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What is Focal Length? Clarifying Some Misconceptions

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I see a lot of statements in forums and get questions here that really overcomplicate focal length. This post will explain what focal length is, including the relationship between focal length and field of view, conversions for different sensor sizes (crop factor), and how aperture relates to focal length.

Even with all of this information, in the end, you’ll see the only thing you really need to understand when it comes to focal length and different sensor sizes.

Defining focal length

What exactly is focal length?

Let’s start with how light is focused in a lens.

The light from the scene in front of you is going to be reflected into your lens. That light is going to pass through different pieces of glass and converges at a point called the optical center. It will then continue through your lens and fall onto your sensor, where it can be recorded as an image.

Focal length is the distance between that optical center (where the light converges) and the sensor. Note that this is when the lens is focused at a distance of infinity, for standardization purposes. Focal length is typically measured in millimeters.

measuring focal length
Rough diagram of measuring focal length (50mm), the distance between the optical center and the sensor plane. Note that the size of the sensor has nothing to do with how the focal length is measured.

A short focal length would be something like 12mm, 16mm, or 24mm (wide-angle lenses). A long focal length would be something like 90mm, 200mm, or 400mm (telephoto lenses).

Now knowing what focal length is, does the size of the sensor affect that measurement? No. Focal length is a characteristic of the lens, not the camera.

“I wish Fujifilm [or other APS-C camera] would name their lenses for the APS-C focal length and not the full-frame focal length” is a common sentiment among photographers.

Well, there is no such thing as an APS-C focal length and a full-frame focal length. It’s just focal length, regardless of the sensor size.

Focal length and field of view

What sensor size does affect is the field of view.

Look at this comparison of a 50mm lens with both a full-frame sensor behind it and an APS-C sensor behind it.

full frame versus aps-c focal length
A smaller sensor will capture less of the total field of view.

It’s the same lens and the same focal length, but the APS-C sensor is capturing less of the image. It will appear to be “zoomed in” more because the edges are cropped off. This is a smaller field of view.

A smaller sensor will have more “cropped off” when compared to a full-frame sensor, and this is known as the crop factor.

full frame sensor vs aps-c sensor
Using the exact same lens, an APS-C sensor will “crop” the image compared to what a full-frame sensor will record, giving you a smaller field of view.

Crop factor equivalence

For a given focal length (or the same lens), smaller sensors will capture a smaller field of view compared to a full-frame sensor.

A 50mm lens on an APS-C camera, where the sensor is 1.5x smaller than a full-frame sensor, will capture a field of view equivalent to what a 75mm lens would capture on a full-frame camera.

That 50mm lens has an equivalent focal length of 75mm on that APS-C camera (50mm x 1.5). But this equivalence is based on the resulting field of view, not the focal length. The physical focal length is still 50mm.

Put that 50mm lens on a medium format camera and it will capture a larger field of view thanks to the larger sensor. Fujifilm GFX medium format cameras have a crop factor of 0.79x, meaning the 50mm lens has an equivalent focal length of 39.5mm. Or, a field of view equivalent to what a 39.5mm lens would capture on a full-frame camera. The physical focal length is still 50mm.

focal length equivalents
(Above) This is what you would capture with a 50mm lens on three different sensor sizes.
different focal lengths
(Above) And these are the different focal lengths you’d need to get equivalent fields of view with a full-frame sensor.

Not sure what your camera’s crop factor is? This Wikipedia article has a table of crop factors for most cameras.

Sensor size and lens size

A 50mm lens has a focal length of 50mm on a medium format camera, a full-frame camera, an APS-C camera, and a micro four-thirds (MFT) camera. So then why is a 50mm lens designed for MFT so much smaller than a 50mm lens designed for medium format?

Think about the image area – the physical size of that sensor. The image that pops out the back of that medium-format lens needs to be much larger than the image that pops out of the back of that MFT lens to ensure sensor coverage.

sensor size image areas
Required lens diameters for different sensor sizes, regardless of focal length.

This requires a larger rear element, which requires larger glass, which requires a larger diameter lens barrel. These are all going to contribute to increases in size, weight, and cost.

The actual physical length of the lens, while somewhat related to focal length, has more to do with optical design and the number & type of glass elements included in the lens. Some 50mm lenses are much longer than other 50mm lenses, just based on what the engineers decided to include to optimize sharpness and handle optical characteristics.

fujifilm 18mm lenses
Schematics of two Fujifilm APS-C 18mm lenses (f/2 on the left and f/1.4 on the right). They both have the same focal length but one is physically much longer due to the extra glass.

Aperture is related to focal length

The f-number that you adjust on your lens, like f/2, f/4, or f/11, refers to how much light the lens can gather. This is actually a ratio, where the light-gathering capability is equal to the focal length divided by the diameter of the aperture opening.

Let’s look at a 50mm lens:

  • An f-number of f/16 means the aperture opening is 3.125mm across (50mm/16).
  • An f-number of f/11 means the aperture opening is 4.54mm across (50mm/11).
  • An f-number of f/5.6 means the aperture opening is 8.93mm across (50mm/5.6).
  • An f-number of f/2 means that the aperture opening is 25mm across (50mm/2).
  • And so on…

On a 200mm lens, however, an f-number of f/5.6 means the aperture opening is 35.7mm in diameter. Compare that to an opening of only 8.93mm on a 50mm lens, for f/5.6.

This shows that shorter focal lengths can gather more light for a given lens opening diameter. An aperture opening of 9mm in diameter would give you an f-number of f/2 on an 18mm lens and an f-number of f/6.3 on a 56mm lens. The 9mm opening wouldn’t let that 56mm lens pass as much light.

calculating aperture
The 56mm lens will need a much larger aperture opening to let in as much light as the 18mm lens (the diameter would need to be 28mm instead of 9mm).

This is also why you don’t see telephoto lenses with really small f-numbers, like f/1.4 or f/2. A 600mm f/2 lens would require an opening diameter of 300mm (12 inches)!

What you really need to know about focal length

As long as you’re staying within the same camera system/sensor size, you shouldn’t really be doing any math. Calculating equivalent focal lengths is more work than you need to do.

All that matters is that short focal lengths give you larger fields of view (wide-angle lenses) and long focal lengths give you smaller fields of view (telephoto lenses). It’s as simple as that. Don’t overcomplicate it by introducing crop factors.

Crop factors are more important if you’re working between different systems. Like if you love your 24mm lens when photographing landscapes on your full-frame Sony a9, and you just got an Olympus MFT, you’ll probably want to know which focal length will give you an equivalent field of view on that Olympus (it’s 12mm).

Or, you’ve been with the Fujifilm X APS-C system forever and are transitioning to the Fujifilm GFX medium format system. You’ll want to know which medium format focal lengths will give you the fields of view you were used to working with in that APS-C system.

There are depth-of-field implications between different sensor sizes also, but that’s a topic for another day. It’s again something that’s only important when going back and forth between systems.

Questions? Comments? Let me know below!

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David Hampson

Tuesday 18th of July 2023

Loved this explanation, thank you.

John Peltier

Tuesday 18th of July 2023

Oh good! I appreciate the feedback.