Lens Basics: Telephoto Zooms and Primes

Palm Warbler, Canon EF 400mm f/5.6L

Telephoto lenses allow you to get close to your subject without getting close to your subject.  In this post, I'm considering prime lenses 300mm or longer or zoom lenses that extend at least to 300mm. These lenses are designed for nature, wildlife and sports photographers.  There are many factors that can influence which lenses you'll buy but here are what I consider the 5 main factors.

1. Focal Length.  The longer the focal length, the larger you can make the subject in your frame.  This means you have more of your pixels on your subject, and therefore more detail.  Of course, the longer the focal length, the larger and more expensive the lens.  For extremely long lenses (500mm and longer), it is impractical to hand-hold the lens.
2. Sharpness.  This for most is the most important factor.  The sharper your lens, better image quality (IQ) you will have, and this will allow you to crop your image more or magnify it larger.  Unfortunately, many consumer grade telephoto zooms get soft at the telephoto end of the zoom.  
3. Image Stabilization (IS).  The longer the focal length of your lens, the faster the shutter speed you'll need to get a sharp image.  Image stabilization (or Nikon's Vibration Reduction, VR) will allow you to get sharp images at slower shutter speeds. 
4. Maximum Aperture.  A larger maximum aperture will give you faster shutter speeds and shallower depth of field.  Both are extremely valuable for wildlife and sports photography.  But the larger the maximum aperture, the larger the lens.  For instance, the difference in size between a 400mm f/5.6 lens and a 400 f/4 lens is tremendous.  The f/5.6 lens can be hand-held without much difficulty.  The same cannot be said for the f/4 lens.  On most cameras, autofocus will only work when attached to a lens with a maximum aperture of f/5.6 or wider (that is, smaller f/stop number).
5. Tele-Extenders. Tele-extenders are basically magnifiers that you can put between your camera and lens.  A 1.4x extender will make your subject 1.4x larger.  A 2x extender will make your subject twice as large.  In effect, with a 1.4x extender, your 400mm lens becoems a 560mm lens with a 1.4 extender and a 800mm lens with a 2x extender.  But these extenders also lower the maximum aperture of your lens.  A 1.4x extender lowers it by one stop, and a 2x extender by 2 stops.  And this is one of the reasons why people shell out a lot of money for big telephoto prime lenses.  If you put a 1.4x extender on a 400mm f/5.6 lens, your maximum aperture has shrunk to f/8, and your autofocus will not work on most cameras.  Put a 2x extender on it, and the maximum aperture shrinks to f/11 and no camera will autofocus with this setup.  On the other hand, if you have a 400m f/4 lens, you can add a 1.4 extender, and shoot with autofocus on what effectively is a 560mm f/5.6 lens.  And if you have the right camera, like a Canon EOS 1D mark IV, you can put a 2x extender on the lens and have limited autofocus on what is effectively an 800mm f/8 lens.  And you'll shell out big bucks to do it.

Giant Swallowtail, Canon EF 400mm f/5.6L

So photography is an art of compromise, and this is no exception.  There are basically three options you can take here:

1. Consumer Grade Telephoto Zooms.  These lenses are the least expensive and most convenient option.   They are a good choice for nature photography when you are going to display your images without much cropping.  But consumer grade telephoto zooms are generally not adequate for serious wildlife photography.  You're starting out with a softer lens with an f/5.6 maximum aperture, and it probably doesn't extend beyond 300mm.  This means slower shutter speeds and less IQ.  And when shooting wildlife with these lenses, you will almost always have to crop significantly, magnifying the softness of the lens. With these lenses, you'/re limited to the occasional time when wildlife is very close to you or when you're content to have the wildlife occupy a relatively small portion of the frame.
2. Professional Grade, "Small" Lenses.  Both Canon and Nikon make prime and zoom lenses that are sharp, professional lenses but smaller with a maximum aperture of f5.6.  Using these lenses means slower shutter speeds than larger, faster lenses, and it also means that on most cameras you will need to manual focus when adding tele-extenders.  But these lenses are sharp enough to crop significantly, especially prime lenses. Compared to larger, faster lenses, what you lose in IQ you can sometimes make up for with maneuverability.  These lenses are smaller, so you can hand-hold them when when you need to and more quickly take a photograph.
3. Professional Grade, Large Prime Lenses.  Here I'm considering lenses from a 400mm f/4 to the 800mm f/5.6.  These offer you the best IQ available, and the longer telephoto primes allow you to get even closer to your subjects.  These lenses are for those who can afford not to compromise with IQ,   But this option is cost-prohibitive for many photographers, so I won't spend much time on them here.  A Canon EF 500mm f/4 costs about $6800.  A Canon 2x extender costs $300.  A Canon EOS 1D mark IV (a camera that will autofocus with both the lens and extender) costs $6000. That doesn't count the tripod/head combination you need for such a large lens. It's a great option if you can afford the money to buy it and effort to use it.  I suspect, though, you'll want to read more than a post titled "Lens Basics" before spending $13k on a camera/lens/tripod outfit.