A prime lens is a lens with a fixed focal length, so it's pretty much stuck at say 200mm or 50mm. There's a whole range of prime lenses at different focal lengths. Zoom lenses come in a variety as well, from some that can only zoom a short focal distance, say 18-55mm or 70-300mm to the more impressive 18-200mm zooms. The question remains, would you want to have a set of good prime lenses or a just one good zoom lens? To test the theory out, we decided to grab three different lenses, 2 prime and 1 zoom lenses and take some photos just to compare them.

For the zoom lens, we picked the Sigma 18-200mm F/3.5-6.3 and for the prime lenses, the Sigma APO 150mm F/2.8 Macro and the Sigma 30mm F/1.4 EX DC HSM. Since both prime lenses are faster than the zoom lens, to make thing a little fairer, we'd pick an aperture that all three could handle and just take photos of the same thing and see if we could see a difference. We picked up a D300 and took them all out for a spin.

It was a bright sunny day, and with so much light outside, the question of comparing apertures would be hard, but there's more to image quality than aperture. Buildings are easy enough to take pictures of, they don't move and often the architecture and design makes them interesting subjects. We set the apertures at f/5.6 and compared the 30mm with the 18-200mm lens set at about 29mm, which was as close to 30mm as we could get. Both images seem remarkably similar except that there is only the slightest hint of barrel distortion with the zoom lens. The building seems to bulge outwards towards the bottom of the picture and the image from the zoom camera is just a shade darker, but then the light does have to travel further down the barrel to reach the sensor.

The same can be seen when comparing the 18-200mm with the 150mm lens. The aperture was set at F/8 and then pointed to the building. The difference here was pretty minimal, with hardly any difference between the two photos. The barrel of the 150mm lens was similar to the 18-200 and barrel distortion was hard to detect when the lens was zooming so close to the building.

The real test for the 150mm and 18-200mm was a macro image. We set the aperture at f/8, and then took a picture of a statue mounted inside a makeshift cave. It was much harder for the 18-200mm camera to capture details within the shadows, while the 150mm lens took a brighter image. That said, there was only the barest difference between two images taken with the zoom and the 30mm lens.

Prime lenses have less moving parts and lens elements. This means that they are lighter, and less complicated so have less chance of distortions and better image quality. They also have the advantage of being faster, that is to say they have bigger apertures. While most zoom lenses have apertures that wont go any lower the around f/3.5 or for more expensive lenses f/2.8, prime lenses can often go as low as f/1.4. The difference from one f/stop to another is like halving or doubling the amount of light that passes through.

The one thing that these photos say very clearly, is that each prime lens has a sort of specialised use. Smaller focal lengths are often very good for landscapes while anything in the 50mm area works great for portraits. Lenses like the Sigma 150mm we used, would be good for macro photography. You can get prime lenses at 200mm or 300mm to take great shots at night portraits.

When compared to zoom lenses, the prime lenses will take better images, according to the task it was made for. If you are serious about taking portraits of people or landscapes or just macro and you don't mind moving in closer instead of turning the zoom ring then prime lenses are a good idea. But the zoom does trump the prime lenses in versatility if nothing else. At the end of the day if you are keen on specialising in one type of photography then a prime lens or two is a good investment. If you are keen on travelling and need portability then invest in a good zoom lens instead.