Crop Factor: How It Changes Your Lens's Effective Focal Length

What is crop factor?

Crop factor is a number that tells you how much smaller a sensor is compared to a full-frame sensor. It is calculated as the ratio between the diagonal of a 35mm full-frame sensor and the diagonal of the sensor in your camera. The resulting number lets you convert a lens's real focal length to its full-frame equivalent, which is sometimes called the effective focal length.

A full-frame sensor measures 36 by 24 mm, giving it a diagonal of roughly 43.3 mm. An APS-C sensor, which is what most entry-level and mid-range interchangeable lens cameras use, is smaller. Its crop factor is typically 1.5x for most brands. A Micro Four Thirds sensor is smaller still, with a 2x crop factor. The Micro Four Thirds system standard is an open standard, detailed by the Micro Four Thirds group (four-thirds.org), and it is used by several manufacturers for both cameras and lenses.

Because the crop factor is a ratio, it is a simple and reliable way to compare how the same lens will frame on different formats. A 50mm lens on a 1.5x APS-C camera gives the same field of view as a 75mm lens on full frame. On a 2x Micro Four Thirds camera, that same 50mm lens gives the field of view of a 100mm lens on full frame.

• Full frame: crop factor 1.0x
• APS-C: usually 1.5x (some systems are 1.6x)
• Micro Four Thirds: 2.0x
• 1-inch sensor: about 2.7x

APS-C: the 1.5x standard

APS-C sensors are fitted in a wide range of cameras, from entry-level mirrorless models to professional workhorses. The crop factor for APS-C is most commonly 1.5x, which means a 35mm lens behaves like a 52.5mm lens on full frame. That makes a 35mm lens a standard lens on APS-C rather than a wide lens, which matters if you are choosing glass for street photography. A 16mm lens becomes a 24mm equivalent, which is why kits and primes for APS-C often start around 16mm for wide-angle work.

The 1.5x number is an approximation. Some systems use 1.6x for specific sensor sizes, and older or video-specific modes can crop further. The most useful interpretation is that a 1.5x crop factor means you get a similar level of reach on the wide end as you would get from a lens about 1.5 times as long on full frame, and a tighter field of view on the wide end. This behavior is why APS-C cameras are popular for wildlife and sports photography, where a 300mm lens gives a 450mm full-frame equivalent view, as explained in guides like the best APS-C cameras.

It is worth understanding what crop factor does not change. It does not change the aperture, the lens's optical characteristics, or the exposure. A 50mm f/1.8 lens is still a 50mm f/1.8 lens regardless of the sensor it is mounted on. The crop factor only changes the angle of view and therefore how the subject is framed.

Micro Four Thirds: the 2x system

Micro Four Thirds is an open standard that produces distinctively small and light camera and lens combinations. The system's crop factor is 2x, meaning a 25mm lens produces the field of view of a 50mm lens on full frame. Because the sensor is much smaller than APS-C, the system has a particular strength in telephoto reach. A compact 40-150mm zoom, a common kit lens for the system, becomes an 80-300mm equivalent, which is a versatile telephoto range without the size of a full-frame 300mm setup.

The Micro Four Thirds standard is managed by the Micro Four Thirds standard group, which publishes specifications that allow different manufacturers' lenses and bodies to work together. This interoperability is one of the system's distinguishing features. As a photographer, you can switch between brands of lenses on the same body without an adapter, while still relying on the standard for consistent performance.

The 2x crop factor also affects the wide end. A 12mm lens is a 24mm equivalent on Micro Four Thirds. This means to get an ultra-wide view, you need lenses with very short focal lengths, which can be more expensive and more difficult to design well than the equivalent full-frame ultra-wides. However, for those who shoot wildlife, the best micro four thirds cameras guide highlights how the 2x crop delivers easy reach with modest lens size.

How to compare lenses across formats

The most practical use of crop factor is comparing lenses between systems. When you know a lens's true focal length and the crop factor of the sensor, you can compute the full-frame equivalent and decide whether that lens will give you the angle of view you are looking for. A 17mm lens on APS-C and a 23mm lens on full frame will give similar framing, for example, and those two lenses would be alternatives even though their focal lengths are different numbers.

If you are moving between full frame and APS-C, multiply the focal length by 1.5 to find the equivalent on full frame, or divide the full-frame focal length by 1.5 to find the lens you need on APS-C. For Micro Four Thirds, multiply or divide by 2.0. This calculation works for any focal length, so you can plot out your kit with confidence before you buy.

