The introduction

The purpose of a camera lens is to collect light into a useful image on a piece of film or an image sensor. The more light the lens can capture, the faster the shutter speed can be and the more the photographer can freeze the motion of events. Lens which let in a lot of light are called "fast" because of this, while "slow" lenses let in smaller amounts of light. How much light a lens is able to collect has to do with the relative aperture diameter, the f-number of the lens. The lower f-number a lens has, the more light it can bring in, and the faster that lens is.

The basic lens which is included in new SLR kit deals will generally be a very basic lens with no outstanding characteristics. For the Canon Rebel XT kit, the included lens is the EF-S 18-55mm f/3.5-5.6. This has a maximum aperture of f3.5 to f5.6 depending on the zoom setting. This is plenty fast of a lens for hand-held shooting on a sunny day or in other brightly lit environments. You will need to use a tripod or a flash to get shake-free photos in places which are not so brightly lit. For those who want to do available-light photography in less than ideal lighting situations, the faster the lens the better.

I do most of my shooting with a Canon EOS series digital SLR able to use EF-S lenses. The fastest lenses currently being made by Canon are two f1.2 lenses (50mm and 85mm) in their "L" series. These lenses routinely sell new for between one and two thousand $US. I enjoy photography and wanted a very fast lens for night time shooting, but I don't have that kind of spare change sitting around for one of the new (or even used) superfast lenses made for my camera system.

Canon made a nice 55mm f1.2 lens for their older manual cameras. These lenses can be had for a pittance compared to the new lenses. The lens has a FD mount, which has a lens registration distance of 42mm. My camera has an EF mount, which has a lens registration distance of 44mm. The old lens would have to be inside my camera for it to focus properly.

Buy an expensive new lens or get an old lens which won't work. There has got to be a easier way.

You got it, there's an easier way. Get the old lens and scrape 2mm off the backside of it. Maybe it's not an easier way, but it is a more fun way.

Tools:
	A dremel tool and various bits.
	A dremel-enabled drill-press.
	A center punch.
	A good small screwdriver set, with awl.
	Soldering iron and solder.
	Epoxy, various flavors.
	Ear/eye/respiratory protection.

Parts:
	A functioning FD f1.2 lens:			Canon FD 55mm f1.2 SSC, $135.50.
	A broken EF lens:				Quantarray Tech-10 28-200mm, $? (free for me).
	Focussing rail.					from my grandfather's camera kit, $0.
	Rubber pads.					from RadioShack, <$5.
	Thin sheet steel and aluminum.			from hardware store, cheap.
	3 angle brackets.				from hardware store, cheap.
	3 wide-headed wood screws.			from hardware store, cheap.
	1 2" 1/4-20 stove bolt.				from hardware store, cheap.
	Several 1/4-20 nuts.				from hardware store, cheap.
	1 1/4-20 wingnut.				from hardware store, cheap.
	Short wood 2x4					extra piece from an Ikea chair set, $0.
	Focus confirmation chip				Ebay, ~$70.

You've got two lenses. Rip their mounts off. It helps to be polite about it.

The FD mount on the Canon is held in place by three screws. The silver ring is rotated to lock the lens to its intended camera body. This needs to go. The locking ring will need to be rotated until each screw in turn is accessible through the holes. The locking ring has a spring-driven lock, so you'll need one of those screwdrivers to disengage the lock so you can rotate the locking ring. You'll need to keep those screws. The locking ring is threaded onto the mount, so you can remove it with a few more rotations.

The broken EF lens will generally be easy to strip. On the Quantarray Tech-10, there were four screws holding the mount to the lens. the back-element lens collar is held to the mount by three screws. The contact assembly is held to the mount by two small screws.

On this lens, the contacts of the contact assembly are held in place by pressure from the back-element lens collar. It's not elegant, but I didn't build it. Whatever lens you're using, you'll have to figure out how to maintain the contact assembly without it getting in the way of the rear lens element from the f1.2 lens. There's plenty of room, so be creative.

Continue with the lens breakdown and set aside the section of lens exterior next to the mount. This is the part with a little colored spot to indicate how to attach the lens to the camera body. The rest of the broken EF lens isn't needed for this build. Keep the parts for a later project.

The mount you removed from the Canon lens was responsible for mechanically linking the external aperture selection ring to an internal aperture control ring. Though the lens attracted me because of how wide open it could be, I did't want it always to be wide open. We need some way of coupling the outer aperture ring to the inner aperture ring.

At least with this Canon lens, the solution is almost already present. There is a small piece of metal along the top surface of the lens wall which acts as a stop for the inner aperture ring. The inner aperture ring has two upright tines which are kept pressed against this stop by a spring on the mount we already removed. You could epoxy the tines to the stop, or as I did, you could fasion a new stop which also holds the tines in place. Sheet aluminum proved to be too flexible for cutting the thin shape of the aperture link, so I ended up using steel instead. With a little work you can put holes through your new aperture link into which the screws holding the stop in place can be tightened.

There is one further control lever in the lens. When it is engaged, the iris opens completely. It is used by the old canon camera body to hold the iris open until a picture is taken, when the lever is disingaged and the iris closes down to the selected aperture for the photo. Unless you want to install a solenoid into the available lens space and some control ciruitry which speaks the Canon lens language, it will just have to be ignored. Without this, we will have to focus before stopping down the lens to take a picture. I don't think this is a terrible loss.

