Explanation of focal length

[mdaddyrabbit]

I was reading within this forum and I wonder if what I was reading has any truth to it. I read that with a Canon 20D and a 70-200mm F4 that the actual focal length with my 20D would be somewhere around 280mm, true or false. Also when buying a 1.4x extender does it have to be a canon if not what other brands are compariable to this extender. I am really confused about this and I want to get totally clear before I make purchases.

[pcasciola]

Your statement is both true and false. While it's true on a 20D that your 70-200mm will "kind of" be like a 100-320mm because of the 1.6x crop of the 20D's sensor, it's also false is because it's still just a 70-200mm lens. Since you are cropping from the center of the lens because the 20D's sensor is smaller than the image the lens projects, it yields approximately the same field of view as a 100-320mm lens would on a full frame camera. Get ready for lots of other opinions.

Belmondo's visual aid explains it best I think:
http://photography-on-the.net/forum/showthread.php?t=18880

As far as extenders, you don't have to get Canon's. I have Sigma 1.4x and 2.0x extenders, and they work well with my Canon lenses. I have not compared them to the Canon one's but I'm pretty happy with the quality of the Sigmas.