the rear at f/6). Each subsequent aperture setting results in the blades closing further, so it's working just fine - but I'm wondering, is it normal for it to be stopped down slightly at f/6?

Thanks in advance, all Smile



PostPosted: Fri May 05, 2017 1:02 pm    Post subject: Reply with quote
Mine is like that too and I remember looking into this when I got it and it does seem to be normal.


PostPosted: Fri May 05, 2017 1:49 pm    Post subject: Reply with quote
MartinV wrote:
Mine is like that too and I remember looking into this when I got it and it does seem to be normal.


Thanks, Martin - very good to know. I appreciate the reply Smile

It does make you wonder why they limited the lens to f/6, doesn't it? Seems like it could open up to f/4 or thereabouts if the diaphragm adjustment wasn't limited in this way.

Anyway, I'm happy! Very Happy


PostPosted: Fri May 05, 2017 5:07 pm    Post subject: Reply with quote
BigMackCam wrote:
MartinV wrote:
Mine is like that too and I remember looking into this when I got it and it does seem to be normal.


Thanks, Martin - very good to know. I appreciate the reply Smile

It does make you wonder why they limited the lens to f/6, doesn't it? Seems like it could open up to f/4 or thereabouts if the diaphragm adjustment wasn't limited in this way.

Anyway, I'm happy! Very Happy


You wonder that? Why would a lens designer do that for a lens with that intended purpose?
Possibly to achieve the design goals for such a lens? And to keep the cost down? Wink


PostPosted: Fri May 05, 2017 5:15 pm    Post subject: Reply with quote
BigMackCam wrote:
It does make you wonder why they limited the lens to f/6, doesn't it? Seems like it could open up to f/4 or thereabouts if the diaphragm adjustment wasn't limited in this way.


my guess is that if the max aperture was bigger, then the blades should be larger to achieve minimum aperture and then the diaphragm wouldn't fit inside the lens barrel?

theoretically, if you cut the blades you get a f/4-f/16 or f/4-f/11 lens instead of f/6-f/22 lens?


PostPosted: Fri May 05, 2017 6:18 pm    Post subject: Reply with quote
kds315* wrote:
You wonder that? Why would a lens designer do that for a lens with that intended purpose?
Possibly to achieve the design goals for such a lens? And to keep the cost down? Wink


Sure I wonder that. No harm in wondering - maybe someone has the definitive answer, and then I learn by asking Smile

Plenty of lenses were / are designed to provide acceptable performance outside the parameters of their primary design brief. A specialist lens that can also be used in a wide range of general situations is likely to be more attractive to potential buyers. Macro lenses spring to mind, where most will be used primarily at f/16 or smaller, but with many being perfectly usable much wider than this.

I'm not saying it would or should have been possible to have it open wider, nor that I'm in any way disappointed that it doesn't. I'm just curious as to why it was limited to f/6, and if it would have compromised the performance at f/6 - f/22 by allowing it to open up wider. Curiosity has driven a great deal of my learning thus far...

One thing I'm sure of - the designer had his reasons Smile

Last edited by BigMackCam on Fri May 05, 2017 11:01 pm; edited 4 times in total


PostPosted: Fri May 05, 2017 6:19 pm    Post subject: Reply with quote
kansalliskala wrote:
BigMackCam wrote:
It does make you wonder why they limited the lens to f/6, doesn't it? Seems like it could open up to f/4 or thereabouts if the diaphragm adjustment wasn't limited in this way.


my guess is that if the max aperture was bigger, then the blades should be larger to achieve minimum aperture and then the diaphragm wouldn't fit inside the lens barrel?

theoretically, if you cut the blades you get a f/4-f/16 or f/4-f/11 lens instead of f/6-f/22 lens?


Interesting ideas. Thanks for that Smile


PostPosted: Fri May 05, 2017 11:49 pm    Post subject: Reply with quote
All the Orion lenses have this particularity, as far as I know (Orion-15, Orion-18, Orion-1a...). So I guess it's to limit the decreasing of IQ in borders. On Orion-18, I have "unlocked" the diaphragme (originaly locked @f8, now @f4), quality is still great, but this lens is designed for large format, that's why I guess it avoids problems on borders.
Now, why they did not simply put a shape that would make a diaphragm hole limited (like it is often done on repro lenses and so), I don't know.

Btw, do Topogon's have such apperture limit too?


PostPosted: Sat May 06, 2017 4:51 am    Post subject: Reply with quote
I'm not sure of the reason they make some lens aperture diaphragms not fully open, I have around 5 like this, so it's not that rare.


