Date sent:        Wed, 31 Jul 1996 22:45:11 +0000
From:             Willem-Jan Markerink 
Subject:          A 8mm EOS fisheye was born! (long)
To:               eos@avocado.pc.Helsinki.FI
Priority:         normal
Send reply to:    eos@avocado.pc.Helsinki.FI

Hi gang,

Here is the hack report of mounting an Sigma 8mm/f4.0 in Pentax-K 
mount to the EOS system.


Ingredients:
(1) cheap Sigma 8mm/f4.0 in Pentax-K mount
(2) metal lens mount from an EOS 35-70/3.5-4.5 
(3) metal surgery
(4) several dabs of silicone rubber 'glue'
(5) metal surgery

Note 1) 
Background: a German mailorder company offered the entire manual
focus range of Sigma for dump prices; Sigma Germany skipped this
manual line apparently, and sold their entire stock. I just sat
there, waiting to give in to this last stage of wide angle
addiction....8-)) 

This 8mm has only been available in manual focus mounts, and only in
Nikon, Canon FD, Pentax-K and Contax/Yashica. I would have preferred
a Nikon mount version, but those were already sold out. Only FD and
Pentax-K were left, and knowing that FD wouldn't give me enough play
between the mount systems, I went for the Pentax-K version. This
mount results in 1.5mm play, enough to fiddle around towards a
workable solution. 

Next critical point was the protruding rear lens element; David
Ruether (aka Bob Neuman on Usenet) owns a Sigma 8mm in Nikon mount,
and he measured a rear protrusion of 10mm. Given the fact that the
Nikon register is 46.00mm, and the EOS register 44.00mm, this would
result in an EOS protrusion of 8mm. Bad luck, since my 16mm/f2.8
Russian fisheye in EOS mount clears the mirror by less than 1mm, and
this one protrudes 6.5mm. So 7mm is the maximum protrusion allowed
on an EOS-1 (other models can and do vary due to differently sized
mirrors!). Darn, I thought, I need either mirror lockup, or a
pellicle mirror camera (well, any reason to buy one is valid,
right?....;-)). Upon receiving my Sigma, I happily measured only 7mm
protrusion on the Pentax mount, less 1.5mm for the EOS adaptation
(thanks Roger!) makes a convenient 5.5mm protrusion on the EOS
mount. Another hurdle taken!

Note 2)
I once bought a scrap EOS 35-70 for a few bucks, and I used this
metal lens ring to adapt a Minolta MD micro tube with RMS front
thread to EOS. I dismounted this ring, to see if the mount of the
Sigma would fit inside this EOS ring. Yep, it did, its axial play was
less than 1mm....good start. So I needed a second ring, preferable
from an identical scrap lens. After an intensive worldwide search, a
Singapore lens ring was swiftly exported to the Netherlands (thanks
Kelvin!....:-)). 

Note 3) 
The ring was 2.5mm thick, and I needed only 1.5mm between Pentax and
EOS mount, so I had a friend machine it down to 1.3mm, to account for
the silicone rubber inbetween.

More metal/plastic surgery: the removal of the aperture lever on the 
lens. It sticks out behind the lens, and is protected by a piece of 
plastic. After removing the plastic rear cover, I simply cutted both 
plastic tab and steel aperture lever with a pair of nippers (sp?). 
Voila, a smooth, unobstructed rear lens surface!....:-))

Note 4)
Read the label on the tube carefully. I thought I had a tube of
generic silicone rubber, but I ended up with Solvacryl, a
never-completely-hardening nasty sticky substance. A PITA to work
with, and a nightmare to remove. Attempt 2, with the right stuff, 
worked better, but infinity focus was off-spec, I needed more 
thickness inbetween the mounts. After some experiments, I cut several 
pieces of 0.15mm thick plastic, and stuffed it between the mounts, 
again glued with silicone rubber. Focus is now dead on, amazing what 
0.15mm extraction can do for focus on a fisheye!

Note 5)
After mounting the lens on my EOS-1, and firing the first shot, the
camera locked up, shouting 'bc'.  After lots of behind the ear
scratching, and trying to see if the metal of the Pentax mount
shortened the electronic contacts on my camera, I saw my stupid
mistake....the EOS lens ring still had its 'I am an EOS lens' shape
on the mount, so the camera expected EOS lens communications, which
it didn't get, and therefore locked up. So, I had to remove this
piece of metal, an advice which I gave to another listmember some
months earlier....8-)) 
Better said than done....how on earth could I get so close to the 
mount with a metal saw?? I couldn't, so I went shopping for some 
mini metal-tools, a cutting/grinding wheel with a diameter of 22mm [I 
really wouldn't know how to do this differently....whom did I gave 
that advice to, some months ago? How did you do this??....8-))].
Anyway, this last stage of metal surgery succeeded, and I am now the 
lucky owner of a 8mm EOS lens! 
This full 180 degrees angle of view is an amazing 
perspective....point the camera down, film plane horizontal, and you 
still have the horizon in the picture....same if you point it 
upwards....all sky, *with* a full 360 degees horizon! I love it!
[Adam, I am sure you would like this perspective too!]

Filter note: this lens has one of the strangest filter solution I 
have seen sofar; after unlocking, the front half of the lens 
dismounts, and you can access the screw filter (M22.5x0.5mm).
To see how image would be degraded without filter, I removed it. Not
much difference, something I had seen before on my Russian 16mm.
Will have to run some tests to be sure (darn is it hard to see
whether stuff is in focus or not....I vote for an AF EOS circular
fisheye!). Glad to see it doesn't make much difference, I can now
design other filter solutions without considering glass thickness. I
have IR filters and a enhancement filter waiting to be cut (also for
the Horizon filters).

--
Bye,

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     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
                     _/_/_/  


The difference 
between men and boys
is the price of their toys


[note: 'a-one' & 'en-el'!]

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl