Canon EF 300mm f2.8 L I.S. USM Telephoto Lens
Having switched from Nikon to Canon, one of the biggest losses involved was the parting of company from my beloved AF-S 300mm  f2.8 VR lens. Combined with a teleconverter, I find 300mm f2.8 lenses an ideal walkabout lens... they really aren't too heavy and when combined with a good neoprene strap. They are no hardship on a good hike/walk and the performance and potential rewards are well worth the effort. In reality, you really won't be holding the camera up to the eye for prolonged lengths of time.

Anyway, to replace my Nikon, the Canon EF300mm f2.8 L I.S. USM is a perfect replacement. There really isn't much to choose between these two flagship lenses on any level, although similar to my EF600mm f4, the exceptional performance of the EFII 2x teleconverter does edge overall flexibility in favour of Canon.

The lens arrives in the CS30028 case, which is a compact version of the one supplied with my 600mm f4 lens and looks a bit feminine if truth be told (reminds of a vanity/make-up case that an ex used to have). Also supplied is the obligatory slip-on lens cover, the E-145 in this case. Fortunately this is a non-elasticated cover, so not quite as awkward to remove as some. The Canon EF300mm  f2.8 is also supplied with the ET-120 lens hood.

Controls & Operation
As with other Canon I.S. lenses, there is a comprehensive control panel to the side of the lens body.

Uppermost on the switch control panel of the Canon EF300mm L I.S.lens is the focus limit control. 3 options here, the 2.5 metres - infinity (full) setting allows the lens to focus from minimum distance to infinity. Then the 2.5 metres - 6.4 metres and finally 6.4 metres to infinity. Basically, if you're target is expected within a known distance from you, it speeds up autofocus if the lens isn't hunting through it's entire range to get a lock-on.

Next we have the AF/MF switch. Says it all really, though you can always operate the camera in manual focus even if it's set to AF, being FTM (full time manual) design. Personally I would prefer the option of totally disabling the focus ring, as it's can shift if you're working from something like a beanbag.

Next we have the magic switch, Image sabilisation on or off. After countless questions from people, can I just say here that the I and 0 originates from binary, I is on and 0 is off.

Underneath the magic I.S. on or off switch, we have the Image stabilisation mode. mode 1 controls movement vertically and horizontally, mode 2 just counteracts vertical movement... so as to allow you to pan along with a subject without the I.S. system trying to compensate.

Lastly in this panel is the focus preset functions. Basically, you can set a focus distance on the lens, press 'set' and you can recall this distance at any time via the twist ring (the ring with the serrated edge) just in front of the focus ring. It's handy feature if you're likely to be shooting at 2 vastly different distances. I would prefer it if focus recall was an option on the 4 focus lock buttons and in a really ideal world, a focus recall button would be situated far closer to the camera body, or indeed controlled via the camera itself, as some lens functions are on the Canon 1Ds MkII.

The tripod collar can be simply removed from the Canon 300mm f2.8, and this is my choice for use as a walkaround lens.
EF mount; super telephoto lens
Fluorite and Ultra-low Dispersion-glass;
image stabilizer;
internal focusing;
full-time manual focus
300mm focal length
f/2.8 maximum aperture
UltraSonic Motor (USM)

Lens Construction  17 elements in 13 groups
Closest Focusing  2.5m
Filter Attachment Size  52mm Drop-in
Hood  ET-120 (included) 
Weight  2550g
Diameter 128mm
Length 253mm

Below: Dunnock and Black Redstart taken with EF300mm f2.8 & EF 2x teleconverter
                           Canon EF300mm f2.8 L I.S. USM In Use
To be honest, I havent had this lens very long, so this is just a first impression after 2 weeks or so, and 2 weeks or so of some pretty grim weather. My main use for this lens is with teleconverters, so most of my initial use has been with the Canon EFII 1.4x and EFII 2x teleconverters attached, and quite frankly we all know what these flagship 300 f2.8 lenses are capable of used bare.

With the Canon EFII 2x Teleconverter (also see my Canon extender review page )
Having already used the EFII 2x with exceptional results on my 600mm f4 lens, I wasn't disappointed with the results when attached to the 300mm f2.8. Yes,  Autofocus speed does drop slightly, it's fast and reliable but not exceptional as it is with a 1.4x teleconverter. As the whole combination is giving f5.6, all focus points on the Canon Dslr are available. Stopping down to f8 does reap the rewards over having the aperture wide open.

With Stacked Teleconverters
To get the ball rolling, I though I'd try stacked teleconverters both 1.4x and 2x mounted between Canon 300mm f2.8 and Canon 1Ds MkII
This is a combination to be uses in desperation, even then I do wonder whether there's much more absolute resolving power and maybe upprezzing a photo with just a 2x teleconverter would be simpler.

Without a Canon professional Dslr body i.e the 1d series this is strictly manual focus territory. With my 1Ds MkII you do still get auto focus (only with central focus point) and it will lock-on but only after a bit of hesitation, a 'bit' varying upon light and contrast of the subject. You are best off using manual for most situations.
                                       Both of the images below (of a friendly but lost Racing Pigeon) open up to 1024pixels
Here's the full frame photo. Canon EOS 1Ds MkII
Canon 300mm f2.8, Canon EFII 1.4x , Canon EFII 2x
Exposure Time = 1/160" Not Ideal
Aperture = F8 Wide open due to light, Not Ideal
ISO Speed Ratings = 640 Not Ideal
Exif data only recognises the 2x teleconverter
This is a unprocessed 100% crop from the original. Unfortunately quite severe jpg compression to get it to a reasonable file size.
Birding Top 500 Counter
A flight shot of a Common Gull using the 2x extender, AF keeping up with the action, even with the loss of 2 stops.