If your interest is wildlife, as someone who photographs wildlife and birds myself and uses this very lens, the Canon EF 100-400mm f/4.5-5.6 L IS is the lens you want. From a price/performance standpoint, it packs a LOT into a relatively easy to handle package, offering the benefit of a zoom and the bulk of L-series quality (lacking only wide-open sharpness at the long end.) This is a fairly old lens design...over a decade, if I remember correctly. There are a lot of anecdotes on the net that are frequently regurgitated on forums by people who dislike the push/pull design, however most of them are unfounded and based on speculation.
The Wildlifers Lens
First off, the good stuff. The EF 100-400 is well-known as a key lens for wildlife shooters. It has an ideal amount of reach, especially when paired with an APS-C camera for extra reach (it effectively becomes a 162-648mm lens on say a 7D, 60D, 650D, etc.) It also doubles as a decent lens for bird photography, however its about the minimum you would really want to use for that type (a 500mm f/4 or 600mm f/4 L-series lens, preferably with a 1.4x TC, or a 300mm f/2.8 with a 2x TC, is far preferable for birds as you need all the reach you can get, but the 100-400 will definitely do if you can't afford to drop seven to ten grand on a lens. ;) At 400mm, you are pretty much at the sweet spot for a lot of wildlife types...including ungulates (elk, deer, etc.), canines (foxes, coyotes, even wolves if your brave), and a whole host of rodents and other smaller creatures.
The EF 100-400 has IS, or image stabilization, which is pretty much essential for wildlife and bird work. It is not a top of the line, modern IS system...those will get you about four additional stops of hand-holdability (but it costs!) The EF 100-400 will get you around two stops, which means you can shoot fairly sharp hand-held shot at a 1/100th second shutter speed. For wildlife in the evenings, thats extremely handy (although you might have to use an ISO setting a stop higher than you might prefer.)
Generally speaking, the 100-400 is a sharp lens. At less than 400mm, its pretty sharp up to 1/3rd of a stop down from wide open. At 100mm, f/4.5 is sharp enough. At 400mm, this lens is tack sharp at f/7.1. For most things, unless you are shooting in very low light (such as wildlife 15-20 minutes after sunset), you should be able to shoot at f/7.1 without issue. Wide open @ f/5.6 at 400mm, this lens does get a little soft, however it is still acceptably sharp enough that it can serve in a pinch. A bit of post-process sharpening can do a lot to improve results as well.
When it comes to functionality, personally I find this lens to be excellent. There are a lot of nay-sayers on the net who trash the push/pull zoom design with an often astonishing gusto and glee. I would venture that very few of those individuals have actually used this lens for long enough to actually realize the potential such a design has. Even if you are used to lenses that have dual rings for adjusting focus and zoom, it does not take long to get the hang of a push/pull design. There are several benefits to this design as well. For one, it allows for very quick adjustments in zoom, much faster than can generally be achieved with a ring-based design (you can zoom from 100mm to 400mm in less than a second if you really need to.) The design also makes it easy to adjust both zoom and focus simultaneously since the adjustments are decoupled and require very different actions to perform, something that is rather difficult to do on a dual-ring design. The USM focus with full-time manual focus works superbly by a focus ring. The ability to give focus a quick manual adjustment to get it within range of your subject on the rarer cases where AF does act up means you spend less time hunting and more time shooting. With the push/pull design, you can even make those manual focus adjustments while zooming...a rather unique capability of this lens.
The Potential Detractors
There are a lot of nay-sayers who greatly dislike this lens, and several anecdotes that are regularly repeated about why this lens should be avoided. I believe only one might actually matter.
- "Its push/pull design is horrible and completely unintuitive."
- This is really a matter of preference and personal choice.
- Learning the push/pull design is not hard, and once mastered, is quite flexible and powerful.
- "Its a 'dust pump', and will suck tons of junk into your lens!"
- Completely false. This lens is not a weather sealed lens, however it is FILTERED to prevent dust in the barrel.
- Unless your working in a dust bowl and are getting pummeled by extremely fine, micron-sized dust, its unlikely your ever going to accumulate enough particulate inside the lens to matter.
