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Having said this, I have to make a clarification: in digital photography, there are cameras with sensors of various sizes. As a reference, we use the size of analog camera film from back in the day. It is for this reason that the full frame sensor corresponds to the size of the film, 35mm (35mm is the length of the larger side of the rectangle). In 2012, only professional digital cameras have these large sensors due to their cost. All reflex cameras, semi-professional or entry level ones have smaller sensors. With a smaller sensor, the normal lens varies its visual angle thus reducing it.

Therefore, a normal lens mounted on a reflex camera with a sensor smaller than 35mm will become a modest “telephoto” lens. This is important because every lens is sold with a focal length in millimeters using 35mm as the standard. If you have a camera with a smaller sensor, this should be taken into consideration. The dimensions of the sensor are standard, however, but, in the case of digital cameras with small sensors (such as the APS-C format), the size of the sensor requires us to multiply the value of the focal length by 1.5. Therefore, we can again summarize that the lenses with focal lengths of 50mm have a visual angle near to that of the human eye and are called normal while those with a focal length under 35mm have a greater visual angle and then are considered wide-angle and those between 100 and 200mm are light telephoto lenses. Those over 200mm are true telephoto lenses. We need to realize that a camera with an APS-C sensor, in order to have a visual angle equal to 50mm, must use a 35mm one that, when multiplied by a factor of 1.5, actually corresponds to 50mm.

Variable focal length: zoom length

Up to now we have talked about fixed focal length lenses. In order to include a detail on the edge of the landscape of the photo or to exclude it, we will have to either get physically closer or farther away. 

However, there also exists a variable focal length lens called “zoom” (figures 05 and 06).

A typical example can be an 18-105mm or a 24-120mm as in figures 05 and 06.

This caption makes us immediately understand that the lens is a zoom one because I have two different numbers instead of one. The first indicates the minimal focal length while the second one indicates the maximum. I can use all the focal lengths in between by simply rotating the wheel (figures 05 and 06) without interruption. Varying the focal length generally varies also the length of the lens (figure 05 and 06) but there are also some internal zoom lenses where varying the zoom does not cause any changes outside the camera. The focal length is always a little shorter than the length of the lens for reasons of construction. A 50mm lens will be about 5 cm and little more. The zoom lenses are very versatile and using them is like being able to make use of numerous lenses without having to mount or unmount different ones.

Undoubtedly, they are easy to use and represent the easiest solution by taking money saved into consideration because we purchase just one lens instead of many. However, this is not the best solution for those who require the best quality, and we will understand this better later on when we discuss other characteristics of the lenses.

With a zoom lens, we can include or exclude details in the photo simply by rotating the wheel and without having to move. We should all be asking ourselves how many millimeters when talking about zoom lenses. Everyone is always wondering, what is the "x" zoom factor of a zoom lens.

This question does not make much sense, but, for the moment, I will limit my response by stating that the “x” factor of zoom on a lens are derived by dividing the maximum focal length by the minimum. In the case of 18-105, we get a zoom factor of 5.83x. Therefore, I have a zoom of 6x. This question does not make much sense because with the zoom factor people want to know how much the lens in question can zoom in on a particular detail. It does not make a lot of sense, because I could have a zoom of 6x also on a zoom lens of 50-300mm: it would zoom by the same factor as 18-105mm. Now that you have a better idea, you will immediately understand that we would be speaking two different languages. When we talk about zoom by the same factor, we are talking about different initial sizes. It does not make sense to talk about the zoom factor in professional photography: it is only to understand how much the focal length varies in a zoom lens.

Many take for granted that a compact digital camera (at its minimal focal length) sees what the human eye sees and from there begin to think that the camera is able to zoom in by 3x, 5x and so on. This is not correct. Usually, compact digital cameras start from a wide-angle (figure 04a and 04b).

Turning instead to the interchangeable lenses of the reflex, a fixed focal length lens of 50mm has a zoom factor of 0x.

Then the focal length of a lens is important for the type of photography that we have to perform. If I like ample panoramas, I will tend to shoot using a wide-angle lens or a zoom lens that has wide-angle capabilities. If I like the details in the distance, animals that I can’t get close to, a sport being played at a distance, I will opt for a telephoto lens or a zoom lens that at least starts from a minimal focal length of 50mm. For portraits, you would tend to remain in the range of lenses between 50 and 135 mm, but there is no fixed rule: for example, I love taking portraits with a focal length between 200 and 300 mm.

At any rate, on a zoom lens, the greater the variation in the focal length (that is, having factors of zoom: 1x, 2x, 3x, etc.), the greater the loss of characteristics that influence the quality. In order to obtain the maximum quality, we would have to have many fixed focal length lenses rather than fewer zoom lenses. But everyone searches for their own compromise between versatility, savings and quality. Generally, zoom lenses with a great zoom factor are not very bright and if they are, they cost a lot. There is often a loss of quality and clarity and issues with the minimum and maximum focal lengths. They also cause geometric distortions and weigh more. I will give you an example. If we like landscapes, portraits as well as zooming in on details every so often, it would be better to have a focal length between 20 and 200mm. If instead we like to photograph birds and animals as well as sports photography from a distance and make use of other types of shots less frequently (such as portraits and landscapes), we could get by with a focal length between 50 and 300mm. In both cases, we can choose various solutions to cover the entire range. In the first case, there are some varifocal lenses with a great zoom factor like 18-200mm. With this type of lens, we can cover the entire range.

Otherwise, another solution would be to buy a 20mm, 35mm, 50mm, 105mm, and a 200mm lens, which would cover a large part of the focal requirements one might need. In the first case, we spend less, we do not have to change lenses, we have a total weight that is lower, but we most certainly lose a bit of quality and brightness to such an extent that, in some cases, it is necessary to use a tripod. In the second case, the weight goes up but if we bring with us only the one lens we need to use, the weight is less of a factor. We have to spend more and we lose some convenience, but we gain in terms of quality. In some instance (but not all), where we would have to use a tripod, we can allow ourselves to shoot freehand with a brighter, fixed focal length lens. These are two extremes, but the point is made. In the reality of a photographer who wants an optimal compromise, the solution would be that of buying a 18-70mm lens and a 70-200mm one. Therefore, it is important to understand the preferred subjects of your photography to help us to determine that a very bright, fixed focal length lens might be preferred if we would use it heavily. For example, if we shoot a lot more portraits, we would buy a 135mm lens with an elevated brightness and use the zoom lenses only when needed.