Crop Factor and Angle of View: A Beginner's Guide

What is crop factor

Crop factor is a number that compares a camera sensor's size to a full-frame 35mm sensor. Full-frame is the reference: its crop factor is 1. A sensor that is smaller than full-frame has a crop factor greater than 1, and a sensor that is larger, such as medium format, has a crop factor below 1.

The name comes from what the sensor does to the image. When you mount a lens on a camera with a smaller sensor, the sensor captures only the central portion of the image circle that the lens projects. The outer area of the image is effectively cropped away. That crop makes the angle of view narrower than it would be on a full-frame camera with the same lens.

The practical result is a magnification effect. A 50mm lens on a camera with a 1.5x crop factor produces an angle of view similar to a 75mm lens on full-frame. The lens itself does not change: its focal length stays the same. Only the field of view changes.

Different camera systems use different sensor sizes. The Micro Four Thirds system, for example, is an open standard that lets you combine camera bodies and lenses from multiple manufacturers, and it is built around a sensor smaller than full-frame. You can learn more about how sensor size affects image quality in our sensor size guide.

• Full-frame: reference format, crop factor 1x
• APS-C: smaller than full-frame, crop factor around 1.5x
• Micro Four Thirds: smaller still, crop factor around 2x
• 1-inch: compact cameras and some bridge cameras
• Medium format: larger than full-frame, crop factor below 1x

How angle of view changes with sensor size

Angle of view is the extent of the scene that a lens captures, measured in degrees across the diagonal, horizontal, or vertical dimension. It depends on two things: the focal length of the lens and the size of the sensor. Keep the focal length constant and change the sensor size, and the angle of view changes.

A smaller sensor always sees a narrower angle of view with the same lens. This is why a wide-angle lens on a Micro Four Thirds camera may look like a standard lens on full-frame. Photographers often talk about effective focal length or equivalent focal length to describe this relationship, but the actual focal length marked on the lens does not change.

The crop factor affects every lens you mount. A telephoto lens gains extra reach on a smaller sensor, which is one reason wildlife and sports photographers sometimes prefer APS-C or smaller formats. A wide-angle lens, by contrast, loses its wide field of view, so you may need an even shorter focal length to achieve the same framing. For a deeper look, see our focal length and angle of view article.

When you compare cameras across formats, remember that the same focal length lens on two different sensor sizes produces two different images. A 35mm lens on full-frame is a wide-angle lens. On APS-C it becomes a mild wide-angle to standard lens. On Micro Four Thirds it is close to a standard lens. This is why knowing the sensor size matters when you choose a camera.

Equivalent focal length and lens choice

To estimate the equivalent focal length on a full-frame camera, multiply the lens focal length by the crop factor. A 50mm lens on a camera with a 2x crop factor gives a 100mm equivalent field of view. A 20mm lens on the same camera gives a 40mm equivalent.

The equivalent number is not a claim about image quality or lens construction. It is a way to compare fields of view across different sensor sizes so you can predict how a scene will frame. If you are used to full-frame and moving to a smaller format, or the reverse, this mental math helps you choose lenses that suit your composition style.

Manufacturers design lenses for specific sensor sizes. A lens made for a full-frame camera can often be used on an APS-C camera from the same mount, but the angle of view will be narrower. Lenses designed specifically for APS-C or Micro Four Threes are smaller and lighter because they only need to cover a smaller image circle. The lens mount guide explains how mounts and sensor formats work together.

For example, Sony's Alpha system covers both full-frame and APS-C cameras on the same E-mount, as described on the Sony Alpha Universe site. You can mount a full-frame lens on an APS-C body, but the resulting angle of view will be that of a longer lens. You can also mount an APS-C lens on a full-frame body, but the camera may automatically switch to a cropped mode.

Crop factor, depth of field, and background blur

Crop factor also affects the look of depth of field, although not because the lens changes. A smaller sensor requires a shorter focal length to achieve the same angle of view as a larger sensor. At the same aperture and subject distance, a shorter focal length produces more depth of field: more of the scene appears in focus.

Background blur, the creamy separation that portrait photographers often want, is therefore harder to achieve on a smaller sensor with the same framing. You can compensate with a faster lens, a wider aperture, or by moving closer to your subject. But for the same framing and aperture, a larger sensor tends to produce shallower depth of field.

This is not an absolute rule. A fast prime lens on APS-C or Micro Four Thirds can still produce pleasing blur. But if shallow depth of field is a priority, a full-frame camera gives you more room. Our depth of field article explains the mechanics behind this in more detail, and our full-frame guide can help you compare cameras.

Do you need to care about crop factor

Crop factor is most important when you plan to use lenses designed for one format on a camera with a different sensor size. If you buy a complete system with lenses matched to the sensor, the crop factor is simply part of how the system works. You do not need to calculate anything unless you are switching systems or mixing lenses.

If you are choosing between a full-frame, APS-C, or Micro Four Thirds camera, think about the kind of photography you do. Wildlife and sports benefit from the extra reach of a smaller sensor. Landscape and architecture often favor the wider field of view of full-frame. Video shooters may prefer larger sensors for shallower depth of field or smaller sensors for longer battery life and portability.

The crop factor also explains why a bridge camera with a small sensor can offer an enormous zoom range in a compact body. The small sensor makes a modest lens behave like a much longer telephoto. If you want a camera with a wide field of view, however, you need to check the actual focal length at the short end, because the crop factor works against you there.

For a practical summary: know the crop factor of your sensor, estimate equivalent focal lengths when comparing lenses, and choose lenses by the angle of view you need, not by the number printed on the barrel.

Putting it into practice

If you are just starting out, do not let crop factor intimidate you. Every camera system has lenses that suit its sensor size. A kit lens on an APS-C camera is designed to give you a useful range of fields of view, from wide to short telephoto. The camera works as intended, and the crop factor is only a reference for comparing to other formats.

If you already own lenses from a full-frame system and want to use them on a smaller sensor body, check the crop factor to know what field of view you will get. If you want a specific angle of view, such as a true wide-angle for landscapes, make sure the lens is short enough on the sensor you plan to use.

For buying decisions, start with the sensor size that matches your goals. Our APS-C camera guide shows the range of options in that format. If you care about the shallowest depth of field and widest angles, the full-frame camera guide is a good place to start. For compact travel cameras, see the 1-inch sensor cameras guide.

Micro Four Thirds cameras offer a compact, lightweight system with a 2x crop factor, and the open standard means you can mix bodies and lenses from different makers. If that flexibility appeals to you, the Micro Four Thirds guide covers the options available.

What to pick for your use

Questions

What does crop factor mean for a beginner?

Crop factor tells you how much narrower the angle of view is compared to a full-frame camera. A sensor that is half the size of full-frame, like Micro Four Thirds, shows about half the scene width with the same lens.

Does crop factor change the actual focal length of a lens?

No. Focal length is a physical property of the lens and does not change. Crop factor only changes the angle of view you see because the smaller sensor captures only part of the image circle.

Is higher crop factor better or worse?

Neither. It is a trade-off. A smaller sensor gives you extra reach for telephoto shots and allows smaller lenses, but it also makes wide-angle shots harder and increases depth of field for the same framing.

How do I calculate effective focal length?

Multiply the lens focal length by the crop factor of the sensor. A 50mm lens on a 1.5x APS-C camera is equivalent to a 75mm lens on full-frame in terms of field of view.

Why do different sensor sizes have different crop factors?

Each sensor size has its own physical dimensions. The crop factor is simply the ratio between the full-frame sensor diagonal and the diagonal of the sensor in question.