Crop factor is the ratio of the diagonal of a 35 mm full frame sensor (about 43.3 mm) to the diagonal of another sensor. In photography, and especially in the field of lenses and optics, it is used to express how much narrower the angle of view becomes when a lens is used on a smaller sensor, and to translate focal lengths between formats.
Multiplying a lens’s focal length by the crop factor gives its full frame equivalent focal length. A 50 mm lens on a sensor with a crop factor of 1.5 frames the scene like a 75 mm lens on full frame. The term is sometimes called the focal length multiplier, although the lens itself does not change.
How crop factor is calculated
Crop factor is calculated by dividing the full frame diagonal by the diagonal of the sensor in question. The full frame reference measures 36 x 24 mm, which gives a diagonal of about 43.27 mm. A sensor with a 28.8 mm diagonal therefore has a crop factor of roughly 1.5.
The diagonal is used because it describes the image circle a lens must cover. When two sensors have different aspect ratios, such as 3:2 and 4:3, the diagonal comparison is only approximate for the horizontal or vertical angle, so careful comparisons should check the relevant dimension as well.
The table lists the crop factors of common sensor formats.
| Sensor format | Typical dimensions | Crop factor |
|---|---|---|
| Medium format (44 x 33) | About 44 x 33 mm | About 0.79 |
| Full frame | 36 x 24 mm | 1.0 |
| APS-C (most brands) | About 23.5 x 15.6 mm | About 1.5 |
| APS-C (one major brand) | About 22.3 x 14.9 mm | About 1.6 |
| Micro Four Thirds | 17.3 x 13 mm | 2.0 |
| 1-inch type | 13.2 x 8.8 mm | About 2.7 |
| Typical smartphone main camera | Varies by model | About 3 to 7 |
What crop factor changes and what it does not
Crop factor changes the angle of view for a given focal length, but it does not change the focal length, the f-number or the exposure. A lens marked 50 mm f/1.8 remains a 50 mm f/1.8 lens on any body. The smaller sensor simply records a smaller central section of the image circle the lens projects.
Because the smaller frame must be enlarged more to reach the same print size, crop factor also affects depth of field, total light and diffraction when images are compared at equal output size. The table separates these effects.
| Property | Changes with crop factor | Explanation |
|---|---|---|
| Focal length | No | A physical property of the lens |
| Angle of view | Yes, narrower | The sensor records a smaller part of the image circle |
| Exposure at a given f-number | No | Light per unit area stays the same |
| Total light on the sensor | Yes, less | Smaller area collects fewer photons |
| Depth of field at equal framing | Yes, deeper | Shorter focal length or more distance is needed |
| Visible diffraction | Yes, earlier | Greater enlargement magnifies the blur |
Equivalent focal length and equivalent aperture
The equivalent focal length is the focal length that would give the same angle of view on full frame, calculated as focal length multiplied by crop factor. It is the most common and least controversial use of crop factor, and it helps photographers choose lenses across systems.
The equivalent aperture extends the idea to depth of field: the f-number multiplied by the crop factor gives the full frame f-number that produces similar depth of field at the same framing. It does not describe exposure brightness, which depends on the actual f-number. The table shows worked examples.
| Lens on sensor | Crop factor | Equivalent focal length | Equivalent aperture for depth of field |
|---|---|---|---|
| 35 mm f/1.8 on APS-C | 1.5 | About 52 mm | About f/2.7 |
| 56 mm f/1.4 on APS-C | 1.5 | About 84 mm | About f/2.1 |
| 25 mm f/1.2 on Micro Four Thirds | 2.0 | 50 mm | f/2.4 |
| 300 mm f/4 on Micro Four Thirds | 2.0 | 600 mm | f/8 |
| 8.8 mm f/1.8 on 1-inch type | About 2.7 | About 24 mm | About f/4.9 |
Equivalence assumes the same framing, viewing size and sensor technology. In practice, differences in sensor generation, processing and lens quality can matter as much as the format itself.
Using crop factor when choosing lenses
Crop factor is most useful as a planning tool when photographers switch formats or buy lenses. A standard field of view requires about 50 mm on full frame, about 33 mm on APS-C and 25 mm on Micro Four Thirds, so the focal length labels shift with each system.
- Wide-angle work becomes harder on small sensors, because very short focal lengths are needed to reach an ultra-wide view.
- Telephoto work becomes easier, because a moderate focal length gives a long equivalent reach at lower weight.
- Full frame lenses on crop bodies use only the center of their image circle, which often gives good corners but loses the wide view.
- Crop lenses on full frame bodies can cause dark corners unless the camera switches to a crop mode.
Related Terms help complete the picture. Angle of view describes the result directly, focal length is the fixed lens property, and depth of field explains why equal framing on different formats does not give identical background blur.
Common misconceptions about crop factor
Crop factor is widely misunderstood because the label focal length multiplier suggests the lens itself changes. The most common errors are listed below.
| Misconception | Accurate view |
|---|---|
| A crop sensor magnifies the subject | It records a smaller section of the same projected image |
| An f/2.8 lens becomes darker on a crop body | Exposure at f/2.8 is identical on every format |
| Crop factor creates extra detail at distance | Detail depends on pixel density and lens resolution |
| Equivalent aperture should be used for metering | It applies only to depth of field and total light |
Keeping the lens properties separate from the sensor properties removes most confusion and makes comparisons between systems fair.
The idea in brief
Every lens throws a round picture onto the sensor, and the sensor records a rectangle from its middle. A smaller sensor cuts out a smaller rectangle, so the photo looks more zoomed in, even though the lens is exactly the same.
Crop factor is simply the number that says how much tighter the view becomes. With a crop factor of 2, a 50 mm lens shows the same view a 100 mm lens would show on a larger, full frame camera.
A practical scenario
A portrait photographer who works with a full frame camera and an 85 mm lens buys a lighter APS-C body for travel. To get the same framing and working distance, the photographer divides 85 mm by the crop factor of 1.5 and chooses a 56 mm lens.
At a session in a city park, the photographer sets f/1.4 and keeps the familiar distance of about 2 meters. The framing matches the full frame results closely, and the exposure settings stay the same. The background blur is slightly less pronounced, close to what f/2.1 would give on full frame, which the photographer accepts as a fair trade for a lighter travel kit.