Reverse engineering the EF mount

 

Today I did a very basic reverse engineering of the Canon EF lens mount. There are many ways I can study and analyse the results from my optical rig, but I think recording the image circle on a digital sensor allows me to get a more "real" perspective on what the final image quality of my optical designs are like. To make life easier, I have decided to make use of the imaging equipment I already own. 

The camera I will use is my own Canon 5D Mark II, which features a full frame 35mm sensor packaged in a DSLR format. Using an SLR is not my default choice, as it is severely limited by the flange distance from the lens mount to the focal plane. However, using a camera such as this does provide me with a good place to start learning about designing your own optics, as I am able to attain information on the construction of many 35mm format lenses built to fit SLRs. It also allows me to compare any results to professional lenses that I already own (I have professional grade glass from 24-200mm covered at f/2,8).

To fix my 5D to my lens rig however I will need to attain a mount. For a single installation I could simply go on ebay and buy a cheap extension tube and simply unscrew the mount and use that. However, I anticipate making prototype lenses in the future that will need to be fitted to a DSLR independent of my rig. So for that reason I thought it was necessary to reverse engineer Canon EF mount. I searched online expecting it to have been done before, but after poking around on google all I could find were requests for technical drawings on forums. Better do it myself then! Luckily, I have plenty of EF mounts to have a good look at.

Because I moved abroad with only one suitcase, I annoyingly don't have my calipers to hand, so dimensioning the mount was done with a ruler..... It's not as accurate as I like, but I can see what sort of tolerances the lens mount has so I think I'll be able to get away with my rough measurements. If not, a bit of tweaking will suffice. I measured up the mount and built a replica in SolidWorks (seen rendered at the top of the page). The mount is very simple. It is a bayonet mount with a location hole for a sprung loaded pin on the body to hold the lens in place preventing it from rotating. It has 3 bayonet "wings" which drop into the body and slide into the mount through about a quarter turn.The construction of the mount is obvious, a CNC lathe could make this in a matter of minutes from a blank aluminium tube. When i looked closer at one of my 3rd party mounts (not official Canon), I noticed the tell tale marks of CNC lathe tools which had been used to mill out the bayonet.

Now for all those people out there googling "Canon EF mount dimensions", here they are. Unless Canon ring me up and tell me to take it down (not sure why they would, anyone could pick up a lens and measure the mount), it's here to stay. The dimensions are a guide, you might find they need tweaking to make your mount fit. I intend to export my SolidWork file as a STEP and run it directly onto my universities CNC lathe. Once my first one is made and verified to fit, I will most likley make a run to use up my aluminium blank, but non of this will happen until I'm back in the UK, so for now I am going to continue designing my rig and premiere optics.