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  #41  
Old 23-09-2015, 09:48 AM
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Quote:
Originally Posted by Shiraz View Post
just a thought, but some optics designs use the aperture placement to help control aberrations in addition to limiting the amount of light (eg a MN uses the aperture placement to control coma). I have no idea if it makes sense with your lens, but it might be just be possible that it would work better at f2.8 if a full open aperture stop was placed where the original one was.
I was thinking about it.. and I will try it soon.
Especially because it seems this lens should behave a tad better at full aperture, according to another owner.
However, based on experience with 200m and 300mm Canon lens (similar optical design),
(http://www.iceinspace.com.au/forum/s...ht=canon+200mm), (and some russian lenses like Tair 11A (135mm) and Tair 33 in particular), the external aperture is way better.. Canons 200mm f2.8 and 300mm F4 have a lot of coma in corners (APS sensor.. and lenses were designed for full frame!) with internal iris, which is not there when external aperture is applied.
I think the reason why iris is where it is in Canon large telephoto lenses is simply because it would have been mechanically very complicated (for manual design.. with motors it is easier of course) to place it near front group.. so the current position is a compromise IMO.
However, smaller lenses are different (symetrical tessar-like designs). The external aperture doesn't work with them, it actually worsens the coma and/or CA.

I converted this lens to EOS by removing everything FD from it (including iris), so I can't really tell.. and I don't want to bother the only other person I know to own this lens to do the test for me (it is not really hard to cut the aperture.. but still.. )
In order to put iris back, I need to redesign the whole iris mechanism.. and this is not easy to do.

Last edited by bojan; 23-09-2015 at 03:15 PM.
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  #42  
Old 23-09-2015, 10:45 AM
Stefan Buda
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Thanks Bojan for the detailed feedback. It is good thing that we were able to rescue such a nice piece of optics.

Regarding aperture stops, please note that with camera lenses the iris is an integral part of the optical design and its location is important - especially for wide angle lenses. As Ray suggested, the placement of the aperture stop at the front is only valid for image points that are near the centre of the field. In the corners some of the aberrations sneak back because the aperture stop lo longer lines up with the entrance pupil as defined by the iris set for the same f-stop.
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  #43  
Old 03-10-2015, 09:01 AM
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Yesterday I put back the internal iris and tried again (at f/2.8).
There was no improvement at f2.8 internal iris stop compared to no iris at all.
Now I am working towards the full EOS modification (3D-printing some required parts from ABS).

Last edited by bojan; 03-10-2015 at 09:44 AM.
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  #44  
Old 03-10-2015, 12:15 PM
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What is the shape of the internal iris stopped down to f/3.5 Bojan?

My tokina 300/2.8 gives fairly bloated stars at f/2.8, but tiny pinpoint stars at f/4. The only problem at f/4 due to the rough shaped internal iris I get slightly odd star shapes with some weird aberrations. I'm testing some home made aperture masks to see where best performance lies then I will get something 3D printed.

Unfortunately my lens goes directly from f/2.8 to f/4 with the internal iris. I suspect f/3.5 will give me the performance I'm after.
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  #45  
Old 03-10-2015, 12:20 PM
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Not sure because I still have to design and build mechanism to control iris now that the m42-EOS adapter is in place.
Try with external aperture.. it is 5 minutes job to cut it from black paper or folder plastic.
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  #46  
Old 03-10-2015, 02:02 PM
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I've been doing tests. Just trying to nail down a method of cutting a perfect circle.
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  #47  
Old 03-10-2015, 02:12 PM
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I am using one of those:
http://www.ebay.com.au/itm/30mm-to-1...item3ab155cf3a
Also, round aperture doesn't produce diffraction spikes.
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  #48  
Old 03-10-2015, 02:52 PM
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That brilliant. I've been finding I can get star images better than using the internal iris but I get flares and spikes due to the holes being slightly out of round or not perfectly central.

I'll buy one of those cutters and see how I go then.
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  #49  
Old 03-10-2015, 05:38 PM
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Well, this $10000 lens is indeed really good.

But I bought a 200 f/2.8L for only $500 (used) but its image quality is excellent. Pinpoint stars to the corners at full aperture.
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  #50  
Old 15-10-2015, 11:05 AM
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Quote:
Originally Posted by AlexN View Post
What is the shape of the internal iris stopped down to f/3.5 Bojan?

My tokina 300/2.8 gives fairly bloated stars at f/2.8, but tiny pinpoint stars at f/4. The only problem at f/4 due to the rough shaped internal iris I get slightly odd star shapes with some weird aberrations. I'm testing some home made aperture masks to see where best performance lies then I will get something 3D printed.

Unfortunately my lens goes directly from f/2.8 to f/4 with the internal iris. I suspect f/3.5 will give me the performance I'm after.
Hi Alex,
The shape of internal iris at f/3.5 is not so good.. f/4 is OK.
External aperture behaves much better, as far as diffraction is concerned.
I will do comparison between internal and external and post the results as soon as possible.
Attached Thumbnails
Click for full-size image (F_3.5.jpg)
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Click for full-size image (F4.jpg)
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  #51  
Old 29-12-2016, 01:59 PM
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With some delay, I thought it may be useful (for archival purpose) to add images of my conversion of this lens to EOS.
Attached Thumbnails
Click for full-size image (Parts_printed.jpg)
187.1 KB22 views
Click for full-size image (sub_assembly.jpg)
179.9 KB20 views
Click for full-size image (Iris.jpg)
201.2 KB20 views
Click for full-size image (Iris_holder.jpg)
195.4 KB18 views
Click for full-size image (EOS adapter.jpg)
170.7 KB16 views
Click for full-size image (Cog.jpg)
163.9 KB15 views
Click for full-size image (Assembled.jpg)
174.3 KB18 views
Click for full-size image (Threaded Ring.jpg)
182.1 KB15 views

Last edited by bojan; 29-12-2016 at 02:24 PM.
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