Design

Here's the 200-400mm with its natural companion, the EOS-1D X. It is huge - over 36cm / 14" long - and a heavy beast too, weighing in at no less 3.6 kg (~8lb). Add in that not-insubstantial 1D-series body and you're looking at a distinctly unwieldy combination - it can be used hand-hold, but for any kind of extended shooting you'll want to use a monopod at least. The barrel is festooned with control rings, buttons and switches - we suspect it has the most controls ever seen on an SLR lens.

Here we're looking down onto the top of the lens. The tripod collar rotates, with click stops every 90 degrees, but it doesn't detach. There's a choice of two 'feet' - the longer one shown here doubles as a padded carry handle. The hood is a substantial metal cylinder which locks in place using a large screw, and is nearly half the length of the lens. It has a chunky rubber bumper at the front for protection against knocks.

On APS-C the lens offers an equivalent range of 320 - 640mm, or 450-900mm with the extender engaged. We can't imagine anyone's likely to use this lens with an EOS 100D, but we couldn't resist seeing what it looks like.

The hood reverses over the barrel for transport, and the front cap is a fabric 'bucket' that's held in place by velcro fastenings. There are also lugs on the side of the barrel for a shoulder strap.

Built-in 1.4x Extender operation

The built-in 1.4x teleconverter is engaged by flicking a chunky lever on the side of the barrel. This swings the optical unit out of its housing on the side of the barrel into the lens's light path. A sliding switch can be engaged to lock the extender in either position, and a red line on the switch allows you to check its position at a glance.

This lever on the side of the barrel engages the built-in extender. It's big and chunky, and should be easy to operate while wearing gloves. The sliding switch on the side of the lens barrel can be used to lock its position.

The movie below shows how this works - it's a deceptively simple mechanism. But don't underestimate the technical accomplishment here - to maintain image quality the Extender has to be centered within a fraction of a millimeter, time after time after time. When it's engaged the maximum aperture selectable by the camera becomes F5.6, which gives an indirect indicator of its position.

Canon EF 200-400mm f/4 L IS USM Extender 1.4x - operation of built-in teleconverter

It's possible to tell whether your images were shot with or without the extender engaged simply by looking at the EXIF data. Without the extender, the lens reports as 'EF 200-400mm f/4L IS USM'; when it's in place, the EXIF says 'EF 200-400mm f/4L IS USM EXT'.

Optical diagram

The lens's optical diagram below shows how the Extender fits into the overall construction. It slots in front of the rear lens group, unlike a conventional teleconverter that goes between the lens and camera body. Here you can also see all the special glass used in the lens, and the location of the IS group.

Controls and body elements

Here's a closer look at the main lens controls. From left to right we have a set of focus stop buttons arranged around the barrel, so that in principle one should be within reach no matter what orientation you're holding the lens. Next to this is knurled metal ring that's used to perform the lens's powered focus functions: twisting it drives the lens to its preset focus distance.

The widest ring on the lens is the zoom control, which is marked at the 200, 250, 300, 350 and 400mm positions. Next to it is the manual focus ring, which is linked to simple distance scale.
Nobody could accuse Canon of skimping on the lens barrel switches. This set next to the mount deals with focusing - the top allows selection of Auto, Powered or Manual focus. As usual with a top-end Canon lens, you can tweak the focus manually at any time when using AF. Power Focus is designed to give smooth, controlled focus pulls during movie shooting.

Below this is the focus distance limiter switch.
These switches are set further forward on the barrel, and deal with the image stabiliser.

Mode 1 is the 'standard' setting that stabilises in both dimensions. Mode 2 automatically detects panning, and turns off stabilisation in the direction of movement. Meanwhile Mode 3 is a relatively recent addition to Canon's super-telephoto lenses, and only activates the IS system at the point of exposure.
The lower set of controls deals with the focus preset function. This allows you to set a particular focus point of interest - perhaps the goal line when shooting sport, for example. A quick twist of the metal ring in front of the zoom ring then quickly resets focus to this distance.
As with other super-telephotos, it wouldn't be practical to use screw-on filters with the 200-400mm's huge front element. Instead it uses a drop-in filter holder behind the Extender unit.