Converted 85mm f/1.2 breech to EOS (Canon30D)

Started Aug 23, 2008 | Discussions thread
OP jrmcvey Forum Member • Posts: 62
Re: 85mm f/1.2 convert and Bravo! Question...

Thanks to those that responded to my post. Here are some additional comments:

I've been quite happy with the optical quality so far including color balance and saturation. Of course I'm using an APS-C sensor, so I'm using the "sweet spot" of the lens. The samples I posted were shot wide open at f/1.2 with no post-processing so you can judge for yourself.

As mentioned by another poster, non-computerized lenses such as this should be used in Av or M modes. My 30D seems to meter quite accurately in Av mode with manual lenses.

Any wide-open fast aperture lens will challenge the camera to have a fast enough shutter speed to prevent over exposure outdoors on a bright day. The "sunny 16" rule suggests this lens at f/1.2 would need a shutter speed of something like 1/16000 sec to correctly expose at ISO 100. Canon's new EOS 85mm f/1.2L II is no different in this regard. The Canon 50mm f/1.4 needs 1/12800 sec!

Converting the lens did not affect the MFD or infinity focus. The focus distance scale is still accurate. On a FF camera the DOF scale would also still be accurate and the overall illumination (MFD) is not materially changed.

The lens conversion was quite a challenge and it never goes as easily as hoped. I always select the older "breech" style lenses to convert rather than the (I believe) more complicated New-FD style with the internally rotating mount. Each lens seems to have a unique structure to it. In fact I have learned that Canon has several diameters of the basic silver breech locking ring.

I begin the conversion by removing the FD mount and measuring it's thickness. In this case the FD mount was 2.5mm thick. Remember the EOS mount must be 2mm thinner to preserve infinity focus. If it was possible to get a strong EOS lens mount (ring) that was only 0.5mm thick then it could be attached in place of the FD mount and the job would be mostly complete. But a strong EOS mount is at least 1mm thick, so then you are forced to remove metal from the lens barrel itself to help make up that 2mm back-focus reduction. This requires disassembly of the helicoids and you must make careful measurements and documentation so that reassembly goes correctly.

If this was not enough, the f/stop is dependent on linkage contained in the FD mount, so once that is removed, a bit of ingenuity is required to relink the f/stop ring to the iris mechanism.

About 8 hours of work for the mechanically inclined hobby-machinist and you're done. Just don't let the little f/stop click-stop balls roll under the fridge!


 jrmcvey's gear list:jrmcvey's gear list
Panasonic Lumix DMC-G6 Panasonic Lumix DC-G9 Panasonic Lumix G Vario 7-14mm F4 ASPH Panasonic Lumix G 20mm F1.7 ASPH Panasonic Lumix G Vario 45-200mm F4-5.6 OIS +6 more
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow