I recently converted my FD 85 1.2L to EOS mount. It sits resting quietly in its soft protective case waiting to be used. I have adjusted it for infinity focus and done some initial tests on a borrowed crop body but have not found the time to shave the mirror on my 5D so that I can start actually using the lens.
I elected to fix the rear element in position (I confirmed the 10.40 mm positioning) rather than use an adjustable positioning method. Although I think it is a nice idea to be able to adjust the lens for use on an unshaven 5D; I decided not to employ this method as IMHO there are a couple of caveats to this method that I wanted to avoid.
Firstly, by virtue of the design of a thread (play between meshed helical wedges) there is a possibility of the rear element faces sitting somewhere off the ideal (designed) 90 degree axis to the light path. The degree of free play and consequent misalignment is reduced when using a fine pitch thread but the risk increases with threads of greater pitch and shorter length. The original threads on the outside of the rear element are kept \"loaded\" when the element is threaded all the way to the bottom of the original holder. The element faces are positioned exactly perpendicular to the light path in this position because the flat face of the holder bottom is forced against the flat face of the outer diameter of the element threads. Without a similar axial locking method being employed for an additional set of positioning threads, positioning accuracy could be compromised.
Secondly, there is the question of securing the rear element in the selected position. I have not read any mention here of how the rear element is prevented from \"working loose\" when an adjustable rear element mount is employed. As there is no room for a thin lock nut or pin I would assume that some type of removable thread lock adhesive is employed. This method would introduce the risk of getting adhesive on the glass and possibly getting the rear element \"stuck\" in one position or the other.
Something else of note that should be mentioned about this particular conversion is the clearance between the rear element holder and the components within the camera body mirror box. Jim Buchanan gave me a \"heads up\" on the potential clearance issue but I decided to quantify it before I tackled the conversion.
When the lens is mounted on the camera the outside edge of the rear element extends all the way to the focus screen/superimposing plate retention latch plate (not sure if this is the correct name). By virtue of this the supporting material for the rear element is limited to a depth that does not interfere with this plate. It was difficult to get an exact measurement of this depth but it is VERY close to the 2mm that the rear element must extend into the mirror box. Furthermore the mouth of the mirror box tapers inwards and reduces the available width of any supporting mount for the rear element. The combination of these factors severely limits the width and depth of any fabricated rear element holder. I found that any material extending beyond approximately 1.5mm of the diametrical edge of the rear element interfered with the mirror box on my 5D.
Sep 11, 2009 at 12:06 PM
Previous versions of pengland's message #7518224 « FD 85L f/1.2: a real gem »
