Technically, telephoto lenses have a focal length or 60mm or more for a standard 35mm or full sensor size camera, though most people think of telephoto lenses having a focal length of 85mm or more. Lenses with a focal length greater than 300mm are considered to be super-telephoto lenses.
Telephoto lenses help us bring distant subjects closer, fill our frame with the subject, show detail difficult to see, capture action, and produce a shallow depth of field.
Telephoto lenses magnify the subject in the image. Assuming a normal lens has a 50mm focal length, we can say its magnification is 1X, or no magnification. Every 50mm adds a power of magnification. Therefore a 100mm lens is 2X, a 200mm lens is 4X and a 500mm super-telephoto lens is 10X. The more magnification the lens has, the more any tiny movement or vibration of the lens will affect the sharpness of the image.
When telephoto lenses are used, photographers must take into account the focal length of the lens, in addition to the demands of the particular scene being photographed, when determining the exposure settings for the image.
I have five tips to help you maximize the sharpness of your images when you use a telephoto lens.
1. The shutter speed you choose can make a big difference when hand holding your camera/lens in eliminating or at least minimizing blur or softness in the image due to camera/lens movement. A high shutter speed helps you “freeze” the movement. The general rule of thumb for photographers is to keep the shutter speed above the reciprocal of the focal length. For example, keep your shutter speed at or above 1/300 sec. for a 300mm lens or 1/500 sec. for a 500mm lens.
Remember, when shooting with a DX or APS-C camera, multiple the shutter speed by the camera's crop factor, in this case 1.5x. For a DX or APS-C camera, the shutter speed should be at or above 1/450 sec. for a 300mm lens or 1/750 sec. for a 500mm lens.
2. The aperture you choose can make a difference in sharpness with a telephoto lens. It can't help compensate for camera/lens movement as shutter speed helps, but it can help get a sharper image in two ways.
First, if you don't want a large depth of field you might want to open the lens' aperture to its maximum opening, but most telephoto lenses, when wide open, will produce a “soft” image. If you choose a somewhat smaller aperture you will improve the sharpness. This is called stopping-down.
Second, when shooting wildlife at distance, for example, the depth of field in a telephoto or super-telephoto lens can be as small as an inch or so. That will often mean part of your subject will be in focus, while other areas of your subject will not. If you increase the depth of field by stopping-down the lens further, you can bring your entire subject into focus and thereby improve the image's overall sharpness. This can also compensate for a small error in focus of the subject.
3. Image Stabilization/Vibration Reduction is a feature found in some cameras and lenses. It is specifically designed to compensate for camera/lens movement due to unsteadiness on the part of the photographer holding the camera/lens and internal vibration from the camera's mechanical actions, while making each photograph. Sometimes it can even help when the camera/lens is mounted on a tripod.
Due to the mechanics and electronics which make this feature possible, a good rule of thumb is to turn it off when your shutter speed is 1/500 sec. or faster. When using fast shutter speeds, if this feature is on, it may actually result in less sharp images.4. Use a carefully chosen tripod with head, capable of holding your camera and telephoto lens steady, without vibration. For help in choosing a tripod, consult my article How to Choose a Tripod. Tripods can be a pain in the neck to drag around with you while traveling or hiking, but they are unsurpassed for holding your camera/lens steady and minimizing the effects of camera vibration when making images. Use of a tripod gives you the flexibility to choose slower shutter speeds than you could otherwise use, and lower ISO settings which can reduce image noise.
5. Use a long-lens-support for super-telephoto lenses on a tripod. Super-telephoto lenses use a lens collar to attach your camera/lens to the tripod's head. Use of the lens' collar centers the weight of the camera/lens better over the center axis of the tripod, compared to attaching the camera directly to the tripod. Nevertheless, a significant length of lens will hang out past the central axis of the tripod.
Using a long-lens-support, such as the one in the photo above, takes the weight of the lens extending past the central axis of the tripod and supports it as a unit with the lens collar to reduce camera/lens movement and vibration by improving the way the camera/lens is balanced, attached to the tripod head, and supported by the tripod.
Using any one or all of these tips can improve the sharpness of your telephoto lens images.