Tamron converter w/ Canon 50mm 1.8?

CanonFan

Can somebody tell me why the Tamron 1.4x converter, when added to my Canon 50mm 1.8, on my 5DM2 won't autofocus? It just vacillates but won't lock on the focus point?
Thanks..

ziggy53

The simplest explanation is that the Canon EF 50mm, f1.8 is not designed to accept a teleconverter at all.

Lenses that are designed for teleconverter use will have a capability to drive the AF motor at multiple speeds. The reason this is required is because AF technology in a lens is a delicate balance of drive power, drive speed and braking. Just as a car does not stop instantaneously, a lens requires that you cut the AF motor before the point of optimal focus. The Canon EF 50mm, f1.8 has the cheapest AF drive motor technology and I believe that it is a single speed micro-motor. With the teleconverter attached the lens is probably consistantly overshooting the optimal focus point.

The other problem is that most teleconverters are designed for longer focal length lenses, but not all long focal length capable lenses are designed for teleconverter use, for a number of reasons. The Canon teleconverters, and I believe the Tamron teleconverters and Sigma, etc., are all designed for 100mm lenses and beyond. Some 70-200mm lenses will also work fairly well with a teleconverter, but they work optically best at the long end IMO.

Another potential problem is the teleconverter choice itself. One of the Tamron teleconverters, specifically the Tamron 1.4x "F" series, is, I believe, specifically designed for "flat field" lenses. I have one of these converters and I use it with the Tamron 500mm, f8 SP (55BB) (catadioptric, aka "cat" mirror lens), which "is" a flat field lens.

It's terribly important to find lens/teleconverter combinations that work together, and not all do work together.

In the specific case of the Canon EF 50mm, f1.8 and any teleconverter, you will probably be able to use manual focus and the 5D MKII center dot focus confirmation, but only the center will reliably focus. I doubt if you like the edges and corners of the image as they will probably show both field curvature and some chromatic aberrations.

CanonFan

Thanks....guess its just a mismatch but manually might suffice until I get a portrait lens. I appreciate the info!