Canon EF 100-400mm f/4.5-5.6L IS USM

Canon built this in 1998 for the EOS shooter who wanted 400mm without hauling a prime the size of a fire extinguisher. Wildlife shooters, the sideline crowd, birders short on budget and short on shoulder for a 400mm f/2.8. Canon's answer was a white L zoom with Image Stabilization at a point when IS was still a novelty on long glass, and it stayed the default "one lens for safari" for the next fifteen years.

The thing everyone argues about is the zoom action. You do not twist a ring. You shove the barrel in and out like a trombone, push for wide, pull for 400. Birders loved it, because you can rack the whole range in a single motion to catch something taking off. Everyone else called it the dust pump. That in-and-out throw breathes air, and dust, straight into the barrel, which on a film body meant the occasional speck waiting on your next frame. A tension collar sets how stiff the action feels, and it loosens with age, so a hard-used copy creeps under its own weight when you aim it down.

Optically it holds up better than the bargain-bin used price suggests. There is a fluorite element and Super UD glass inside, contrast is proper L-series, and the color needs no apology. It stays sharp from 100 through about 300. At 400 wide open it softens and goes dreamy, and you want f/8 to pull it back to crisp. The first-generation IS buys roughly two stops, which on a 400mm handheld is the gap between a keeper and a smear.

The honest catch is that it is slow. Variable aperture means f/4.5 at the wide end and f/5.6 by the time you reach 400, which is exactly where wildlife lives, at the long end in the thin light of dawn and dusk. Thread a circular polarizer onto the 77mm front to cut haze and water glare, normal practice on glass this long, and you surrender another stop and a half. Feed that into the filter compensation in Zone Light Meter so the reading matches what actually reaches the film, not what the aperture badge promises.

Canon replaced it in 2014 with a Mark II that dropped the trombone for an ordinary rotating ring and tightened up the long end, and the third-party 150-600 superzooms and mirrorless RF glass have pulled most buyers away since. None of that buried the original. It still moves cheap as a first real reach lens, and the people who pass on it tend to go to the 400mm f/5.6L prime instead, sharper and lighter with no stabilizer at all. Pick your compromise. Zoom and IS, or bite and a fixed frame.