Camera sensors come in different sizes. The one in your smartphone is a lot smaller than the one in my Canon 5D MKIII, a professional DSLR. For high quality mirrorless and DSLR cameras, there are two main sensor sizes: 35mm (generally referred to as “full frame”) and APS-C (normally called a “crop sensor” or “crop camera”). Let’s look at the difference between the two.

Sensor Size, Explained

The size of the sensor is just that: the physical size of the sensor. A 35mm sensor is actually 36mm x 24mm. It’s the same size as the 35mm film it replaced. A crop sensor is called that because it’s cropped to a smaller size than a 35mm sensor (or piece of film). Exactly how much smaller and what that means we’ll get to in a minute.

The relative sizes of 35mm (pink), APS-C Nikon (red) and APS-C Canon (green).

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Sensor size has nothing to do with the number of megapixels. You can get 20 megapixel full frame sensors and 20 megapixel crop sensors. A 10 megapixel full frame sensor will still be physically bigger than a 24 megapixel crop sensor. The difference is that on a crop sensor, each individual photosite (the tiny little sensors that detect the light for each pixel) is going to be smaller.

Full Frame Cameras Are Better Quality, Especially in Low Light

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Since the photosites on a full frame camera are larger, all else being equal, a full frame camera will be better in low light situations than a crop sensor camera. More photons fall on each photosite, so they have more data to work with.

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Each photosite is likely to be of higher quality as well. Full frame cameras are more expensive and there is just more space on the sensor for high quality components. This means you normally are able to use a higher ISO setting before starting to see digital noise in your photos.

These same effects also hold true when you’ve lots of light to work with: full frame cameras are better at resolving accurate colors.

Crop Sensors Have a Different Field of View With the Same Lens

While the low light performance is a nice benefit of full frame cameras, it is far from the most noticeable difference. Full frame cameras and crop sensor cameras often use the same lenses, and even when they don’t, the crop sensor lenses are described as if they are full frame lenses.

Imagine you have a Pringles tube with the bottom cut out. If you hold it a few inches from your face, you’ll see a circular image. This is similar to what your lens is actually projecting into your camera.

Now take the imaginary lid and cut a 36mm x 24mm rectangle in it. Put the lid on and what you see through the hole is how much of the image projection a full frame camera is actually capturing. It takes a rectangular crop and ignores the rest of the projection.

Grab another imaginary lid and cut a second rectangle, this time make it a little over half the size of the first; around 22.5mm x 15mm. That’s roughly the size of a crop sensor. This time, the rectangular crop is throwing away even more information.

This is where are thought experiment gets a little trickier. If both our full frame Pringles tube and crop sensor Pringles tube have the same number of megapixels, even though the hole in the crop tube is smaller, the image it produces is the exact same resolution as the one produced by the full frame tube. On your computer screen, the images will appear the exact same size.

The difference is, however, the image taken with the crop sensor Pringles tube will appear as if it’s zoomed in.

Let’s look at this with some real photos. Below is an image I shot with my full frame 5D MKIII and a 50mm lens.

And here is an image shot with my crop sensor Canon 650D from the same spot with the exact same 50mm lens.

As you can see, the image shot with crop sensor camera appears zoomed in. In reality, it’s because the sensor has taken a tighter crop from the lens’ projection.

Crop Factor and Focal Length

How a crop sensor camera effects the photos you take is entirely predictable. Crop sensor cameras have a “crop factor” which describes how much they appear to magnify the image they take. For Canon cameras, the crop factor is about 1.6. For Nikon cameras, it’s about 1.5.

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What the crop factor tells us is the full frame equivalent focal length (and thus field of view) that you get from a crop sensor camera. To use it, you just multiply the actual focal length of the lens by the crop factor.

Continuing the example from above, the 50mm lens on my 650D is equivalent to an 80mm lens on my 5D MKIII; just multiply the lens focal length, 50mm, by the crop factor, 1.6, and that’s what you get. We can prove this in practice. Below is a photo I shot with my 5D MKIII and my 85mm lens.

And here it is side-by-side with the photo I took on my 650D with the 50mm lens. As you can see, the photos look pretty similar.

Which Is Right for You?

Full frame cameras, in general, are higher quality and better made than crop sensor cameras. They’re the flagship models with all the latest features. Most manufacturers’ crop sensor cameras are their entry or mid-level models. However, the gap isn’t as big as it used to be. Modern entry level cameras are better than the ones professionals were using just a few years ago. It’s not likely you’ll notice the difference in image quality unless you’re shooting in very specific circumstances.

Since full frame cameras tend to have a lot of extras, like improved autofocus or build quality, sensor size is only one factor in choosing a camera. The biggest reason I bought my Canon 5D MKIII wasn’t that it was a full frame camera, but that it was weather sealed and made fully of metal. It means I can carry it anywhere when I travel without having to worry too much. If you want a small, light camera, then you’re probably better off with a crop sensor. Even the mirrorless full frame cameras are pretty big when you put a zoom lens on them.

There are even professional level crop bodies, like the Canon 7D MKII for sports or wildlife photographers. Rather than a downside, the crop factor actually helps them get closer to the action.

Title photo credit: Michael Toyama/Flickr

Profile Photo for Harry Guinness Harry Guinness
Harry Guinness is a photography expert and writer with nearly a decade of experience. His work has been published in newspapers like The New York Times and on a variety of other websites, from Lifehacker to Popular Science and Medium's OneZero.
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