The Objectives – Fixed Focal
One of the major discriminatory elements with regard to the objectives, it is definitely the focal length. Variable focal length lenses means a greater number of lenses, of mechanisms (for zooming), a lower brightness (for the greater path the light has to do to arrive at the sensor), a considerable weight and a greater predisposition to the aberrations (always for the greatest number of lenses present in the optics.

Among the different marketing objectives, however, we find the focal length lenses. These are objectives that can cover a focal one forget the zoom ring But because a photographer should choose a fixed focal lens rather than a zoom There are various reasons that can be considered.

The first is the price a single focal length lens is made from a limited number of lenses and mechanisms with respect to a variable focal lens. The direct consequence is a reduced price less material, less manpower, less construction time and greater design simplicity.

The second reason is the weight having fewer lenses, a prime lens will weigh much less than any varifocal lens. If we compare the two Canon 18-55mm and 50mm the difference is notelvole first graduation weighs around 200g and the second stops the scale at just 130gr. Difference that tends to worsen with increasing focal length, with gains that can be equal to two thirds for tele zoom (of course do not expect that a 400 mm is a featherweight!).

A third reason is the increased sharpness of fixed focal length lenses than those with variable focal. The reasons of greater clarity are mainly two the simplicity of construction allows the use of different and more efficient solutions (particular shapes of the glasses, additional treatments) and the number of lenses is less than variable which allows to drastically reduce reflections and refractions. A fixed focal length lens will beat any zoom lens of entry level, such as the lens bundled with most of the SLR. The same objective will be able to compete, always on the sharpness (and of course also on color rendering), also with professional zoom.

Do not forget the brightness for the same reasons listed above, a prime lens will be much brighter than a varifocal lens and you realize this difference simply by reading the values ??of f of these objectives. A prime lens of average price will normally have an f  1.8 up to go to f  1.2. In the three photos below are the three objectives Nikkor 50 mm respectively to f  1.8, f  1.4 and f  1.2.