Prime and Full Size

writer: russell j.t. dyer; posted: jul 2009; revised: oct 2017; readers past month: 861

When deciding on which lenses to purchase, choosing lenses that are most versatile (i.e., zoom lenses) and cheaper (i.e., lenses for small frame sensor cameras) is common. In general, for many amateur photographers, this is the way to go. However, if you want to become an advanced amateur photographer or might even want to become a professional photographer, you might want to consider the the less versatile, more expensive choices.

Prime Lenses

There used to be a time when there weren’t zoom lenses. You would have to buy a separate lens for each focal length. Zoom lenses are very practical: one lens can take the place of two or more lenses. This not only lightens your camera bag, but it also can save you the cost of buying multiple single focal length lenses, or prime lenses. For these reasons zoom lenses are popular and because of the first reason, it’s why I have zoom lenses. Nevertheless, prime lenses do have their advantages over zoom lenses. To design and construct a lens that can focus at every point of its focal range requires more engineering and more inner workings. Conversely, if the engineers that designs lenses know that the lens is to only have one focal length, they can put more of their genius and your money into making a much better lens for the one focal length. It’s as simple as that.

To understand the value of a prime lens, let’s look at two photographs taken of the same scene, with the same settings, but different lenses: one with a zoom, the other with a single focal length lens. The two photographs on the right were taken in Riga, Latvia. It was a cold night with very low humidity and no moon overhead. All light was from the city lights. I used a Canon EOS 40D digital SLR camera. I didn’t have a tripod, so I rested my camera on a railing along the river walk. Both shots were taken with identical exposure settings: the aperture was set at f/11; ISO set to 200; and the shutter duration was at 30 seconds. Trying to take a photograph at night with the shutter open this long and without a tripod is extremely difficult. It took several tries to prop the camera on my camera strap so that it was positioned right. Plus, I had to use the timer so that I didn’t shake the camera when pressing the shutter release. I tried using a high ISO sensitivity setting, but I had problems with noise — spots on the sky section of the image. A higher aperture would have helped too, but I had problems with the image’s clarity. Basically, these exposure settings gave me the results I wanted. But the prime lens gave me a better shot.

Same Shot with Two Lenses

Lens: Canon EF-S 17-85mm;
Focal Length: 78mm; Aperture: f/11;
ISO 200; Shutter: 30 seconds

Lens: Canon EF f/1.8 85mm USM;
Focal Length: 85mm; Aperture: f/11;
ISO: 200; Shutter: 30 seconds

The first photograph was taken using a zoom telephoto lens, a Canon EF-S 17-85mm lens. The focal length was at 78mm. The second photograph was taken using a prime lens, a Canon EF 85mm f/1.8 USM. The focal length is, of course, 85mm — its only focal length. So the only difference between these two shots — other than the first is angled lower and shows more of the river, while the other is angled higher and shows more of the sky. — is the lens. Given that all other factors are the same, look at the results of a zoom lens use for the first and a prime lens used for the second. In the first photo, the windows and the clock on the tower are blurry. They’re much clearer in the second shot. To see larger images, click on each photo. For a side by comparison, I’ve provided a <page_link section=’tutorials’ alias=’’>close up of each</page_link> (the zoom shot is on the left, the prime shot on the right).

Full Frame Lenses

Looking again at the photographs of the Riga skyline you’ll notice from the settings that I listed, the focal length of the top one was at 78mm. The shot for the bottom one taken with the prime lens was shot at 85mm. Despite the slightly different focal lengths, the angle of views are almost the same. However, if you look closely that the skyline is wider with the zoom lens: it takes in more on both ends of scene. It may not seem like much at first glance, but it’s more than for which can be accounted by 7mm in focal length. The difference is related to the image sensor in the camera. The Canon EOS 40D doesn’t have a full-sensor. Only the professional cameras like the Canon 5D Mark II have the full-sensor. A full-sensor is basically the size of an image or negative in an old 35mm film camera. The size of the sensor can be a factor when using less than full-frame lenses.

Again, the first photograph was taken using a Canon EF-S 17-85mm lens. This lens was made for the smaller sensor. The nomenclature, EF-S basically stands for Electronic Focus-Small sensor, or something like that. The second shot was taken with a Canon EF 85mm f/1.8 USM lens. Notice that it’s an EF lens, not an EF-S lens. It’s made for a full-frame sensor camera. When used with a smaller frame sensor, the edges are chopped off as you see in this photograph. This may not seem like much of a difference (or very noticeable since the focal lengths aren’t the same), but consider the potential of each lens: in a small frame sensor camera, the Canon EF-S 17-85mm lens set to a focal length of 85mm has a maximum diagonal angle of view of a little more than 18 degrees. Whereas the Canon EF 85mm f/1.8 USM has a maximum angle of over 28 degrees. So by design, the photograph taken by the prime lens should show much more of the skyline and not a little less the zoom lens I used.

On the other end of the focal length, the same zoom lens when at a focal length 17mm has a maximum diagonal angle of view of 78 degrees. The prime lens, Canon EF 20mm f/2.8 USM can photograph a maximum diagonal angle of view of 94 degrees. This is a much wider. To see how the small frame sensor will chop off the edges of an image at this shorter focal length, see the two photographs compared in the article on Understanding Exposure Settings. The photograph on that page that was taken by the Canon EF-S lens is wider since its maximum angle ratings are based on a smaller sensor. The maximum diagonal angles each of the three lenses I mention here are based on them being fitted on a respective camera for which they were made.

In light of this, one might argue that if you own a small sensor camera, you should by lenses made for small sensor cameras. This is reasonable if you plan on always owning a small sensor camera and have no intention of ever moving up to a full frame sensor. However, if you become more interested in photography and find yourself thinking about buying a full frame sensor you may wish you hadn’t bought any lenses for your small sensor camera. On Canon cameras, the EF-S and the EF lenses both work on the small sensor cameras. However, only the EF lenses work on Canon’s full sensor lenses. The EF-S lenses won’t even physically connect to the professional models. If they did, your photos would have a black border around them. Nikon lenses are all made for full sensor cameras. So if you prefer Nikon, you won’t have this problem — or rather, by default you are making the choice I’m suggesting to Canon enthusiasts.

While I agree with the adage of using the right tools for the job, if you know you’re going to be upgrading to a full-frame sensor camera and don’t intend to keep your old camera, you might want to buy only lenses made for that tool. The cost of moving up to a full-frame sensor camera won’t be as great if you can use your existing lenses. If you have to replace your camera and your lenses, it might make it unnecessarily difficult, if not prohibitive for you. It would be better to accept that your images won’t be as wide as they could be than to try to sell your old lenses and buy all new lenses.