When you look at the specifications of zoom and prime lenses, the most obvious difference is the aperture (how much light they let in). The widest aperture offered by most professional zoom lenses is f/2.8. If you search hard enough you will come across a small number of zoom lenses with a wider aperture but these are often extremely expensive or not suitable for 35mm sized sensors. On the other hand, prime lenses have been known to go as low as f/0.7 and most commercially available lenses are around f/1.8 to f/1.2. A fairly standard and inexpensive prime lens can offer 50mm f/1.4 and Nikon have recently improved their offering to include some really excellent wide angle prime lenses such as the new 20mm f1.8 and 35mm.The advantage of this? You can make use of a shallower depth of field and faster shutter speeds than are ever possible using a zoom.
Generally speaking, prime lenses are able to produce sharper, crisper photographs. In all fairness even the cheapest modern lens offers excellent sharpness, in fact it’s probably the last aspect you need to be concerned about. Having said that, the complex arrangement of lens elements moving backwards and forwards through a zoom lens inevitably effects the light travelling through which leads to inferior optical quality.
Pretty much all standard zoom lenses suffer from distortion; barrel distortion at the wide end and pincushion distortion at the long. This is far less of an issue with prime lenses which allows you to create far more natural looking perspectives. Another advantage of the prime lenses is the predictability of their behaviour, which means that, with the right software, you can make mathematical corrections that virtually eliminate distortion.
If you are shooting like me every day, weight makes a very big difference and zoom lenses generally weigh significantly more than the equivalent prime lens. For example one Nikon 24-70mm f/2.8 weighs just over 900 grams whereas a set of 28mm f/2.8, 50mm f/1.4 and 85mm f/1.8 lenses weights around 860 grams. That means three prime lenses weigh less than just one zoom on top of which you get an extra 1 stop of light and they are less expensive, all of which makes a pretty compelling case for the prime. If you think weight isn’t an issue,trust me, if you are shooting all day you will definitely be thankful that your lens weighs 300 grams rather than 900 grams.
As I have illustrated, you can buy three f/1.8 prime lenses, which cover the same range as a single f/2.8, zoom for less money. At professional level, you will pay a similar price for an f/1.4 prime lens as you will for an f/2.8 zoom however, in my view, the extra 2 stops of light that the prime will let through and the enhanced sharpness still make the prime a better choice.
Aesthetic - Bokeh
The term Bokeh (pronounced bo as in toe and ke as in kettle) refers to the aesthetic of the out of focus elements in a photograph. The larger apertures which prime lenses offer create a smoother and less distracting background. For the professional photographer, this is one of the most important advantages of a prime lens over a zoom.
Different focal lengths offer you different perspectives and these are a very powerful element in your photography. You can make use of perspective to exaggerate scale of to control the relative size and proportion of the different layer or elements in a composition. Over-reliance on using a zoom leads to a temptation to stand still use the lens to achieve the crop that you want. What I have come to realise is that using a prime lens creates a real understanding the importance of focal lengths. It isn’t about the distance from your subject or the crop, it’s about choosing your perspective first and then physically moving to achieve the specific crop that you want.
I eventually reached a point in my career when I found myself on a photo-shoot without a single wide-tele zoom lens and that was quite nerve wracking; it shouldn’t have been. The first shoot that I did without a zoom lens went incredibly smoothly. True, I probably walked an extra mile but that was because instead of standing still, I was actually moving around looking for, and finding, those really unusual angles. After that first shoot, there was really no going back for me… I was officially a photographer in his prime.