Every time we talk about the digital cameras, one thing that comes up is the “crop factor” of the sensor. Let’s dig a bit more into it and explain why it matters.
Different Cameras, Different Sensors
Digital cameras don’t all have identically sized sensors; there are a couple of different standards. The leading standard—used by manufacturers in their professional and high-end cameras—is 35mm or full frame. The sensor is approximately the same size as a piece of 35mm film (36mm x 24mm) which was the most popular film format.
Digital sensors, however, are pretty expensive to manufacture. The bigger the sensor, the more it costs. For this reason, manufacturers also make cameras with smaller sensors. By far the most common standard is APS-C which is based on the size of Advanced Photo Systems film size. Exact sensor sizes vary a bit between manufacturers, but they’re typically between 22.5mm x 15mm and 24mm x 16mm.
While 35mm and APS-C are the primary standards, there are other sensor sizes too. The one in your phone is around 9mm x 6mm. Digital medium format cameras (link) can have sensors that are 50mm x 40mm.
Sensors and Field of View
Now, to get crop factor, you need to understand two things:
The focal length of a lens is a physical property that stays constant regardless of what camera you use. The focal length of a lens is what determines its field of view.
But here’s the thing: The field of view you get from a lens of a given focal length doesn’t stay constant. It depends on what camera you use.
Let’s look at this in action. In the image below, courtesy of Sony, you can see how a given lens projects an image circle onto a full frame sensor and the resulting image.
Now, look at how the same lens projects an image circle of the same scene onto an APS-C sensor.
Since the sensor is smaller, the area that it samples from the image circle is smaller. This has the effect of reducing the field of view relative to the full frame sensor.
Nothing about the lens has changed; it’s just that for the image to be in focus, the sensor has to sit a certain distance from the lens which means a smaller sensor will always have a narrower field of view when using a lens of the same focal length.
Crop Factor
So to recap:
Different cameras use different sized sensors. 35mm full frame is the main standard. Smaller sensors have a narrower field of view than larger sensors when using lenses of the same focal length.
Since photography is based around incredibly well understood and predictable optical principles, we can calculate the relative field of view for any combination of lens and sensor size when compared to a full frame camera. This is the crop factor. Fortunately, the maths has already been done for us, so you can put your pencil away.
The most common crop factor you’ll encounter is 1.5x. That’s the crop factor for most APS-C cameras. It means that a 50mm lens on a crop sensor camera has an equivalent field of view to a 75mm lens on a full frame camera (50mm x 1.5 = 75mm). Bear in mind; this is just an approximation. Canon’s crop factor is actually about 1.6x, and most Nikon and Sony cameras are normally closer to 1.52x. If you’re curious about the exact crop factor of your camera, look up its specs online.
Phone cameras have a crop factor of about 7x. The wide angle lens on your iPhone has an actual focal length of 3.99mm; this gives it a full frame equivalent focal length of about 28mm given the tiny size of the sensor.
Crop factor also cuts both ways. Medium format cameras have a crop factor that’s less than 1. For example, the Hasselblad H6D-100c has a crop factor of 0.65x. This means a 50mm lens has a full frame equivalent focal length of 32.5mm. That’s a much wider field of view.
Why You Should Care
At How-To Geek, we believe you should understand how your camera works so you can better control what it does. Focal length is the biggest factor in determining how your images look, so it’s important that you know how different focal lengths work with your camera.
For example, a 35mm lens (super popular with the great street photographers like Henri Cartier-Bresson) is a wide angle lens on a full frame camera but a normal lens on a crop sensor camera. If you wanted to recreate the look of Cartier-Bresson’s photos with your crop sensor camera, you’d need to use a 24mm lens.
