How Zoom (Variable Focus) Lenses Work

How Zoom (Variable Focus) Lenses Work

by William Lulow

As a photography teacher, I am asked fairly frequently about zoom lenses and more specifically, why people can’t seem to get really tack sharp images from them. Well, let’s start from the beginning and see if we can find out why not. First, a zoom lens is a “variable focus” lens. That means that it has some movable elements inside it that allow it to focus on near subjects and far away subjects both, depending on how much you can separate the elements. If you can rack out your lens fairly far, then distant objects will be able to be seen larger. If you can shorten the distance between lens elements, then the lens can include more of the scene. So, it has to do with the lens elements and how far apart or close they are to each other. The closer they are, the more of the scene they can cover. The further apart, the more they can enlarge distant objects. Now those elements moving around could pose a problem depending on how well made the lens is. But, certainly not all the time.

Okay, so we now know what zoom lenses are and a little about how they work. But let’s delve a bit deeper. Here is one example of a good zoom lens:

This is a Canon 70mm-200mm f/2.8 lens. It retails for about $1,800. But what can it do? Well if you stand in one spot the 70mm setting will allow you to make a good portrait. That is, it will let you pretty much fill the frame with a person’s head, if you are about 15 feet away, for example. From the same position, if you zoomed the lens out to its 200mm setting, it would allow you to fill the frame with that person’s eye! Standing in one spot, you could vary your composition almost at will without doing anything except racking the zoom feature on the lens. This is great if you don’t want to do much work and just want to stand there and get a couple of different sized images of your subject. (You will, of course, alter the entire composition by “zooming” the lens in or out). It might also be a great lens to use if you shoot sports, for example, where the distance from camera to subject is always changing and camera might have to remain in the same position.

The problems with these lenses start when photographers ask too much of them. The above lens is expensive mainly because of the quality of the glass but also the size of the glass elements. They are capable of transmitting a lot of light. So the speed of the lens (f/2.8, very fast) is maintained throughout the entire zoom range of the lens. Here’s another zoom lens:

This is a Canon lens as well, but notice that it is marked “1:4-5.6” on the lens barrel. This means that the lens when used at the shortest focal length (75mm) will only open to f/4 (Fairly slow) and, if you racked this lens out to its 300mm setting it would only be capable of f/5.6 (very slow).  Now since this lens is made by Canon, you can be sure that it is of very good quality. But because the lens barrel is so small and the glass elements are small as well, it would not be able to render a very sharp image. Nor would it function well under less than normal lighting conditions. This lens retails for less than $200! It is Canon quality, if you will, and should function well under most “normal” lighting conditions, provided there is plenty of light. If you could shoot a subject and use an aperture of f/8 @1/200th of a second, say, you could come up with a sharp image. But if you had to use this lens wide open, you would most probably wind up with a soft image.

So the moral of this particular story is that you have to be careful when you buy a zoom lens. You need to get the best quality and size lens that you can afford.

The other part of the story that is of equal importance is what kind of photography you do with a zoom lens. If you are constantly moving about a room for instance and you cannot always pick a good place from which to shoot an event, then having a zoom lens can make your life easier. You can simply adjust the zoom to get the composition you want. If you are doing landscapes, on the other hand, you can always or most always pick the spot from which you shoot and so a zoom lens is much less needed.  Many photographers are lazy. They don’t want to keep changing the position from which they are shooting, nor do they want to go to the trouble of switching lenses. So they like to rely on zoom lenses. Also, cheap zoom lenses, as I mentioned, do not carry their speeds through all their zoom settings. So if you have your camera on AUTO, the computer will compensate for the lens’ lack of speed by slowing down your shutter speed or adjusting your ISO speed. Your resulting images will not be really sharp due to these settings, many of which you may not be aware. If you are armed with this information and can adjust the light levels accordingly on your subject, that will solve the problem. But if you simply set your camera to AUTO most of the time, you would be giving up control over your exposures, and you will wind up wondering why your images are not sharp.  In addition, a cheaper lens, even though it is made by a reputable manufacturer, is just not as good as an expensive lens by the same maker.

Again, if you are lazier than most and you want a zoom lens so that you won’t have to change camera positions or lenses, get a really good zoom lens. Also, if you are traveling and want to cut down on the amount of gear you are taking, you might think that a zoom lens is the thing to have. For my money, zoom lenses just can’t compete with fixed focus lenses in speed or sharpness. So another solution would be to have two camera bodies of the exact same make. (The reason you want to do this is because various settings are in different places on cameras even by the same manufacturer. When you’re switching from one to the other, you don’t want to have to think about where the settings are on each). Yes, it will add a bit to the load, but regular, fixed focus lenses make you think more about your compositions and because of that, you will probably wind up with better quality images.