How to Choose a telephoto for my Digital Camera

When using or purchasing a telephoto, it's convenient not to forget some concepts. Although the first virtue of a telephoto is amplifying the image, not for this is a 300 mm telephoto better than a 100 mm. The telephotos have more complications the greater the focal length.

The first problem is the risk for moved pictures, with the camera in hand, rises dizzily. Since the angle of vision is reduced, a camera oscillation produces great movement of the elements in the frame. The precaution to adopt is to use a higher shutter speed (2.3). As a practical rule (for 35 mm) the minimal speed used is the closest or coincides with the focal length: therefore for a 135 mm it isn't convenient to use times shorter than 1/125 of a second; for a 300 mm we won't go below 250, etcetera.

The second complication is the luminance (7.1.8). To maintain a good luminance as the focal gets larger, we'd also have to increase the diameter of the lens, which implies greater optic complexity and higher weights. In practice, the manufacturers usually forget about maintaining high luminance, and therefore, the longer a telephoto, the lower it's luminance is usually. The heavy and expensive ultra bright telephotos are only advisable for sport and theater photography , or for "photographic hunting", were a large aperture is vital.

The third factor to have in account it the minimal focal distance . There is a practical limit for the minimal focus distance of a lens. If the sharp focal planes correspondent to the different distances are more separate to each other, the greater the focal (7.1.7), the focus mechanism of a lens will have to displace the lenses in an equivalent measure. For short distances this supposes such a complexity that the focus mechanism that, in practice, it's expected that the longer a lens, the greater the minimum focus distance will be. For example, we can expect that a 100 mm is capable of focusing at 90 cm but we would hardly have this possibility with a 300 mm.

If we have in account these three factors, it turns out that the complications coming from using a telephoto will be greater, the longer the focal length is. The same goes for the considerations of price, volume and weight, acquiring a 300 mm only because it is more "powerful" and then using it for senseless portraits. Effectively, it's lower luminance and the need of using higher speeds force us to use high sensibility film, with the resulting loss of quality and contrast; add to that the focus difficulties that (low depth of field and a low brightness in the viewfinder), which at short distances (portraits) are maximal, and we will have a 300 that will be a source of problems an will spend most of it's time in the case. If we want to make portraits, a short telephoto covers much better our needs and minimizes complications. Another very different thing will be acquiring a 300 for photographing nature for example. In this case, the increase is vital to photograph animals and other inaccessible things, in who's case the lens is usually focused at great distances. The complication are the price that we have to pay to reach the results we want: therefore the senselessness of using a short telephoto, since the increase won't be sufficient.