What is Focal Length? Understanding Lenses

Understanding camera terminology can be intimidating. When learning about photography for the first time as a fresh 16-year-old, the whole terminology behind the camera, and sitting in a lecture about how the camera works, did nothing for me. I wanted to be out there with my DSLR taking a photo and learning on the go. Not sitting learning the intricacies of what glass was first used for lenses and how it was made! So I have taken it upon myself to keep things a bit simpler when sharing the art of photography. Onto something that baffled me for a while, focal length – what is it?

Focal Length

Focal length is the basic description of a lens. It is not a measurement of the actual length of a lens, instead, it represents a calculation (in mm) of an optical distance from the point where light rays converge to form a sharp image of an object to the sensor at the focal plane in the camera. The focal length calculation is determined when the lens is focused at “infinity.” That’s the scientific explanation of what focal length is. But what does that mean?

Lens focal length tells us how much of the scene will be captured (angle of view), and how large individual elements will be (magnification):

The human eye has an equivalent focal length of 45mm.

flea bitten grey horse at a show jumping competition

The longer the focal length, the narrower the angle of view and the higher the magnification.

This photo was taken at a focal length of 400mm.

The shorter the focal length, the wider the angle of view and the lower the magnification.

This photo was taken at a focal length of 50mm.

fine art portrait edit of a horse

Using Focal Length

It’s all well knowing what it is, but how do you use it in practical application?

image of hardknott fort, UK

Wide Angle Lenses
Usually any lens under 50mm. These are practical for landscape photography, as well as interiors.
The image on the left is of Hardknott Fort in the UK, taken with an 11-24mm lens.

Prime 50mm Lenses
A prime lens refers to a non-zoom/telephoto lens. It has a fixed focal length. 50mm is the closest to our eyesight so is often used in portraiture.
This image was taken with a 50mm f/1.8 lens.

image of couple
show jumping image

Telephoto 70-200mm Lenses
Telephoto or zoom lenses refer to lenses that can move between two focal points. A popular one is 70-200mm or 70-300mm.
The image of the grey mare jumping has a focal length of 100mm. Perfect for pitch sports, or events taking place at a reasonable distance.

Super Telephoto 150-600mm
A focal length over the 300mm mark is normally used for nature photography or sports. Personally, I used the Sigma 150-600mm for bird photography, as well as sports photography.
The image of the chaffinch in the tree has a focal length of around 500mm.

chaffinch in a tree

Finding The Focal Length

The focal length of your lens is marked on the front of the glass, as well as on the side of telephoto lenses. The information is stored within the metadata of your photos and available to see in editing programs, and the raw file itself.

A little bit more Focal Length Info

If you want to get into the details of angles, here is a conversion of focal length to the angle of view:

Focal LengthAngle of View

It is important to remember that the angle of view can cause distortion of an image. This is why using the lens correction function is so important in editing, especially with images of people!

For more photography posts look here:

I am currently taking bookings for shoots and competitions local to me. Feel free to get in touch for information!


Published by Briony-Molly

Photographer & Designer. Horse Owner, Book Fanatic

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