The introduction

The longer a lens you have, the further away you can be from a subject and still get a frame- filling image. If you want to take pictures of little birds, you need a long lens. As with many things, more is better... in long lenses, longer is better.

In the 1970s Vivitar produced a line of lenses using an interchangeable lens mount system, the TX mount. This system allowed their lenses to be used with camera bodies from many different manufacturers simply by using a relatively inexpensive mount converter. This mount converter would sit between the lens and the camera body, providing the proper spacing and aperture linkages. There have been a few different interchangeable lens mount systems produced: T4, TX, Adaptall, Adaptall-2, etc.

My Canon Rebel XT kit came with the EF-S 18-55mm f/3.5-5.6. Not really worthwhile for longer distance photography, so before a major road-trip out through the western states I went and got myself a EF 70-300mm f4.5-5.6 DO IS USM.

The lens is fun and I use it all the time. But longer is better, right? I wanted more reach, more ability to capture images of tiny little birds or other such things.

Longer is also more expensive. A modern 400mm f5.6 made for my camera system goes for $1000-$2000. This puts the long-long lenses out of my reach financially.

Like most of my lens-related pages here, my solution to get the product I want, without cashing out from the bank, is to do some hardware hacking. There are plenty of nice manual 400mm f5.6 lenses made for older camera systems which are now available for relatively cheap.

I'm not going to have autofocus or a cool-big-white-iconic Canon lens, but I'm also not going to have a big-hole-in-my-bank-account to deal with. I think the two even out for now.

Tools:
	Drill and bits.
	Center punch.
	Small screwdriver set.
	Swiss Army pocket knife.

Parts:
	A functioning 400mm lens:	Vivitar TX-mount 400mm f5.6 (with Canon FD mount link) on Ebay, ~$80.
	A new EOS mount:		M42 EOS adapter on Ebay, $8.
	Epoxy putty:			Random-Brand at Random-Hardware-place, ~$4.

Since I was starting with a lens designed for use on many different camera bodies, all I needed to do was modify the mount adapter. As I had already converted a lens from Canon FD mount to their EF mount, I figured it wouldn't be all that hard.

First remove the mount adapter from the lens. I covered the lens with a folded sheet of paper to help limit the amount of dust or metal shavings which might fall into the lens. Take apart the mount adapter starting from the camera side, but leave any lens-side surfaces intact.

What you should end up with is a partial mount converter which can still be attached to the lens without difficulty. Among those other parts you didn't throw away is the blackened throat of the mount converter.

As with my last lens conversion, it almost seemed the parts were designed with the hack in mind. If you invert the mount throat, you'll find it is able to sit flush against the base of your partial mount converter. A protruding pin on the mount will fit into a slot in the throat. By rotating the throat, it will get locked in place as the pin pushes against the sides of the slot. Prepare a small amount of epoxy putty and place inside the lens mount to secure our two parts together.

The TX mount on the lens includes a small number of insertion points for pins from the mount adapter. These insertion points allow any auto-aperture control by the camera body. The lens mount adapter had a pin sticking out the back, but we got rid of that. We're not going to be using auto-aperture with the lens, so we have no problem.

The TX mount on the lens has a pin which disengages manual aperture control when pressed. Since I'll be using manual aperture control, I need to make sure this pin does not get pressed when my new mount adapter is in use. The lens-side face has a notch in place which matches the pin. The inverted throat and epoxy putty have filled in behind this notch. I used a center punch and 1/4 in drill bit to clear out this space.

Theoretically an FD lens mount needs to have 2mm removed from the back of the mount to allow use on a EF camera system. Realistically what we have to do is make sure our new mount adapter is the right length to provide proper focus. At this point I attached the modified mount adapter to the lens and set the lens focus to its minnimum, 13 ft. I found a target 13 ft away and placed the protruding throat of the mount converter against the mirror-box of my camera.

The target was in focus. I confirmed the mirror would not protrude past the mirror-box and took a few pictures. This makes it an EFs instead of EF lens mount adapter. Now I just had to add the new EF mount to the mount adapter.

I attached the new EF mount to my camera and pressed the mount adapter against the camera's mirror-box. There was a gap of a few millimeters between the two pieces. You could measure this gap and then seek out a proper thickness spacer. I decided to use more epoxy-putty instead.

I mixed up a thin roll of the putty and placed it on the camera-side of the mount adapter. I pressed the mount adapter throat against the camera mirror-box. This flattened out the epoxy-putty against the new EF mount, making the proper thickness spacer I needed. The smooth metal of the new EF mount prevented the epoxy from bonding. After the spacer had mostly hardened, I used a pocket knife to trim off the excess around the edges.

Prepare a small amount of the epoxy-putty. Place the new EF mount against the mount adapter. If you want your lens to be oriented some specific way, then you need to take care how the two pieces are rotated gainst each other. Press the epoxy-putty into the gap between the new EF mount and the mount adapter throat. Before the putty hardens, make sure the two pieces are centered relative to each other.

The mount pieces will not be stable at this point. Prepare more epoxy-putty and filly in the gaps between the two pieces. Around the mount adapter throat, I had to later remove some of the putty so the EF mount would fit into the camera body without hitting the electronic connections in the camera.

I have a EFs-mount adapter for TX system lenses. In the future, I can get other TX lenses and know I'll be able to use them with my current camera system. I also have a functional 400mm lens to use as I see fit. On my camera, the lens has an effective focal length of 640mm due to the smaller than standard image chip size of my camera.

Tiny little birds, here I come. Err... well, yeah, that sounds somewhat strange. And really, 400mm isn't that much longer than 300mm. The above image is from my 300mm lens. The red box is an image from the 400mm scaled to fit appropriately. Eh, oh well. I'll probably find some use for the lens.

Well... I can't find any. I'm pretty sure others have done mount conversions on telephoto lenses, but my searches are coming up dry.