PostPosted: Sat May 06, 2017 5:34 am    Post subject: Reply with quote
When I got my copy of that lens quite a long ago, I had the same question. I opened up the lens and removed the aperture just to test it.
The result was ugly vignetting at the 10-15% of the frame.

It is slow, but I like it though as it turns my Zorki-1 literally to a point-and-shoot camera with me worrying less for the focus.


PostPosted: Sat May 06, 2017 11:28 am    Post subject: Reply with quote
It is actually the simplest way when you want to fit the same diaphragm model to different lenses. On older Russian diaphragms it only needs a narrower or wider limiting cut beneath the diaphragm ring and no supplementary pieces.
Now, why the lens opening is made wider than needed and limited by the diaphragm - IMO that's the question! Some repro lenses are build similar. The fixed-opening Rodenstock Apo Rodagon-D 70mm F/4 have a mask limiting its opening, too (probably unmasked its opening is f/2.8 ).


BurstMox wrote:
Now, why they did not simply put a shape that would make a diaphragm hole limited (like it is often done on repro lenses and so), I don't know.

Probably a schizofrenic economy of metal dictated by some politruc of the Comunist era.


PostPosted: Sat May 06, 2017 4:43 pm    Post subject: Reply with quote
BurstMox wrote:
Btw, do Topogon's have such apperture limit too?


Yes, they have and quite massively though, limited to f10 (the ones I have)


PostPosted: Sat May 06, 2017 4:48 pm    Post subject: Reply with quote
BigMackCam wrote:
kds315* wrote:
You wonder that? Why would a lens designer do that for a lens with that intended purpose?
Possibly to achieve the design goals for such a lens? And to keep the cost down? Wink


Sure I wonder that. No harm in wondering - maybe someone has the definitive answer, and then I learn by asking Smile

Plenty of lenses were / are designed to provide acceptable performance outside the parameters of their primary design brief. A specialist lens that can also be used in a wide range of general situations is likely to be more attractive to potential buyers. Macro lenses spring to mind, where most will be used primarily at f/16 or smaller, but with many being perfectly usable much wider than this.

I'm not saying it would or should have been possible to have it open wider, nor that I'm in any way disappointed that it doesn't. I'm just curious as to why it was limited to f/6, and if it would have compromised the performance at f/6 - f/22 by allowing it to open up wider. Curiosity has driven a great deal of my learning thus far...

One thing I'm sure of - the designer had his reasons Smile


Curiosity is good Mike! Maybe it makes you even once read a lens design book (there are good ones out there), as these explain quite a bit, including your question! Rudolph Kingslake's famous book "Lens Design Fundamentals" would be a good one Wink

And yes, the designer indeed had reasons, one being the one Pierre mentioned, to avoid corner vignetting and achieve better corner performance.


PostPosted: Sat May 06, 2017 6:03 pm    Post subject: Reply with quote
kds315* wrote:
Curiosity is good Mike! Maybe it makes you even once read a lens design book (there are good ones out there), as these explain quite a bit, including your question! Rudolph Kingslake's famous book "Lens Design Fundamentals" would be a good one Wink

And yes, the designer indeed had reasons, one being the one Pierre mentioned, to avoid corner vignetting and achieve better corner performance.


Thanks, Klaus Smile I'll try to pick up a copy of that book... I have no books on lens design, and it's an area I have very little knowledge of. I'm aware of the configurations of some classic lens designs, but that's about it...


PostPosted: Sat May 06, 2017 6:27 pm    Post subject: Reply with quote
If I read this right, the diaphragm is acually inside the m39 mount so there is not much space for it

#1


#2


PostPosted: Sat May 06, 2017 7:30 pm    Post subject: Reply with quote
kansalliskala wrote:
If I read this right, the diaphragm is acually inside the m39 mount so there is not much space for it


nope, centered between the two symmetrical lens groups. It is indicated in image 1


PostPosted: Sun May 07, 2017 1:52 pm    Post subject: Reply with quote
AARGH! I hate these edit-quote buttons side by side Mad

Last edited by kansalliskala on Sun May 07, 2017 2:19 pm; edited 1 time in total


PostPosted: Sun May 07, 2017 2:15 pm    Post subject: Reply with quote
damn - I just edited my original answer away ..

orion diaphragm is about 27,96mm from film plane, flange to film plane is 28,8
so the whole rear part of lens is inside the mount like J-12
it's difficult/impossible to construct 4-22 aperture there?

kansalliskala wrote:
kansalliskala wrote:
so the whole rear part of the lens is inside the mount, like the J-12?


J-12 diaphragm seems to be 24,55mm +7,53mm=32,08mm from the film plane so it is 3mm in front of the mount

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