The Crop Factor Explained Simply [+ How to Calculate It]

What does the crop factor mean?

Crop comes from English and means something like “cut off.” A factor is always a number by which you multiply or divide something. The crop factor lets you calculate how the focal length of your lens will behave on your camera.

That’s still pretty abstract, so let’s walk through it step by step.

But first:

Is the crop factor even relevant for me?

Generally speaking, the crop factor isn’t something you need to think about while you’re actually shooting. It only matters when you’re buying a new lens.

Focal lengths on lenses are always stated in relation to full-frame. But if you have a camera with an APS-C or MFT sensor — which is the case for most people — you can use the crop factor to calculate the effective focal length of that lens on your specific camera.

This way, you can figure out what focal length your new dream lens should ideally have.

What was focal length again?

We keep talking about focal length, but what exactly is it? You’ll find the focal length as a millimeter value printed on your lens. To put it in somewhat physical terms, the focal length is the distance from the principal plane of your lens to the focal point — that is, to the camera’s sensor.

This determines the field of view and therefore the framing of your subject. In this context, people also talk about the angle of view. Physicists will probably throw their hands up in despair. But for understanding the basic relationships, this explanation is more than sufficient.

As a rough guide, focal lengths can be categorized as follows:

• up to 35 mm: These are called wide-angle lenses. They capture a particularly large portion of the scene, making them great for landscape photos or group shots with lots of people.
• 35 mm to 75 mm: These are called standard lenses. This focal length range is interesting for portrait photography or street photography.
• 75 mm and above: This is where telephoto lenses begin. They let you bring distant subjects in close — great for sports events, the zoo, or wildlife. Portraits taken with a telephoto lens also look really beautiful.

What sensor sizes are there?

The sensor is the part of the camera that takes the light information coming through the lens and creates the digital image. Most cameras on the market are equipped with sensors of one of the following three sizes.

1. Full-frame sensors, also called 35mm sensors, are the largest sensors. They’re found primarily in professional cameras that often cost more than $1,000. The sensor is roughly the same size as a single frame on a roll of film in an analog camera.
2. APS-C is a common sensor size, especially in entry-level cameras. It’s used in many cameras from Nikon, Fujifilm, and Sony. An APS-C sensor is 1.5 times smaller than a full-frame sensor.
3. Micro Four Thirds is the sensor size used primarily in cameras from Olympus and Panasonic. These sensors are a bit smaller still than APS-C sensors — half the size of a full-frame sensor.

Why are there different sensor sizes anyway?

The sensor is the most expensive component in your camera. The smaller the sensor, the less expensive the camera is to manufacture. Especially in the early days of digital cameras, the price differences were extreme — so APS-C and MFT established themselves alongside full-frame as standard sensor formats.

Smaller sensors also make it possible to build smaller, more compact cameras. Cameras with MFT sensors in particular play to this strength very effectively.

Where does the crop factor come from?

The crop factor comes from how much smaller your camera sensor is compared to a full-frame sensor. For APS-C models, the crop factor is 1.5, and for MFT models, it’s 2.

How do the different focal lengths come about?

To answer that question, we’ve put together an explanatory diagram. Take a look at the image first, then read the explanation below.

The circular area is the image circle — everything your lens captures. Your sensor, in turn, captures a portion of that and saves it as a photo. We’ve already established that cameras have sensors of different sizes.

As you can see in the image, a smaller sensor captures less of the scene than a larger sensor. This creates the impression that you’re shooting with a longer focal length. The focal length itself doesn’t actually change — but the field of view does.

Here’s a helpful analogy. Imagine you’re standing in front of the mirror in the morning, happy that you can see your whole body. If you swap it for a smaller mirror, you can only see part of yourself — even though you haven’t moved an inch. That’s exactly how it works with sensors.

Angle of view is the key concept here. The smaller mirror has changed it. To see yourself fully again, you’d have to step farther back — in other words, change the “focal length.”

How do you calculate the crop factor?

Don’t worry — it’s just simple multiplication.

You take the focal length printed on your lens and multiply it by the crop factor of your sensor. That gives you the effective focal length for your camera.

If you have a camera with an APS-C sensor, 50 mm becomes roughly 75 mm (50 mm × 1.5 = 75 mm). With a camera that has an MFT sensor, those 50 mm become 100 mm (50 mm × 2 = 100 mm).

But to be physically accurate: the focal length of your lens doesn’t actually change. What changes is the angle of view, and therefore the look of the image.

The look of a 50 mm lens on an MFT camera effectively resembles that of a 100 mm lens on a full-frame camera.

What’s the practical benefit for you?

Worth saying again: the crop factor isn’t something you need to think about while shooting. But it really does matter when buying a new lens — because that’s when the sensor size in your camera becomes critical.

Let’s take a practical example: you want to buy a wide-angle lens. If you have a camera with a full-frame sensor, a 20 mm lens might be a solid choice.

But if you have a camera with an APS-C or MFT sensor, you’ll need to buy a lens with a shorter focal length to achieve the same result — because you have to account for the camera crop factor.

With APS-C, you’d want to go for a lens with around 14 mm (crop factor 1.5); with MFT, a 10 mm lens (crop factor 2).

If you want to photograph distant subjects, the crop factor is actually really handy. A 200 mm lens on your APS-C camera effectively behaves like a 300 mm lens. On an MFT camera, it would even be 400 mm.

Here’s a conversion table with common focal lengths:

Note: The crop factor has no effect on the actual focal length of your lens. However, the field of view — and therefore the look of the image — does get smaller.