March 3rd is World Wildlife Day, so why not take it as an opportunity to start shooting some wildlife photography? It may seem like an intimidating hobby to get into, but you don't have to book a safari trip to capture great wildlife shots—you can start in your backyard or at the nearest park.
To take awesome images of animals in the wild all you need is good timing and patience. Of course, having a long lens in your arsenal doesn't hurt either, so let's go through some factors to consider when buying a telephoto lens.
Telephoto lenses generally start around 100mm for full-frame, but if you want to get seriously close, a range of 200mm to 800mm is recommended. This will allow you to keep a safe distance while not disturbing your subject, but still get close-up shots.
Of course, there are disadvantages with going too long, as your depth of field (what's in focus) gets increasingly narrower. Plus, extremely long lenses tend to be quite large and heavy—not to mention prohibitively expensive. A good compromise is settling for a moderate focal length (around 300mm) and if you need more reach, you can always add a teleconverter.
Having a large maximum aperture (generally, an f/ number of 4 or lower) is always a plus for any lens, and telephoto lenses are no different. The wider the aperture, the more subject isolation you can achieve—which means you can blur out distracting backgrounds and make your subject stand out. Luckily, at long focal lengths you don't need super-fast lenses to achieve good separation.
Generally, if you're shooting in daylight, a lens with a maximum aperture of f/4 or f/5.6 is good enough to get the job done. In dimmer conditions, getting down to f/2.8 or even f/2 is a nice luxury. Keep in mind that you'll lose a little bit of light if adding a teleconverter, so having a large maximum f-stop makes a difference in that respect as well. The one downside to lenses with large maximum apertures is that they tend to be expensive and larger in size due to their complex designs.
Autofocus Speed & Noise
While your choice of camera body is the main factor in determining autofocus speed, some lenses have faster motors than others and that can also affect AF speed. If you want to photograph fast-moving subjects, look for lenses that can snap to focus quickly.
Some lenses are also louder than others when autofocusing. If you don't want to scare off sensitive subjects with excess noise, especially when shooting birds or jumpy woodland creatures, opt for a lens that can focus quietly.
The longer the lens, the harder it is to keep steady. If you don't want to end up with blurry images, you can use a support like a tripod or monopod, and additionally, get a lens that has built-in stabilization. This feature can neutralize vibrations to keep your images nice and sharp even if you happen to move slightly.
With image stabilization, you'll be able to get better results at slower shutter speeds and while hand-holding your camera. Also, if you plan to shoot from moving objects like boats or piers, stabilization offers much-needed help.
If you're going to be outside, you have to be prepared for adverse conditions. Rain, fog and humidity are common hazards that can introduce moisture within your equipment. This moisture can lead to fungus and haze within the lens elements. Having some weather resistance built into your lens and camera can help protect your gear.
And it's not just water that you have to worry about—dust, salt and smoke can all damage your lenses as well. While weather resistance is a good general defense, keep in mind that it doesn't make your equipment completely invincible, so you should still use caution.
Weight & Size
Telephoto lenses tend to be bigger than most lenses due to the extreme demands of the optics, but there's still a notable size difference between models. Obviously, if you plan to hike long distances to reach remote subjects, the size and weight of lenses are important factors.
Zoom or Prime?
There's no right answer to this timeless question—it mostly comes down to preference. Prime lenses (those with a fixed focal length) usually offer better optical quality, while zoom lenses (those with a range of focal lengths) offer the convenience of multiple lengths in one package. There are telephoto zoom lenses that can hack it optically with their prime counterparts, but they tend to be quite pricey and large.
If you're shooting subjects that are constantly moving, especially closer and farther away from you, a zoom may prove convenient. If you're shooting subjects that are usually a fixed distance from you, a prime lens would probably work best.