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Photography is full of fancy terms and complicated science, but we are here to explain it all to you in simpler terms. Today we are focusing on focal length. This term gets thrown around often, especially when looking at lenses, so it’s important you get familiarized with it.

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What is focal length?

Focal length is the distance between a camera sensor and a lens' point of convergence.

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Put simply, focal length is the distance between a camera sensor (or film) and a lens’ point of convergence.

The hardest part is understanding what the point of convergence (also known as optical center) is. When light rays enter a lens they travel through glass and bend to converge in a single point. This point is where light data is collected to form a sharp image for the sensor to record. Manufacturers measure the focal length focused to infinity, to keep a standard.

Focal length is measured in millimeters. A 50mm lens will have a point of convergence that is 50mm (or 5cm) from the sensor.

In the graph below, the point of convergence is marked with “F'”, while the focal length is noted “ƒ'”.

Focal length lens graph JiPaul/Wikimedia

How to pick the right focal length

We can sit here and explain the different elements of a lens and all the science behind glass, but ultimately what matters is how focal length affects your ability to shoot an image. Focal length is especially important when trying to pick which lens to use or buy.

Focal length determines how “zoomed in” you are.

Edgar Cervantes

A lower focal length will make your subject appear smaller, while a higher one will enlarge them. In addition, a lower focal length has a larger field of view, which determines how large of an area you can photograph. In simpler terms, focal length determines how “zoomed in” you are. You should pick a shorter focal length to shoot a landscape photo, and a longer one to focus on a tree in the distance.

Types of lenses: 

  • Ultra wide-angle: 24mm and lower
  • Wide-angle: 24-35mm
  • Standard: 35-85mm
  • Telephoto: 85mm and above

Focal length’s relation to bokeh

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You have seen those lovely photos with the subject in focus and blurry backgrounds. That blurred out effect is known as bokeh, and is caused by the shallow depth of field. Most people thank aperture for this, but a longer focal length will also reduce depth of field and isolate your subject with gorgeous bokeh.

Most thank aperture for bokeh, but focal length is just as important.

Edgar Cervantes

Crop sensor equivalent

As per the definition above, focal length stays the same regardless of the size of the sensor. What does change is how the image will look.

A full-frame sensor measures 35mm, a standard which was taken from the size of film. Anything below 35mm is considered a “crop sensor”. A smaller sensor will record a smaller image, which essentially makes a photo look more zoomed in.

Most cropped frame sensors have a crop factor of about 1.6x. This means that a 50mm lens on an crop sensor camera will look like an 80mm lens on a full-frame camera.

To calculate focal length equivalents, you first have to figure out your sensor’s crop factor. This is done by dividing a full frame sensor’s diagonal length (43.27) by your sensor’s diagonal length. You can then multiply the crop factor by the focal length to get the crop sensor equivalent.

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Understanding focal lengths and their effects on photographs will be crucial in your photography advancement. This post should be enough to get you started, but remember photography is all about getting hands on. Go experiment with focal lengths and see what you can capture!

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