A crop factor also helps you understand that the same lens design does not automatically give the same framing on different systems. A 35mm prime is a compact normal lens on full frame but a moderate wide-angle on APS-C, and a portrait lens on Micro Four Thirds. When shopping, know which format you are buying for. If you are considering both APS-C and Micro Four Thirds cameras, read the relevant buying guides, such as the best APS-C cameras and the best micro four thirds cameras, to see which system offers the focal length coverage you need.

Practical examples of crop factor in action

Suppose you want to shoot a portrait and are used to a 85mm lens on full frame. On an APS-C camera with a 1.5x crop factor, you would choose a lens around 56mm to get the same look. On Micro Four Thirds, you would choose a 42.5mm lens, which is a very common portrait prime for that system. These short lenses are typically lighter and less expensive to make than an 85mm full-frame prime, which is one reason smaller formats are attractive for travel and street photography.

Now suppose you want to shoot birds in flight. On full frame, you might be considering a 600mm lens. On APS-C, a 400mm lens gives the same reach, and on Micro Four Thirds a 300mm lens gives the reach of a 600mm on full frame. This is why the 2x crop factor of Micro Four Thirds is often described as an advantage for wildlife and sports photography, where a relatively compact and affordable 300mm or 400mm lens can provide impressive telephoto reach.

For wide landscapes, the situation is reversed. To shoot at an equivalent of 16mm on full frame, you would need a 16mm lens on full frame, an 11mm lens on APS-C, and an 8mm lens on Micro Four Thirds. Ultra-wide lenses in these shorter focal lengths exist, but they are specialty designs and often cost more. If wide-angle work is a priority, a full-frame or APS-C system with a good 10-20mm zoom may serve you better, as outlined in the best full-frame mirrorless cameras and best APS-C cameras guides.

What crop factor does not change

Crop factor only changes the field of view. It does not change the amount of light that reaches the sensor, the lens's maximum aperture, or the depth of field in terms of the physical lens properties. The same amount of light passes through a 50mm f/1.8 lens whether it is on a full-frame or APS-C camera, and the exposure settings will be the same. However, because a smaller sensor captures a smaller portion of the image circle, the depth of field and the background blur will appear different when you frame the same subject and compose the image to match the reduced angle of view.

Crop factor also does not change the lens mount. A lens designed for a full-frame system will physically not fit many APS-C or Micro Four Thirds bodies without an adapter, and even with an adapter, the image circle may not cover the smaller sensor. The lens mount guide walks through which lenses fit which mounts. Always verify compatibility before buying a lens for a different system.

Finally, crop factor can also affect your resolution and sensor characteristics. A full-frame sensor has more surface area, which typically allows for larger pixels at a given resolution, which can result in better dynamic range and low-light performance. Those differences are explained further in the sensor size explained article. When evaluating a camera, look at the sensor and the crop factor as complementary specifications, not as a single rating.

What to pick for your use

Questions

Does crop factor change my lens's actual focal length?

No. The focal length printed on the lens is its true focal length. Crop factor only changes the angle of view when the lens is used on a smaller sensor. A 50mm lens is still a 50mm lens optically, but on APS-C it gives the field of view of a 75mm lens on full frame.

Is a higher crop factor better?

It depends on what you shoot. A higher crop factor gives more reach for a given focal length, as with Micro Four Thirds' 2x, which is helpful for wildlife. A lower crop factor, such as full frame's 1x, makes wide-angle design easier and typically offers more shallow depth of field. The best choice matches your primary subjects and lens needs.

How do I calculate the full-frame equivalent focal length?

Multiply the lens focal length by the crop factor. For example, a 35mm lens on an APS-C camera with a 1.5x crop factor gives roughly 52.5mm; a 25mm lens on Micro Four Thirds gives 50mm. To go from full-frame to a smaller format, divide the focal length by the crop factor.

Does crop factor affect depth of field?

Crop factor does not change the lens's aperture, but it does change the framing when you match the field of view. To fill the frame with the same subject on a smaller sensor, you typically move farther away, which increases depth of field. So the visible depth of field is usually larger on a smaller sensor for the same subject size in frame.

Why do lenses made for APS-C or Micro Four Thirds have lower crop factors?

The crop factor is a property of the sensor, not the lens. APS-C lenses project an image circle just large enough for the APS-C sensor, and the crop factor remains 1.5x. A lens designed for a larger sensor can be used on a smaller sensor with the same crop factor, provided the mount and flange distance are compatible.