Every digital camera, even a point-and-shoot, has a sensor inside of it. For DSLRs, it is hiding behind a mirror and looks like a green rectangle. For mirrorless cameras, you can see the sensor immediately after removing the lens. The sensor is what conveys information that results in an image. It is what we popularly use now to make pictures instead of film. In the simplest of terms, all these sensors do is convert an optical image (light) into an electronic signal which can be read as digital information – an image you download and can see, edit, and share.
Defining Crop Sensors and Full Frame Sensors
A piece of 35mm film measures approximately 36 x 24mm in size and that’s the size of the sensor in full frame cameras like the Nikon D5 and the Canon 5D Mark IV. Full frame sensor cameras are among some of the most expensive DSLRs you can buy.
There are also crop frame sensor cameras. Imagine a 35mm piece of film, crop the edges down, and that’s your crop frame sensor. Cameras like the Nikon D500 and the Sony a6500 have APS-C-sized sensors that measure about 23.6 x 15.7mm (this varies slightly among manufacturers). Most crop sensor DSLRs use this APS-C format (there is also a rarer APS-H format), which is a 3:2 ratio (as is full frame) but approximates the size of Advanced Photo System Classic film, which is closer to 24mm rather than 35mm. You can fit more cropped sensors on a silicon wafer during production than full frame-sized sensors so the yield is higher, making the cost lower. This is why full frame cameras are almost always pricier than crop frame cameras (and will remain so, even as wafers get larger – the yield will always favor a smaller sensor). Since crop sensors are smaller, the cameras they go into can also be smaller – another price point factor.
Crop Sensors, Crop Factors, and Fields of View
The differences in sensors matter because the size of your sensor will affect your perceived field of view when using the same lens on a camera with an APS-C sensor compared to a full frame one. For example, if you put an 18mm lens on a crop sensor camera, it would look as though you were using around a 27-28.8mm lens on a full frame. How do we know that? If you take the diagonal measurement of a 35mm piece of film (or full frame sensor) and divide it by the diagonal of the crop sensor then the result is going to be your crop factor, also known as your focal length multiplier. Most people just memorize the most popular ones:
• Crop factor for Nikon, Pentax, Sony E Mount, Samsung, Fuji crop sensor cameras = 1.5x
• Crop factor for Canon crop sensor cameras = 1.6x
• Crop factor for any Micro Four Thirds sensor camera = 2x
• Crop factor for many point & shoots, including the Sony RX100 Series and RX10 Series = 2.7x
• Crop factor (approximate) for Super 16 = 3x
• Crop factor (approximate) for GoPros and some cell phone cameras: 5.6x
• Crop factor (approximate) for many cell phone cameras: 7.21x
Take any lens’ focal length, multiply it by your crop factor/focal length multiplier, and you will have the perceived field of view that lens produces on your particular crop frame camera that would be considered the “equivalent” on a full frame camera:
18mm x 1.6 = 28.8mm. Your 18mm lens will produce a field of view on your crop sensor camera similar to what a 28.8mm lens would on a full frame one. So if you’re looking for a very wide scene, then you might want to pick out a lens that is wider than 18mm to compensate for your crop.
For any lens, your field of view is reduced by an amount that is proportional to the ratio between your crop sensor and a 35mm-sized sensor. That reduction in field of view is greater the smaller your sensor is. The crop factors listed above help make those calculations quicker for you. Once you know the crop factor of your camera’s sensor, you can then find the actual field of view for any lens attached to it.
When to Calculate a Crop Factor
Full frame users don’t have to worry about this. What they read on the lens is what they’ll get. Crop frame users have to do a little extra math – even with lenses built specifically for crop frame cameras, like DX Nikon lenses or EF-S Canon lenses. The projected image circle on crop frame-specific lenses isn’t larger than what can be used by the crop frame camera (unlike full frame-specific lenses, which project more than what crop sensor cameras can use). This allows manufacturers to make crop frame-specific lenses correspondingly smaller and also more affordable. But the focal length multiplier still exists.
The focal length of the lens is measured based on the standard 35mm film size – a size that lenses are built to accommodate. Why? In the digital photography world, the 35mm size is our reference point for all imagery so it is the standard reference for all lenses, too. This means that when you are using a crop sensor camera, you have to figure out what your crop factor/focal length multiplier is for a given lens to find out what your apparent field of view will be when composing a shot. It’s just the way it is.
You might be asking yourself, “If I mount a 24mm lens on a crop sensor camera and a 36mm lens on a full frame camera and took the same picture, my results would be identical?” No, though your the results will have a very similar fields of view. There are other qualitative differences with sensor sizes, especially for depth of field. Learn more about this in How Crop Sensors Affect Depth of Field.
A Note on Magnification
Sometimes you will hear the term “Magnification Factor” when referring to crop sensor cameras and the effect they have on perceived lens length. Note that your lens isn’t shooting at a focal length that is any longer than what the lens actually is. The image only appears magnified on the smaller sensor because it is producing a narrower angle of view. The focal plane distance between you and a subject is entirely the same whether you’re using a crop frame sensor or a full frame one. It is just that the image is displayed as “enlarged” within the reduced field of view on crop frame sensors to produce a result that matches our “standard” – the standard always being 35mm.
Crop Factors and Built-In Lenses
Sometimes manufacturers will list both “focal lengths” on their lenses – though this is rare (a notable example is the Canon EF 8-15mm f/4L Fisheye, which notes positioning for ranges in both full frame and for crop frame). If you’re shooting with a crop sensor camera, you must be prepared to multiply the crop factor (1.6, 1.5, etc) by the listed focal length. This is especially important for wide angle shooters who are most negatively affected by any narrowing of field of view.
Sensor Comparison Resources
If you’d like to explore crop factors and field of view changes even further, these are some of my favorite resources:
Image Sensor Size Specs Comparison
This page allows you to view camera and phone sensors side-by-side.
You can see visual approximations of sensors side-by-side and if you scroll down (not visible in the picture above) there are more detailed side-by-side specifications where you can easily find your crop factor number to multiple by the focal length(s) you’re using. The only downside to this resource is it doesn’t appear to have been updated with all the latest gear and phones in awhile.
Field of View Comparator
This page gives you a cityscape scene with which to compare fields of view side-by-side for various sensor sizes. It also gives you the option to change lenses and see the results.
As you can see in the example above, it will show you the crop factor and even the focal lengths you’d need to find “equivalent” fields of view for the scene. It’s a wonderful tool. It doesn’t list every camera in its dropdown but that is because so many cameras have the same sensor and crop factor so if you don’t see your camera in the list, find out what size sensor is in your camera and then just choose a camera from the dropdown that has the same sensor size and crop factor as yours – then play around with different focal lengths virtually before committing.
Camera Sensor In-Depth Comparison Tool
This page does a deep dive into comparing sensor quality under a variety of conditions.
This is a great resource for if you’re on the fence between a couple of items and you just want something to tip you over the edge one way or another. This site also does lens sharpness tests. Here I selected some cameras at random but it is best to compare cameras of comparable sensor size to get a more fair sense of which one is the “winner” for you among different brands.
Studio Shot Comparison
This page gives you a mock studio scene that you can “shoot” with up to 4 cameras.
What I love about this tool is that it will automatically select the focal length of the lens for each chosen camera that will best approximate the same field of view as the original camera/lens combo you selected as your standard. Plus, you can “shoot” different parts of the studio scene and compare JPEG and RAW visualizations. It’s a very fun tool.
I hope this primer and these resources help better guide your lens buying/renting decisions for your next shoot!
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