Am i getting the wrong end of the stick here? A 70 - 300 lens on a 4/3 oly or pan will give you 140 - 600 is that correct? If so how close do you Steve or Gergrd need to get to the birds or wildlife to take a picture, when you say that you changed to canon because there isnt the lenses made for birders in 4/3. I am thinking of getting a oly because of the lightness and the fact that i don't have to carry a big heavy Nikon or Canon lens arround. Please corrct me if I'm wrong.
Ian
Ian, I think you have the right end of the stick, & Ron's calculations seem to look correct to my eyes too. See the attached table which I find helps me to compare between systems.
As others have said, the focal length issue isn't the only factor when choosing a long lens. The Olympus 70-300 is optically very good, and performs very well in tests. In fact I recently read a magazine review in which it came a very close second to Canon's 100-400 L IS zoom in terms of its pure optical performance.
But as Adey and gergrd have said, there is more to longer lenses than pure focal length. With something like my Canon 40D and 100-400 zoom, I get a little more focal length (but the difference is negligible in real world terms), but more importantly the focussing is much faster via Canon's USM (ultrasonic motor), and it can reel off 6 frames per second, so birds in flight are quite easy.
The downside to this is that the Canon combo weighs in at around 2kg or more, whereas the Olympus 70-300 lens with a light weight body would be less than half this. I bought this exact lens as my first long lens about 2 years ago for the same reasons that the OP wants it for; length without too much weight, and reach.
However I was never happy with it and quickly returned it in favour of the Olympus 50-200SWD and EC14 converter. With the converter attached this lens competes directly with the Canon 100-400 zoom @ 350mm (so not much different really) with similar fast focussing, widest aperture of F4.9 compared with the Canon's F5.6, and very similar IQ.
However in going this route I lost most of the 4/3 weight advantage and increased the cost to roughly the same as a Canon L series lens. There was no other 4/3 lens with similar performance available to upgrade to, but the main reason I changed over in the end was because the other lenses I wanted from the system were not available either i.e. a longish macro lens for butterflies & dragons. There is a 100mm macro promised on the Olympus roadmap, but there is no sign of it coming anytime soon. Canon, on the other hand have a multitude of lenses in this category that are available now, including a two excellent 100mm macros. There is also very good support for the system from the likes of Sigma, Tamron etc.
So you can see that in theory the 4/3 and micro 4/3 system promises much, but in the real world it falls a little short IMHO. However, if you're sure that all you'll ever need is a 300-400mm lens, and a light weight body at a reasonable cost, without the need for fast focussing and high frame rates, it might just be what you want.
Just be careful when predicting your needs
now that you don't under estimate your
future needs as I did. The problem with photography, especially when quite inexperienced, is that you don't really know what you will want and how much you will ultmately be prepared to spend until you have already commited yourself, by which time it is too late, and it gets very expensive to change later.
Steve