The most requested information about photography, after someone is ready to proceed in getting a little more serious about their images, is lens selection. I remember wondering the same thing when I started having a little money burning a hole in my pocket.
Camera lenses do only two things: control the amount of light entering the camera, and focus the light entering the camera. The cost of a lens depends on how much light the lens will let into the camera and how well it focuses that light. The method of how the lens measures the amount of light entering the camera is similar to how the pupil measures how much light enters your eye. When it’s dark our pupils dilate (open) to let more light in our eye so we can see better in dark situations. Similarly, when we go outside in the bright sunlight in the snow our pupils constrict (close) so the light doesn’t hurt our eyes and we can see better in those bright situations. The “pupil” of the lens is called the aperture or f-stop.
The numbers which reference the aperture are often confusing for beginners; the big number is a small opening of the lens and a small number is a big opening of the lens. The reason for that is that the numbers, which are denoted as whole numbers, are really not whole numbers. The “f” means factor. It is the factor of the size of the sensor (film), the focal length of the lens, and the measured amount of light entering the lens. Although not totally correct, it is sometimes easier to remember that the f-stop numbers are fractions, like f4 is a lens that is 1/4 open and f 16 is a lens that is 1/16 open. Something that is ¼” open is a bigger opening something 1/16″ open. Think of your turning the faucet on your sink. Do you always want it open full blast or do you sometimes just want a little water?
So you’re looking at lenses in a catalog and you see a bunch of numbers that don’t make any sense (for example: 18-200 f/3.5-5.6). The first numbers are the focal lengths of the lens (a 50mm is often thought of as a normal angle of view lens for 35mm film or sensor photography, what your eye sees as a normal view). Any number higher than 50mm will be a narrower angle of view, and any number smaller than 50mm will be a wider angle of view. So the 18-200mm is a zoom lens that includes a range of angles of view from wide angle (18mm, wider angle of view than your eyes see) and including focal lengths up to telephoto (200mm, a narrower view of what your eye sees). A telephoto lens brings things closer, whereas a wide-angle lens pushes things back.
Back to the numbers; the second set of numbers are the widest open the lens will be at the different focal lengths. f3.5 is the maximum (largest) aperture opening at 18mm and f5.6 is the maximum aperture opening at 200mm. As the photographer zooms the lens from 18 to 200, the lens gets longer. The lengthening of the lens decreases the relative size of the aperture. You will notice that telephoto (longer) lenses will have smaller (bigger numbers) maximum opening than wider angle lenses. The reason for that is that longer lenses need to be bigger around to let more light in. The bigger around a lens is, the larger the lens elements need to be and the more expensive the lens becomes. It’s all a trade off.
Take another example from the catalog: a 50mm f/1.8 costs $114 and a 50mm f/1.2 costs $1,619. What!? The second lens is about 16 times more expensive than the first. Why would you ever want to spend so much money on the f/1.2 when both lenses are 50mm and why would you need to? Well the f/1.2 lets in 4 times more light than the f/1.8, and if you regularly photograph in very dimly lit situations it’s worth it. Because of optics, lenses are generally sharpest in the middle of the aperture range. But lenses that are considered fast (large maximum opening) are designed to be sharpest at their maximum opening because those who buy those lenses want them to be sharp at the aperture they use most often. Another reason someone may want a “fast” lens is its ability to have everything that is not in focus blurry. Perhaps you’ve noticed this effect in films. The camera focuses on one person as they speak, then refocuses on another more distant person when they start to speak. You can also see this effect in this month’s photo, which was taken in the suburbs of Molt, Montana. The lens was a 180mm and it was set on f2.8 or wide open. Only the fence post and bird are in focus, everything else is out of focus. A photographer uses this effect to draw the viewer’s eye to one specific point of interest, which in this photograph are the fence post and the bird.
So what lens should a person buy? The most expensive one they can afford. When I was starting out, I was young and poor, and zoom lenses were not as good as they are now. I photographed with a 50mm normal lens and soon bought a 28mm wide angle because I spent a lot of time in the mountains and I wanted a wider view. Lenses that have only one focal length are sometimes called prime lenses, and are generally sharper than zoom lenses at the same focal length. I also think that many times zoom lenses make a photographer lazy because a certain image usually demands a certain focal length lens and a photographer may not notice what focal length is best. Many photographers compose and shoot a photograph without even thinking about what is included in the image. They also don’t tend to move much for a different perspective or angle because they shoot a variety of shots from the same place at different zoomed settings. A good photographer is usually someone who can see an image and capture it with their camera with little guess work.
What is the best focal length lens to buy? It depends on the type of shooting you will be doing. If you love wildlife like Merv Coleman and Dick Forehand, you’ll be shelling out over ten grand for a 500mm or 600mm. Now their photographs don’t seem very expensive. If you want to do weddings you’ll probably need two zooms on two cameras. A wide angle zoom and a telephoto zoom. You won’t have time to change lenses. If you like to do quality landscape photos, start getting prime lenses. By using prime lenses you begin to understand the benefits and drawbacks of each focal length, and you’ll soon know what lens to pick for each shooting situation. I think using prime lenses starting out makes you a better photographer because their limitations force you to see a potential image.