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How to Calculate the Focal Length of a Camera Lens

This tutorial breaks down how to calculate the focal length of a lens.

Knowing the focal length of your lenses is crucial for choosing the right one for a given shot.

To help you get started, I’ve covered the basic method for calculating the focal length for various camera brands.

In addition to the focal length calculator, I’ll also cover how to convert your lens’s focal length into its x-times optical zoom.

What Is the Focal Length of a Lens and How Is It Measured?

Knowing the focal length of a given lens is essential for selecting the right lens and image distance for a particular shot.

There are two main types of lenses used in photography: concave and convex. Each has a different effect on incoming light rays.

Concave lenses are thicker on their edges and cause light to diverge away from the focal point while having a negative focal length.

On the other hand, convex lenses are thicker in the middle and bring light to a point to create a positive object distance and an inverted image.

Whether using a converging lens or diverging lens, the focal lengths represent the distance from the optical center to the image plane in the camera.

Typically, this is indicated by a Φ symbol located on the top plate of the camera’s body, with different focal lengths used to accomplish different shots.

How to Calculate Equivalent Focal Length (Cropped Sensor Conversion)


Figuring out the equivalent crop factor for different camera formats is a handy way to quickly convert the size of media you can work with.

Fortunately, there’s a straightforward way this can be achieved, using a quick formula based on the full-frame diagonal size compared to the smaller sensor’s diagonal size.

This formula is as follows when applied to a full-frame camera or a Sony camera with an APS-C sensor:

Full-Frame Diagonal Size divided by the APS-C Diagonal Size (43.3mm / 23.5mm) = crop factor of 1.84x

It’s important to bear in mind that different brands sometimes have different crop factors for their various camera sensors.

Some of the more common crop factors to be aware of that relate to specific camera manufacturer sensors include the following:

  • Canon APS-H sensors – crop factor 1.29x
  • Canon APS-C sensors – crop factor 1.61x
  • Micro 4/3rd camera sensors – crop factor 2.0x
  • Medium format sensors – crop factor 0.79x
  • APS-C sensors (general brands) – crop factor 1.5x

If you don’t have the mathematical know-how or time to calculate focal length, these statistics can help you save time and cut to the information you need.

How Do I Know My Lens Focal Length?


Finding the focal length of a specific camera lens is easy, and is typically printed directly on the barrel of the lens for ease of access.

If you’re using a zoom lens, you can also find the range of your lens’s focal length on the zoom ring located in the middle of the barrel.

This allows you to accurately adjust the focal length while framing your shot, while also understanding how much zoom leeway you have either way.

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If you’re using a prime lens, the focal length will be fixed to the lens’s native setting, with no adjustable scale to work with.

How Do You Guess Focal Length?

Most photographers who are dedicated to improving their skills will spend a lot of time checking out work from professionals and their photography peers.

It’s a great way to gain inspiration and come up with new insights into the shooting process, composition, and technical considerations.

But how can you guess the focal length used in an image to better understand how to replicate similar results?

By developing the ability to guess what focal length has been used for an image, you can further understand how lenses control depth and perspective.

For example, wide-angle lenses create a sense of space and immersion, while telephoto lenses work well to flatten perspective.

Ultra-wide-angle lenses, as well as fish-eye lenses, create a sense of distortion in horizon lines and other straight lines that other lenses won’t produce.

Conversely, super telephoto lenses bring the foreground and background elements together and can be used to create interesting patterns and textures.

The more you experiment with different focal lengths, the easier it will be to spot images taken with a longer or shorter focal length.

How to Use the Lens Equation to Find the Focal Length of a Lens


Another way to calculate focal length is by using the lens equation technique, for which you’ll need to know the distance an object is away from the lens and the image distance.

Here’s the equation in its core form:

1/do + 1/di = 1/f

For example, if an object is 50cm away from a lens and produces an image of 100cm, the focal length of the lens will be 33.3cm.

How to Convert Lens Focal Length (mm) to X-Times Optical Zoom?


Unless you’re using a fixed focal length prime lens, you may wish to figure out how to convert your lens’s focal length to its optical zoom.

To do so, you’ll need to factor in the minimum and maximum focal length of the lens’s zoom capabilities in this simple equation:

Maximum focal length / minimum focal length = optical zoom

For example, if you have an 18 – 55mm lens, the sum would be as follows:

55 / 18 = 3x optical zoom

A larger zoom lens with a coverage of 18 – 200mm would have a larger optical zoom coverage, with the equation revealing this as an 11x optical zoom.

This information can be useful when shooting wildlife or sports photography to give a clearer idea of which lens is needed to reach the action.

At the same time, bear in mind that crop factor may be something to consider when working out the optical zoom capabilities of a given lens and sensor combination.

Focal Length FAQs

How do I know what MM my lens is?

Most lenses include information about their focal length on the outside of the lens barrel, as well as being listed on the product’s specifications.

If you’re using a zoom lens, the minimum and maximum focal distance should be printed on the barrel’s zoom ring, indicating your position between these focal lengths.

What is the formula for calculating focal length?

1/do + 1/di = 1/f

What is a 10x zoom equivalent to in mm?

The equivalence of a 10x zoom in millimeters is dependent on the initial starting point and whether you’re using a crop or full-frame camera.

For a standard 35mm equivalent lens, 10x zoom will deliver an effective focal length of around 240mm.

What is the mm equivalent of 20x zoom?

At 35mm equivalent, a 20x optical zoom will provide a focal length of 26.8mm to 576mm, giving plenty of scope for long-distance shots.

What is 50x zoom equivalent to in mm?

The same principle applies when working out the mm equivalence for a 50x zoom for your camera setup.

For example, a crop-framed camera with an equivalence of 35mm would result in a 50x zoom lens of 1,750mm.

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