Hi there,

I can't show a direct shot for shot comparison but I have used both the f4 and 2.8 with the 1.4 converter. You would be hard pushed to notice the difference in the final shot, with the loss of quality being the same as using them side by side without the 1.4x. Where you will lose out a little more is in the focusing speed and of course the brightness of the viewfinder image.

Being a wonderfully helpful and kind person I'd be happy to help with your burden and swap my f4 for your 2.8.  ::

From http://www.wlcastleman.com/equip/reviews/300mm/index.htm:

"For most photographic applications, differences in photographs produced by the EF 300mm f/4 L IS USM lens compared to those produced by the EF 300mm f/2.8 L lens are going to be photographically insignificant".


HTH.


Happy shooting,
Yakim.

The coldsiberia.org tests yeild some interesting results, as the lens does not appear to be that sharp at f4.  I know that the f2.8 is very nicely sharp even at 2.8.  Last fall someone posted results from three different canon f4 17-40L lenses and one was significantly sharper than the others.  I wonder if this could be the case in this test, could it just be a slightly "bad" L lens, or would all of the 300 f4s behave like this?  If there are good and bad L lenses in the same models how hard would it be to get Canon to warranty it?  Buying a professional lens at a professional price shouldn't have to be a gamble.

I was just curious, does anyone have a direct comparison between the canon 300 f2.8 with the 1.4, and the canon 300 f4 with the 1.4?  I Have the 300 2.8 and get great results with the 1.4 teleconverter.  I was thinking of switching to the 300 f4 to save pounds, as I shoot mostly mountian biking and with my pack is getting way too heavy.  I don't want to loose much quality however when the 1.4x teleconverter is used.  If anyone knows how the f4 does in comparison please let me know.  A photo comparison would be great.  Thanks

Thanks for the info, and for being such a kind person.  Maybe if you've got a few other items we could work something out.  I need a new wide angle, and a flash or two.

As a counterweight (you can find this test and several others via Google):

http://www.coldsiberia.org/phototest/EF_300_IS_test.html

"The differences between the EF 300mm 4.0 non-IS and the EF 300mm 2.8 IS are rather negligible, whereas the differences between the IS and non-IS versions of the 300mm 4.0 are very significant. This disparity in optical performance between the two latter telephoto lenses is readily apparent both without and with the use of the Canon 1.4x Extender."

One person's "photographically insignificant" is another's "very significant". And that is the only significant difference I can find at a glance in the two tests. Even the one Yakim quotes clearly shows that the f/4 IS is inferior, but whether it matters or not is up to the photographer and his/her audience, if any.

The weight considerations and IS/non-IS are of course of importance to whether the lens will be used, or whether you'll get the shot. But the non-IS versions (f/4 or f/2.8) are not manufactured anymore.