- I've used this lens for over two years in heady rain, snow, sleet, hail, dust storms and other blowing particulate, and never had any issue...so despite not being weather sealed, it still holds up quite well against the onslaught of nature!
- "Its a dog to focus because of its aperture, the 400 f/5.6 is better."
- This is not as fast as the top-shelf f/4 and f/5.6 L-series supertelephoto lens like the 500mm, 600mm, or 800mm lenses from Canon (but neither does it cost $10 Grand or more!!)
- This lens is plenty fast enough to perform continuous-tracking AF on birds in flight when using a Canon 7D in low sunset light.
- This lens is plenty fast enough to acquire focus lock on wildlife in post-sunset light (which is dimmer than BIF AT sunset!)
- This lens is even capable of autofocusing at f/8 with a 1.4x TC in daylight or sunlight (something not officially supported, but which is possible using a Kenko 1.4x DGX TC).
- AF can be spotty at f/8, however thats kind of run of the mill for f/8 AF on most Canon equipment, even 1-series bodies.
- "Its unusably soft (not sharp) and useless for wildlife or bird photography."
- It CAN be soft at 400mm f/5.6 (max aperture), more so in the corners.
- Stopped down to f/6.3 it gets quite acceptably sharp.
- Stopped down to f/7.1 it gets tack sharp.
- Given that you'll probably need to be shooting at f/8 most of the time for wildlife and birds anyway except when in very low light, your shots should be tack-sharp pretty much all the time.
- Canon AF is always performed at maximum aperture, the shutter only stops down electronically after AF lock is in and the exposure is taken.
- "Its too heavy for walk-around use for more than an hour."
- This lens weighs only three pounds. Just slightly more than the 400/5.6 (2.7lb) or 300/4 (2.8lb).
- In contrast, the other most popular lenses for wildlife and birds weigh considerably more.
- The EF 300mm f/2.8 weighs 5.7 pounds.
- The EF 400mm f/2.8 weighs 8.5 pounds!
- The EF 500mm f/4 L weighs 8.5 pounds!
- The EF 600mm f/4 L weights 11.8 pounds!!
- Without any kind of strap at all, I've found you can walk around with this lens for several hours without it being a problem. Holding it at waist level by the tripod mount can help extend walk-around time.
- With a shoulder-strap that rests the camera at your waist, you can walk around with this lens all day long without issue.
- A 300mm f/4 might weigh less, but you also lose 100mm worth of reach, a non-trivial amount when it comes to wildlife, and unacceptable for Birds/BIF (unless you have some serious skill in sneaking up on them without being noticed!)
For its price, which streets at around $1400 these days, the Canon EF 100-400mm f/4.5-5.6 L IS USM lens is hard to beat. You get a 4x zoom range, L-series build and optics, 2-3 stop IS (about 2 stops @ 400mm), a very sturdy build, and relatively low weight compared to Canon's other supertelephoto lenses.
Ok, now that sales pitch for the 100-400 is over (sorry, have to put some facts strait when there is so much misinformation and nay-saying about that lens!). There are alternatives to this lens. Canon does offer the 300mm f/4 L IS and 400mm f/5.6 L. Both of those lenses weigh slightly less, however they have their own potential drawbacks. There are also third-party zoom lens options, however I do not believe any of them actually compare to the price/performance/features ratio of the Canon 100-400mm.
Regarding the Canon EF 400mm f/5.6 L lens. This is frequently listed as an ideal option for wildlife and bird photography. It has a single, and in my opinion fatal, detractor: It lacks any form of IS (image stabilization). For birds, thats a total dealbreaker, and for most wildlife, its usually a dealbreaker as well. If you have extremely stable hands or are already a skilled hand-held, non-IS panner, this might actually be the lens for you. It might also be a good lens if you always intend to use it on a stable tripod with a gimbal-type tripod head and lens mount. Its slightly sharper than the 100-400 wide open, but usually still needs to be stopped down to f/6.3 or f/7.1 for maximum sharpness. I've met a fair number of Canon 7D users using the 400mm f/5.6 out in the field as well, and heard their surprise when they tried out my 7D+100-400mm setup with its image stabilization. The list price on this lens is $1799, which deters most people, however the street price (actual sale price in most cases) is closer to $1400 (I believe I picked mine up for around $1580 minus a refund deal from Bing Shopping a few years ago, so it ended up being around $1420 or so). Thats just in line with the 400/5.6 L, and the 100-400 does offer IS...making it a more attractive option for 400mm handheld wildlife/bird photography. To keep things simple, you really want image stabilization for wildlife!
