Another unique advantage to using supertelephoto lenses is their narrow angle of view. Distracting elements in a shot can ruin the image, but using a longer lens can help eliminate this issue. Imagine a scenario where you're photographing a street mime in Paris. If you shoot using your 50mm at 10 feet, the mime is the right size in your shot, but there are distracting elements on both sides of your subject. If you move back to 20 feet and use a 100mm lens, the subject is the same size, but you've narrowed your angle of view and eliminated the distracting elements. My 400mm has an angle of view of nearly 6º, while my 600mm has an angle of view of about 4º. By comparison, a 200mm has an angle of view of almost 20º, and a 50mm is 46º. If you want to eliminate distracting image elements, use the narrow angle of view provided by a telephoto lens.
TELEPHOTOS AND AUTOFOCUSChoose the right autofocus. Many people buy supertelephoto lenses to photograph wildlife and action sports, which means fast-moving subjects. Choosing the right autofocus will improve your big-lens experience. You have four variables to set: autofocus button, focus pattern, frame rate and focus range.
Choosing the autofocus button is your first decision. Pressing the shutter halfway down starts autofocus on most cameras, and works fine for many photographers. But some cameras allow you to set a custom function and use a button on the back of the camera for autofocus.
Why? Back-button autofocus separates the shutter and focusing operation, eliminating the chance for an unwanted element changing your autofocus in the middle of a sequence. Imagine photographing a hawk in flight, and as the hawk flies past, a tree comes between you and the bird. If you're using shutter-button autofocus, then the camera will try to autofocus on the tree when it comes between you and the hawk. If you're using back-button autofocus, you can stop autofocusing while you pan past the tree, but still keep shooting frames holding the shutter button down. Back-button autofocus isn't for everybody and takes practice to master, but it can offer advantages in dynamic shooting situations.
Another choice to make is the autofocus pattern. I normally leave my camera on single-point autofocus, as this works best for most of my shooting situations. If I'm photographing a model with a 300mm lens at ƒ/2.8 at 36 feet, I have seven inches of depth of field. I need to make sure my focus point is right on her face or I may get a soft shot. With moving subjects, I use a nine-point autofocus pattern. This pattern has more autofocus area to track moving subjects, but is still small enough for fast autofocus. For subjects that are erratic and hard to follow, try using larger focus patterns to capture the movement.
Frame rate is the third variable to consider using telephoto lenses. Similar to single-point autofocus, my default setting is the single-shot setting. I use flash a lot in my work, and the flashes pop for one frame and then need to recycle. Multiframe bursts result in the first shot lit by flash and the rest are dark.