Do you think that using an 18 mm lens or a 100 mm one will only zoom in or out your scene? Why are camera lenses are measured in millimeters? Do you know what those measurements mean for your photo? If you’re not sure which lens to use and why I invite you to keep reading and learn about focal lengths and how to use them.
The most common consideration when choosing your lens is whether or not you need to zoom in or zoom out. Therefore logic dictates that you would use a wide-angle lens for landscape photography and a telephoto for a detail of that landscape. Another well-known factor is the distortion of wide-angle lenses, so for example, if you want to do a portrait you would instead use a normal or a telephoto lens.
But how about shooting objects or photographing still life subjects? Which lens is better? I’ll use this subject to illustrate the characteristics of different focal lengths that normally get less attention.
What is focal length?
When light comes in through the lens, it passes through a small hole called a nodal point. The distance from that point to the sensor when your lens is set to infinity is called the focal length and this is measured in millimeters. A smaller distance gives you a wider angle of view and that’s why it’s called a wide-angle lens. Therefore a bigger distance gives you a narrower angle of view which is called a telephoto lens.
What is normal?
When you say a normal lens, it means that it will see more or less the same angle of view as the human eye. Anything longer than the normal focal length is a telephoto and everything shorter is a wide-angle lens. This measurement depends on the size of your sensor because the measure of its diagonal is what determines “normal” for that camera.
For example, in analog photography, it was a very standard measure because there were only so many negative film formats. A 35mm film had a normal lens of 50mm, this can be translated into digital cameras that have a full frame sensor because it’s about the same size as 35mm film. If you have a cropped sensor camera, that “normal” lens becomes a telephoto.
Why is this important?
As I mentioned before, zooming in or out is the most obvious impact of the focal length. But what happens when you are shooting something where you can achieve that by getting closer or further from your subject? How do you choose your lens? Well, that’s where the other characteristics of the focal length come into play.
A photograph is a two-dimensional representation of a three-dimensional world. By changing the focal length you can compress or extend the distance between two objects, or between the subject and the background. Let me show you with a series of images taken of the same scene but with different focal lengths.
I put a measuring tape next to the objects so that you have a reference and see that they were separated by the same distance even if it doesn’t look like it in the various images.
Depth of field
As you probably know, the depth of field (area in focus) depends on the aperture. A small aperture gives you a greater depth of field than a big one. But there is another factor involved and that is the focal length.
A wide-angle lens appears to have a greater depth of field than a telephoto at the same aperture. It is a common misconception that wide-angles have more depth of field than longer lenses. The reason it appears so has to do with the subject to camera distance, not focal length.
This effect is intensified by the fact that you will be physically closer or further away with each lens to achieve the same framing. Allow me to illustrate with this photos in which I maintained the same aperture but changed the focal length.
In conclusion, there is no such thing as the best lens for the type of photography you are doing. It really depends on the results you want to get.