What is Focal Length?

The concept of focal length is something that isn’t necessarily easy to understand.

You may notice the focal length of your lenses by the number that’s with it that has mm next to it. For example, if you have a lens that says 75mm, the focal length of the lens is 75mm.

But what does that even mean? What does it mean to have a focal length of 75mm?

To put it in easy terms, focal length has a lot to do with the zoom of a lens. However, it goes way deeper than that and that’s something that a lot of people don’t really understand.

Knowing exactly what focal length is may not help your overall photography skills, but it will allow you to have a better understanding of your camera and lenses.

And yes, focal length is something that can apply to any camera. So, you don’t need to worry about having a specific camera to understand focal length.

Understanding focal length can improve your photography skills with any camera.

And with a deeper understanding of your own photography skills, you’ll have a much easier time deciding on what type of lens you’ll need for your shots.

The Canon RF 50mm and the Canon 16-35mm are my personal favorites for lenses.

A great artist needs to understand their tools as much as possible. As a photographer, you are an artist and understanding your camera is something that you have to do.

What is Focal Length?

The 100mm is the focal length of this lens.

All lenses are named by their focal length. This bit of information probably doesn’t help you too much.

However, once you really understand focal length, that information will actually become very useful.

Focal length measures the distance, in millimeters, between the optical center of a lens and the camera’s sensor.

Again, that’s a piece of information that may not make too much sense right now, but by the end of this article you will be able to have a full understanding of what focal length actually is.

Optical Center

What is the optical center of a lens?

Put simply, the optical center of a lens is where all the light from the view of the lens meets to form the image you see.

So that means if you have a 75mm lens, all the light is going to hit the optical center of the lens 75mm from the sensor of the camera.

That distance, although it may seem very small is going to make a huge difference with how your image is seen.

What Focal Length Tells Us

Knowing what the optical center is compared to the actual focal length of your lens is something that you don’t necessarily have to understand. What you should understand is what focal length really tells us.

Focal length is actually something that describes the angle of view of a lens. This essentially means how much of a scene is seen by the lens.

You can relate this to the size of objects that are in your scene as well. This relates back to how much zoom a lens has.

Focal length – 24mm
Focal length – 35mm
Focal length – 50mm
Focal length – 70mm

A certain focal length is going to have a certain amount of zoom to it. So a 75mm lens is going to have its objects appear much larger than a 25mm lens.

The objects in the 75mm lens are going to appear larger because the zoom of this lens is more than the 25mm lens.

Field of View

Focal length – 24mm
Focal length – 50mm
Focal length – 70mm

Angle of view and field of view are two different terms. They often get used interchangeable but they really shouldn’t be. Both are similar but refer to different things.

While angle of view is solely based on the lens itself, field of view has to do with the combination of the lens and camera. Field of view is dependent on the focal length and the camera’s sensor size.

Angle of view is fixed based on the lens. The camera you’re using doesn’t change the angle of view that is seen by the lens.

This is important to remember because a lot of digital cameras can come with a wide variety of sensor sizes. Because of this, your field of view is going to be different from other cameras that have different sensor sizes.

Because of the difference you can have with your field of view based on the lenses you use, you’re going to have to adjust other aspects of your camera. For example, ISO is something that you’ll have to look at changing.

You need to be able to understand ISO though so that you can make the proper adjustments.

Focal Length Classifications

Lenses can fall into 5 different categories based on their focal length.

There’s ultra wide angle lenses. These are lenses that have a focal length of less than 24mm. You can get extremely broad views with these types of lenses.

You have wide angle lenses. The focal length of these lenses are between 24mm and 35mm. Although you won’t be able to get the same type of wide view as you would with an ultra wide angle lens, you can still get a good wide view with these lenses.

There are standard lenses that have a focal length between 35mm and 70mm. These lenses are good for photos that you want from a real world point of view. By that, I mean these lenses will most closely show the world from how it would look with your eyes.

Telephoto lenses have a focal length between 70mm and 300mm. These lenses are going to be a bit bigger than most other lenses you see. You’re more than likely going to need to actually hold the lens with one hand while the camera is in the other.

Using these lenses are great because they allow you to get closer photos of your subject without having to actually get close to your subject.

There are also super telephoto lenses that have focal lengths more than 300mm. You’ll use these lenses when you want to shoot smaller objects but they are a distance away.

Every type of focal length is going to have some sort of effect on the composition of your photos. Composition can make or break your photos, so that is something that you need to keep an eye on as well.

Prime vs. Zoom

Some lenses are going to be considered zoom and others are going to be considered prime.

This is only based on whether or not you can change a lenses focal length.

If you have a lens where you can change its focal length, this will be considered a zoom lens. A prime lens is one that has a fixed focal length.

Focal length – 24mm
Focal length – 35mm
Focal length – 50mm
Focal length – 70mm

Both types of lenses have their advantages and drawbacks. Deciding on one is going to essentially come down to personal preference a lot of the time.

As I mentioned earlier, my two favorite lenses to use are the Canon RF 50mm and the Canon 16-35mm. The 50mm is the prime lens and the 16-35mm is a zoom lens.

I understand the drawbacks and advantages of both so it’s nice to have a favorite for a lens that’s a prime and zoom.

Understand Focal Length

Knowing about focal length can really help you understand your equipment that much more.

Understanding your equipment will make things better for you in the long run. It could very well help you become a better photographer.

Focal length is an integral part of your lenses and understanding that will allow you to understand what you need to help develop your photography skills.