In order to obtain sharp images it is necessary to have good optics, film and technique. Assuming that these three items are of sufficient quality, there are typically two other variables that contribute to unsharp photographs. They are subject movement and camera movement. The first variable can be controlled to some degree by selecting a higher shutter speed. Shutter speed is dependent upon film speed, lens aperture and available lighting conditions.
Camera/lens movement is more difficult to control (this is especially true when trying to follow a fast moving bird as it hops from branch to branch). Proper technique is paramount when using long telephoto lenses as the high magnification will amplify vibrations and cause soft images.
First and foremost, a heavy tripod and sturdy tripod head are essential. When you first set your tripod up, you should "lean" on the tripod to make sure the legs are planted firmly on the ground. This is necessary as we are often setting up on grass or soft ground. By putting some weight on to the legs, we will ensure the legs are anchored well. To further stabilize things, it is helpful to press your face against the back of the camera and place your arm on top of the lens. For additional support, consider hanging your camera bag between the tripod legs to add a bit of mass to the system.
Wind is quite troublesome and will result in blurred images unless your shutter speeds are high (ie.1/250second). If you can't find a way to shelter yourself, then make sure you grip the camera and lens firmly and put some of your body weight on to the equipment. If your camera has a lens hood, consider taking it off or retracting it if the sun is not falling directly on the front element (the sun will cause flare, or fogging if it does). The reduction in surface area presented to the wind will help to reduce vibrations.
So what do we do if we are handholding? Unlike the Great Blue Heron standing stone still even in gale force winds, we wobble............a lot! To have a chance at sharp images, somebody came up with a "handholding" rule of thumb to help us. This rule tells us the minimum shutter speed that will be necessary for the average user to obtain a sharp image when handholding a camera lens. It states that the minimum shutter speed is found by taking the reciprocal of the focal length. For example, if you are using a 400mm lens, then 1/400mm or 1/400 second is the minimum shutter speed you must use in order to counteract the camera movement caused by you handholding the lens. Some people are sturdier than others and will be able to hand hold at slower speeds, but this rule serves to give us a reference point.
When handholding is the only option available to us, the best stability is obtained by pressing the camera back firmly against our face, supporting the lens from below and placing our elbow against our chest. If you are photographing a flying bird, try to pan or follow the motion by turning at the waist versus stepping. Additional support can be had by using a "gunstock" such as that produced by L.L.Rue at http://www.rue.com/gunstocks.html (this is another item which can be homemade fairly easily).
A major advancement for telephoto users was introduced a few years ago by Canon with their line of Image Stabilized lenses. This system utilized a moving lens element driven by motion sensing motors to redirect the light path so that the image hitting the film plane was stable (phew!). This system allows handholding at much slower shutter speeds than before. The first generation allowed the handholding rule of thumb to be modified by two "steps" or "stops". This meant that the same 400mm lens that required at least 1/400second to handhold could now be handheld at 1/100second and give the same results. (i.e. one stop or step is from 1/400second to 1/200second, another is from 1/200second to 1/100second) The latest generation of Image Stabilized lenses has allowed a three-stop reduction so that the same lens can now be handheld at 1/50second!
The greatest benefit comes when using these lenses with subjects that are relatively still, in low light or windy conditions, when using Teleconverters, or when shooting from a car.
In summary, there are several factors that contribute to making sharp images. Good equipment, while important, is not sufficient without good technique. Image Stabilization has been a great advance for telephoto users to reduce camera/lens movements, especially under adverse conditions such as windy days. It is important to keep in mind however, that even with technological innovations, proper technique is still helpful.