Oh yes, I mentioned scraping 2mm off the f1.2 lens? It turns out that after removing the mount componants there was no further need to trim the lens. I instead had the opposite problem. I needed to add a spacer between the lens and the new mount if I wanted the lens to focus properly at the distances indicated on the lens exterior. To figure out how much of a spacer was required, I needed a new tool for adjusting the spacing between the camera and the lens while I took test photos.

The focussing rail is attached to the wood block by a tripod screw made from the stove bolt with one wingnut and several nuts tighened up to the bolt's head. The nuts act as a handle on the bolt, which is then inserted through a hole at one end of the wood piece and cut off with a cutting wheel on the dremel tool. This allows the focussing rail to be reversibly, but securely, attached to the alignment tool. To use the adjustment wheels of the focussing rail without destroying my fingers, I needed to carve out a small notch in the wood under each.

The lens platform is constructed at the opposite end of the wood piece. Ideally the tool should be able to move the lens from being in contact with the camera to several inches away. The maximum distance will depend on what the travel of your focussing rail is. I cut a rectangle of aluminum matching the combined size of the three angle brackets and lined the edges with rubber pads. This way the lens would not be able to roll or slide once placed. The present version has this plate secured with a plastic tiedown, but it will be easier to use the tool when I epoxy the plate to the brackets. The brackets are attached to the wood using screws with nice wide heads. Centering the lens vertically requires loosening these screws, adjusting the brackets, and tightening the screws again. A future version may have some sort of mechanism with a simple adjustment knob.

At this point I discovered that the optimal distance between the lens mount attached to the camera and the lens was 0mm. Designing and building the distance alignment tool could have been bypassed and would have been if I had bothered to check infinity focus with the lens held against the new mount. Oh well, I have a good tool for future lens conversion projects.

You could either epoxy the mount to the lens body, or you could be ambitious and drill some holes to attach it using the original mount screws like I did. I needed to transfer their placement from the original lens mount to the new lens mount. The holes are not equally spaced around the mount. I first made a cardboard jig which had a hole for the mount's extensions, so I could attach the mount flush to the cardboard's surface. I used an awl to punch through the mounting holes (here highlighted in red), transferring the pattern to the jig. I also traced the outline of the mount onto the cardboard. I attached the new lens mount to the jig, centered it, rotated it, and tapped small holes with my center punch.

I put the mount on the drill press and drilled the three holes just wide enough for the mounting screws. Being careful paid off and I was dead-on accurate. The mount was now attached to the f1.2 lens. This revealed to me the need to countersink the screw heads. Simple, I think, and slowly mill out a wider hole centered on the screw holes. Two worked as planned. The third broke through before the hole was deep enough for the screw head. The underside of the new lens mount had a space milled out for its original lens componants to fit. This gap happened to be under one of my new holes, so the material was too thin for the holes I was making. To avoid having a poorly secured lens, I simply filled in this gap (and the holes) with a kevlar-reinforced metal epoxy. Once this hardened, I reformed the hole and the mount holds securely to the lens.

The next step was to attach the broken EF lens's mount-end exterior shell to the new mount. This took some trimming with the dremel, comparing to the space available, then more trimming. The, now much reduced, plastic ring was then glued to the lens mount with a thin layer of a quick-set epoxy. I made sure to align the mount indicator dots before letting the epoxy harden. There is a gap in the pastic ring where a switch was placed. I plan on filling this gap, but the lack of a troublesome light leak suggests it is not required.

A quick dremel-polish of the new mount made sure there were no burrs to prevent smooth lens mounting.

I ordered one of the canon focus confirmation chips you can find on ebay. I've heard reports that these chips are somewhat fragile and my personal examination of one corroberates this. Having the chip in a more protected location within the lens body would help to prevent this from becoming an issue during use. The chip comes in a little piece of plastic to hold the contacts in their appropriate locations. The two contacts on the lowered end of the piece are not required and can be trimmed off with a sharp pair of wire cutters. These contacts have no electrical connections with the chip. The the five contacts at the end of the raised portion away from the wide end are the connections we need to make with a lens contact assembly.

My first attempt was to try and surface mount the chip to the backside of the contact assembly. This didn't work out well. I lifted a few of the contacts from the chip while trying to solder the connections. I then found there was not enough room in the lens body for the chip mounted in this way. The aperture link happens to be exactly where thie chip wants to be when mounted like this.

I used the lens for a few weeks without the focus confirmation feature before deciding it really was worth ordering another chip and trying again. The succesful attempt started by trimming the backside of the contact assembly. This was in part to clean off all the garbage from my first attempt and partly to give me more room to attach wires. The wires used to connect the chip and the contact assembly were a short section of rainbow-colored ribbon cable. There was plenty of room (after trimming the contact assembly) to route the ribbon cable around the lens interior to where there was enough room to hotglue the chip in place.

I've got a tool for helping me with later lens conversion projects or lens builds. I've got a lens which lets me take handheld photos in areas too dark for normal work. The lens even maintains aperture control and the focus distance guide is accurate (as far as my testing can show) from two feet out past infinity.

The focus confirmation chip is almost required to make this lens usable wide-open. The very short depth-of-field makes it near impossible to focus effectively without a focussing aid. I suspect you could go with a microprism focus screen instead, but I didn't feel up to modifying my camera body for this.

Others have done this sort of conversion, with varying technique and results. Here's what I've found.

• http://www.flickr.com/photos/skink74/sets/72157594179626948/
• http://www.flickr.com/photos/rodolfonovak/229655951/
• http://www.rit.edu/~andpph/text-eos-to-fd-adapter.html