The Canon EF 300mm f/4 L IS is another option. If you want a sharp prime, this is a great lens. Its quite sharp wide open at f/4, and about as sharp as it gets at f/5.6. The big improvement with this lens over the 100-400 would be corner sharpness...its one of Canon's best corner-sharp lenses for the price (the next best thing would be the 500mm or 600mm f/4 L, which are two of the sharpest lenses Canon offers...but they sell for around ten grand.) The key drawback with this lens is the focal length. The loss of 100mm is quite a big loss, and will mean you either have to get a fair bit closer to your subjects (which, in the case of wildlife, is difficult...you either scare it off or increase personal risk.) At 300mm, outside of using a bird blind and sitting and waiting for the birds to come to you, your probably going to be out of useful reach of most birds and BIF.
There is an option for this lens to improve its versatility, and still be a bit sharper than the 100-400. Slapping on a 1.4x teleconverter converts this lens into a 420mm f/5.6 lens. The benefit of this lens over the 400mm f/5.6 is that it has image stabilization! Even with a TC, at 420/5.6, this lens would make a great wildlife and birding prime. The drawback is you'll need to spend an additonal $300-$500 on either a Kenko DGX or Canon 1.4x teleconverter, which makes the total price around $2000. If you have the money and want the sharpness, I'd say this is the better choice over the 100-400. Even with a TC attached, the EF 300 f/4 L has a size advantage over the EF 400mm f/5.6.
The Canon EF 70-300mm f/4-5.6 L IS USM is a decent lens. It is one of Canon's newest lenses, so it is a more advanced design than the 100-400, 300/4, or 400/5.6. It has great optics, sharp to the corners wide open, and has superb IS. As a general purpose telephoto lens, its probably one of the best options out there. It could serve as a wildlife lens if you needed it to, as well as a portraiture lens. However it doesn't really have ideal characteristics for a wildlife lens. It maxes out at a 300mm focal length. That is generally not quite enough reach to really get good frame-filling wildlife shots (400-500mm is better). Its also f/5.6 at 300mm, so if you tried to use a 1.4x TC to get 98-420mm out of it, you'll be stuck at f/8 on the long end (and probably be unable to AF, although you might be able to with a Kenko DGX, like on the 100-400.) Its certainly an option, however being a newer lens its likely to sell closer to its list price of $1599.
There are also a bunch of third-party options. Sigma, Tokina, Tamron, etc. all make lenses for the EF mount. Most of these lack the optical, build, and functional capabilities of Canon lenses, and usually have tighter maximum apertures (f/6.3 in many cases.) They are usually cheaper, sometimes quite a bit cheaper. You might be able to pick up a Sigma "Bigma" 50-500mm lens for a decent price, however its going to be just a bit worse characteristically than the EF 100-400mm lens.
In conclusion, if I had to recommend two options, these would be them:
- The Prime Option: Canon EF 300mm f/4 L IS USM + Kenko 1.4x Teleplus Pro 300 DGX
- For better sharpness and ideal reach for wildlife
- A bit higher cost (needs an extra TC, for about $300) and less versatility (no zoom)
- The Zoom Option: Canon EF 100-400 f/4.5-5.6 L IS USM
- For the best versatility at a great price/performance ratio
- Not quite as sharp as the EF 300+1.4x TC
Both options weigh the same, and offer roughly the same reach. If you don't care about zooming and have the cash, I'd say go with the prime option. Better optics, 20mm extra focal length for more reach, and slightly better IS. The whole getup will probably run you around $2000, but its probably worth it.
If you don't have the cash, the EF 100-400 really is the best lens for what you want to do. Its relatively cheap, it only weighs 3 pounds, and at 400/7.1 its as sharp as any other Canon L-series lens. It'll serve you well.