Canon 200mm f2.8 FD->EOS lens conversion

[bojan]

After a very successful conversion of Canon 50mm f1.4 FD lens (optically way better that nifty-fifty), my appetite opened up.. and I decided to have a go at something bigger..
After some deliberating about image quality and affordability of various lenses, I decided it will be Canon 200mm f2.8 FD lens.
So I searched and found on ebay one earlier model, 5-element-version.The reports and images other people obtained with this lens were impressive..

The conversion procedure was standard, I removed FD breach lock (3 screws) and replaced it with one M42->EOS adapter (there was some machining needed to be done on this adapter (if anyone is interested PM me and I will send the drawings).
Again, the infinity focus was easy to adjust by loosing and moving the focus stop (accessible under the rubber grip).

And of course, now that everything is ready for star test, it is cloudy !!!

I will post the details of the modification together with test images later when weather permits..

[bojan]

Tonight it was a perfect night for optical test of my new glass.
Attached are the 200% crops from one 5 sec exposure at full aperture, f/2.8.

For comparison, I also attached 200% crop from corner taken with Tair-11A (the lens I was very proud of).
While in the centre both of them are almost equally sharp (that means - sharp), Tair shows significant coma in corners.. also it is worth mentioning that the Tair shot was taken at f/5.6...
Canon 200mm FD S.S.C. is significantly better in corners, even at full aperture
Total cost: around $300

The next step is addition of lever to control the diaphragm.. (I expect a significant improvement at 1 or two stops down) and I will have a very good lens for narrow wide-fields

[DavidU (Dave)]

Fabulous job there mate.You must be happy. $300 !!!!

[bojan]

It is actually less than that.. I obtained the lens for 190 US$, but the postage was a bit expensive... and $15 for M42-EOS adapter (ebay).
Machining was done by a friend (yes, he is still my friend, despite all those jobs done for me in the past.. and there is no hope I will stop doing those things in the foreseeable future )
Yes, I am pretty happy with this lens. It would be interesting to compare it with some specimens from "L" series.. just to see the real cost of eventually better performance.
Of course, here I have manual lens only, which would be pretty hard to use for terrestrial photography (no AF, no IS etc).
Also, because the mod is not reversible, I am not sure if I could sell it again.. not that I intend to, though .

[bojan]

Well, now I am a bit disappointed with the whole exercise around this 200mm FD lens..
After mounting the lever to control the diaphragm, I expected the red halo around the overexposed stars (obvious on frame above, cropped from corner) would go away.. but it did not.
Something like this was never reported for this lens, or I was not aware of this. Interestingly, similar halo I observed on images taken with 50mm f/1.4 at full aperture, but the effect disappeared completely at f/4 - f/5.6 (OK, I am comparing apples and oranges here, I know..)
It seems this particular lens is a dud.. with too much chromatic aberration
Well.. such is life.

[bojan]

I simply could not accept the idea that I was so wrong about this lens.. and that those couple hundreds of bucks were thrown through the window.
So I decided to try another trick:
Instead of using it's internal diaphragm, I placed in front of the lens the black piece of paper with round opening, so the effective f number was 5.6, the same as last night and gave it another go.

The result was astonishing: it is hard to believe that the same lens can perform so drastically better.

The crop from upper right corner is attached...
I am a happy man after all

[DavidU (Dave)]

That's good news. I was disappointed for you however the aperture mask showed up well.
The question remains, what is wrong with the internal iris?

[bojan]

Well, if I were optical engineer/designer, I would say it is a design compromise made somewhere along the process.. Internal iris is obviously not at the optimal place within the system as far as optical distortions are concerned.. Maybe they did not want it to be too far away from camera end, to keep the controls from camera simpler (it is quite complicated mechanical system already..), and/or to keep the smaller size of iris and maintain the lower cost.

http://www.mir.com.my/rb/photography...nses/200mm.htm

Interestingly, my Tair11-A has iris placed immediately after front lens group, but it is 2x larger in diameter, and it is totally manual (that gave me idea to try something similar, by the way), see here: http://www.zenitcamera.com/archive/lenses/tair-11.html

It is quite obvious that the front group of lenses creates the problem. With external diaphragm, only the central area of lens is used, so CA is much more tolerable.
With internal diaphragm, it seems the peripheral parts of the front lenses are engaged, and hence exaggerated CA.

Whatever was the cause of this hassle (CA), the problem is solved, it seems :-)