I think a converteron a 50MM lens is pointless. All you get is a 70 or 100MM lens. One the other lens, it would be worthwhile. Contact Canon and see if they have a TC to suit the lens. They may or may not.
TC's are cantankerous beasts, picky as all hell about which lenses they will cooperate. If Canon doesn't have anything suitable look into a Kenko TC.
The Canon 1.4 and 2X teles are meant (Designed) only to be used with certain L (pro-series) lenses such as the 300 2.8 and 4 up and the 70~200 f2.8...The front element protrudes into a cavity that is built into the design of the lenses, so my 1.4X is incompatible with my 50 2.5 macro and the 17~35 2.8...
The Kenko 1.4X will work with any Canon lens, but you have to compensate by a stop to use this, since its contact points don't have a chip or translator and simply act as an "extension cord" between the electronics in the lens and the camera...
Stop and think about this for a minuge. Teleconverters are not intended to be used with short focal length lenses. Why would you want to use a TC on a 50mm lens when you have a 100mm lens in your bag? If you've got your 50mm on the camera and think a 100mm would be better, yoou have to stop and remvoe the lens, put on the TC, then put the lens back on. Faster, easier and better optically to just remove the 50mm and put on the 100mm.
A TC is intended to extend the range of telephoto lenses. The person who only occasionally needs to reach out with a 600mm can use a 300mm to get that shot.
You're gonna have to go third party for a TC you can use w/ your lenses. Canon TC's are designed for use w/ "L" series glass, and at focal lengths from 135mm up. (except for the 135 S/F lens) I'd caution against using a 2X convertor from anyone, including Canon. I haven't had good results from my 2X Series I or my newer Series II. The 1.4X from Canon is excellent, as is the Kenko 1.4X from what I understand.
The original poster has a 100mm focal length in his telephoto zoom, but it is f/4.5 (Assuming he's referring to EF 100-300 f/4.5-5.6 USM) or f/4 (EF 75-300 f/4-5.6 USM IS)*.
IMHO, I don't think it unreasonable as a cost saving measure to get a 2x teleconverter for primary use with the telezoom and let it also do double duty on the 50 f/1.4 to get 100 f/2.8 for more dramatic/shallower dof than possible with the telezoom. Plus, when you use the tc with the 50 you retain the close focus to 0.5m, where close focus is 1.5m for the telezoom, v. a separate 100 f/2 with close focus of typically 1.0m (v. 0.23m for the more expensive yet 100 f/2.8 Macro).
The tc/50 combo may (or may not) be less sharp than 100 on the tele-zoom, but that's not necessarily a bad thing with portraits. [IMG]/ibb/skins/default/emoticons/wink.gif[/IMG]
* P.S. There is no 100-300 USM IS. If the zoom the original poster has is the EF 100-400 f/4.5-5.6L USM IS, then the Canon's TCs can be used on it, but they will not fit the 50 f/1.4.
Canon TCs communicate the effective/converted aperture to the camera, so that it displays say f/8 instead of f/4 in the viewfinder and LCD panel. Non-Canon TCs do not communicate this conversion, but the camera metering and autoexposure still operate correctly.
I didn't say they couldn't be used on the 50mm. I said they were not intended for that purpose. Most people that will be shooting at the 10mm range with a need for wider apertures can afford to purchase a lens to git that need. That's not always the case when reaching out to 600mm and beyond.
You can purchase a Canon 100mm f/2.0 for $390. A Canon 600mm will cost $7400. It obvious where the benefit of a TC comes into play.
If you use a 2x TC on the tele-zoom, you will loose autofocus, no way around it. But if you use a 1.4x or 1.5x TC it will probably still AF although poorly.
I have a 1.5x TC from Kenko that I have used with a Canon 75-300 USM IS. If there is enough light & contrast, and your subject doesn't move too fast, it will still AF.
Any TC will degrade the image quality, but a 1.4x TC will degrade it the least. The same thing for light loss. A 1.4x TC will cost you one stop & a 2x TC will cost you two. The metering system will compensate for the light loss though, so you really don't have to do anything except bring a tripod.
The TC is something I've been considering since I like to shoot birds and I can't afford a 500mm lens...
the sigma 175-500 looks interesting, but it may need to be retrofit to work with the ELan7. I've left emails with Sigma as to whether that zoom can be bought elan ready...but no response...I don't want to get something I have to send to manufacturer as soon as I get it.
I've used the 75-300 zoom for portraits...but would prefer a faster fixed lens...like a 100mm f/2 which sells for around $380.
so it's really a matter if I can kill 2 birds with one lens so to say...(no pun intended)
I am a little unclear on the Kenko TC though...will the camera meter work? or will it give false readings requiring adjustment?