The Zoom Lens

Note: I did an article a few months ago about digital zoom lenses and why I don’t really like them and consequently, don’t use them. Here, again is a further, in depth explanation.

The Zoom Lens

by William Lulow

Camera retailers want to sell cameras, so they try to make it easier for first-time buyers to get into taking pictures by offering what is referred to as a “kit” which consists of a camera, lens and maybe a lens cleaner package to get them started. These “kit” lenses are normally zoom or variable focus lenses to cover a wide range of picture-taking opportunities for beginners. They are not great lenses because their glass is limited in its ability to transmit light and there are other qualities that make them inferior as well. These lenses are also fairly inexpensive so that the retailers can offer them at somewhat reduced prices if purchased in this kit form. After a while, users of these lenses begin to notice their limitations. But, they offer camera retailers a chance to get their customers up and running toward taking pictures, quickly and for a reasonable cost. As a consumer, you have to keep in mind that “you get what you pay for” is an operative caveat here.

This doesn’t necessarily mean you can’t get good pictures with the kit lenses, but you have to know how to do it. Most amateurs (folks who take pictures occasionally, because they love to) can make do with these lenses. But what happens is that these people will think that a zoom lens is great because it seems like it can do the job of several prime, or fixed focus lenses. Sometimes they actually can. The problem comes when these “week-end photographers” begin to think that these lenses can do it all. They can’t! They have their limitations.

The “kit” zoom lenses consist of a barrel which holds the actual pieces of glass inside it and maybe as many as five or six (or more) lens elements inside that barrel, each designed to function with the others to provide the correct focus of light rays on the digital sensor (or film plane) to make an image. Any lens works this way, but the zoom lens has movable lens elements that allow it to focus on near and far objects both, without changing the lens. It seems very handy to have this ability when you are out taking pictures. You don’t have to carry other lenses with you and it greatly reduces the amount of “gear” you need to carry.

So, the amateur doesn’t think about it much. If they want to fill the frame with just someone’s head and shoulders, they just rack out the lens until the proper composition is achieved. What they don’t think about is that once they rack out the lens to its “telephoto” setting, it now becomes a telephoto lens with all of the characteristics  telephoto lenses have.  You should know about them before you use them. Here are some:

1. Telephoto lenses often have much smaller apertures than normal lenses. (They rarely can open as wide as f/2.8 or f/1.8) because the size of the glass needed to carry that speed would be way to big, heavy and expensive.
2. Telephoto lenses have a much longer barrel because the elements are further apart.
3. A longer lens barrel means that light takes longer to reach the sensor than with shorter lenses, which is why they are deemed “slower” and have smaller apertures.
4. Telephoto lenses tend to compress actual distances in the real world. In order to make objects appear larger on the sensor, the lens elements need to be further apart.
5. Telephoto lenses therefore, have many more movable elements than normal lenses.
6. Telephoto lenses, because of this distance compression, have much less depth-of-field than normal lenses. In other words, the area of the image that is sharp will be less than with regular lenses.
7. Because telephoto lenses are larger, they are more difficult to hold steady. You may need to use a tripod to get really sharp images.

All of these reasons may make for unsharp images. If all these things were too much to think about for the average picture taker, here’s something else: the “kit” lenses that are often bundled with a camera you buy, are not the best zoom lenses on the market. They often have designations that look like this:

1:f/2.8-3.5 or 1:f/3.5-5.6

What this label means is that when the lens is set to its wide angle setting, its relative speed (the widest aperture) will be f/2.8. But when it is on its telephoto setting, its speed will be only f/3.5, a full stop less! In the case of the lens that says f/3.5-f/5.6, it means that in telephoto mode, the widest aperture is a full two stops less! That’s a lot when you are using a longer lens. What this means is that you will need more light in order to make an acceptable image, all things being equal. If you are shooting mostly in AUTOMATIC mode, the electronic chip in the camera will adjust the other elements of exposure so that a proper one will be achieved. It may increase your ISO setting or lower you shutter speed in order to compensate for the overall lack of speed of the lens. These compensations will make for a good exposure, but other parts of the image might suffer, like sharpness. This is something of concern.

If you look carefully at the markings on this lens you will see it says “1:4-5.6” on the inside there. This means that when this lens is set to its 55mm position, its widest aperture is f/4. But when it is used at its 250mm (telephoto) setting, it loses a full stop in speed and thus only opens to f/5.6. Now, if you are shooting in bright sunlight, this may not make that much of a difference. Take the lens indoors and try to shoot in low light and you will be hard pressed to make a sharp image.

This is a relatively expensive zoom lens:

This zoom lens carries its f/2.8 maximum aperture through the entire range of settings it has. This lens costs $2,000.

I have had any number of students who have complained about the quality of the images they get with their “kit” lenses but don’t know why this is happening. I hope I have shed a little light on the use of these lenses and why the results from them are not as good as you might expect. I always recommend that my students begin with one fixed-focus (prime) lens and work with it until they understand how it operates and what kinds of pictures they get from it. It is best to start with a wide-angle or slight wide-angle lens. There are several reasons for this:

1. Wide angle lenses have a wider coverage. They can include more of the scene.
2. Wide angle lenses have greater depth-of-field which means more of your image will be in focus.
3. Prime (fixed-focus) lenses are generally better quality ones.
4. Wide angle lenses make you think more about the images you wish to make.
5. Wide angle lenses are smaller and much lighter to carry around with you.
6. If you want an object to appear larger in the frame, you will have to get physically closer to it.

Each lens you might use has its own purpose and “reason for being,” so to speak. That’s why manufacturers offer so many different ones. There are telephoto lenses that focus on one part of a scene bringing it closer and rendering it larger. There are “macro” lenses that can focus on one object and render it almost the same size as it is in real life. It can allow you to focus sometimes as close as a couple of inches away. There are “fish-eye” ultra wide lenses that can encompass almost 180 degrees of a scene. And, there are those in the “normal” range that tend to render the objects or people we see in the world, much as they are in the correct proportions. The reason that many professional photographers have different lenses in their arsenal is because they often are faced with many different kinds of demands from clients for photographs that only special lenses can produce.

There is no one lens that can do it all, yet that’s what many people expect of their zoom lenses. Those who are truly interested in learning how to make better pictures should study what each focal length lens does, how it works and what its limitations are. I often include this information right here in my blog pages.

Here is an image actually made with my wide-angle lens:

This image was made with a Canon 20mm f/2.8 lens on a Canon 90D (cropped sensor) body. Notice how it can focus on near and far objects in the same frame. It’s really an ideal lens to carry around.

The takeaway from this article is that zoom lenses are sometimes a good compromise when you are in a pinch and cannot physically get closer to your subject and you need the different focal lengths to complete your assignment, but they are not the “one lens” solution to your various composition problems.