Although it definitely is difficult to achieve focus with the 70-300mm lens (with or without the EC-14 converter), I have set up this little album to show that it can be done fairly reliably, using center spot focusing and precise positioning of the focus dot.

The pictures below demonstrate that you can overcome the focus "hunting" that the 70-300 loves to do. Once you get it to focus approximately where you need it to be (for example, by focusing on a tree near your intended target), you can then focus again and get it to isolate on a small target with a lot of accuracy. This is possible even with a quickly moving animal like a warbler.

All of these shots were taken with an Olympus E-520 with Oly 70-300mm lens at maximum zoom, plus an Oly EC-14 converter. They all retain their original EXIF data. They are not cropped, just downsized for web display. They are all lousy pictures - part of my "shoot anything that moves" series! ;-)

The 2 chipmunk pictures were in pretty low light (shutter speeds of 160 and 200, at ISO 800), taken with a monopod and Image Stabilization. He was pretty still, so there was plenty of time to focus.

The 2 with the bird on the ground (a Towhee) were also taken in pretty low light (shutter speeds 200 and overexposed 60, at ISO 800). These were tripod-mounted, no IS. This bird was quickly moving around, and there was very little time to focus.

The 2 with the bird in the tree (a Back-and-White Warbler) were in bright light (both at shutter 1000 at ISO 400), tripod-mounted, no IS. This bird was quickly moving around, and there was very little time to focus