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An FD to EF Converter
Adapting the older FD series lenses to work on the newer EF series cameras is not as simple as just changing the mount. As was mentioned in my lens sections, the FD series uses a mount and linkage which is purely mechanical, while the EF series uses a linkage which is purely electronic. Even so, lenses are lenses, and it would seem that there should be a way to mount one style of lens on a different style of camera. To a certain extent, you can, but there are some trade offs.
I should first mention that, though FD lenses can be made to function on EF cameras, there is no way to get EF lenses to work on FD cameras. There was never even an attempt made to produce a converter for this purpose. One main reason for this is market demand. There were a number of photographers who were unwilling to abandon their extensive lens systems, when Canon introduced it's new series of cameras. They wished to avail themselves of some of the new offerings, but not at the expense of an entire new system of cameras and lenses. A converter would be just the thing, and was even considered to be a necessary offering in order to prevent lagging sales of the newly introduced cameras. On the other side of the coin, there is really little need for the old FD user to wish to use the EF series of lenses. FD lenses are plentiful, and less expensive, while being the equal, optically, of the newer models. In addition, the new features, autofocus, and electronic integration, would be unusable on the older style cameras. Even standard full aperture metering, and use of the full range of F-stops would not be possible. A close look at the lens, and the mount makes the reason plain. The EF lens has no aperture control; there is no way to stop the lens down. These functions are done electronically, via signals from a compatible body. Even if properly mounted to an FD style body, the lens would only be capable of shooting at it's widest aperture.
Canon introduced an adapter unit for the EF lenses, at about the same time the lenses themselves were introduced. the unit was to be sold to working pros only, and was offered in limited quantities, for a short period of time. The Canon adapters are now very expensive, on those rare occasions when they might be found. Some after market companies also offered converters (some still do), but all have limitations. The Canon unit is the best, not surprisingly, and permits the use of the lens, in manual focus mode, requiring stopped down metering. Neither of these are serious limitations, particularly for the experienced photographer. The adapter itself was simple in concept. One end had an EF mount, while the other used an FD mount. There were no linkages, since the mechanical FD system, and the electronic EF systems can not really be linked. One thing which did need to be added to the basic metal mount, was a small single element lens. This lens is used to make up for the fact that the converter changes the distance between the film plane, and the FD lens.
Most of the units offered by other manufacturers come with the warning that the adapters will impair the ability of the lens to focus at infinity. They are generally offered as macro adapters. Having purchased, and used one, I found myself pleasantly surprised. The model I tried, retains the infinity focus of the lens, though the focus scale is no longer accurate. It is quite possible, with this unit, to focus "past" infinity. Other than this minor, but irritating quirk, the converter is perfectly satisfactory. Other than the focus issue, the adapter requires all metering to be done in stopped down mode. Since there are no mechanical linkages with the camera, the lens must be set to stop down, before it is installed into the adapter.