What is Crop Factor? Here is What You Need to Know

“Crop factor” is a term that is often heard in the world of digital photography, especially when discussing entry-level cameras. If you have no idea what it means, this article is a basic explanation that should help fill you in.

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A Term Rooted in Film Photography

After the early plate era of photography transitioned into the era of film photography, 35mm film became the dominant still photography film format in the photo industry. Also known as 135 film, it was introduced by Kodak in 1934 and was based on an earlier 35mm cinema film that photographers had been using for still photography. By the mid-to-late 1900s, 35mm film had overtaken the medium format 120 film format in popularity, and it continues to be the most widely used format among analog photographers even today.

35mm film features a frame size of 24×36mm and an aspect ratio of 2:3.

When the film photography era transitioned into the digital photography era in the late 1990s and early 2000s, the standard 35mm frame size became known as “full-frame” in the world of digital cameras, and this is the reference point by which the “crop factor” of “crop sensors” is discussed.

The size of the image sensor in a “full-frame” digital camera is the same as a frame of 35mm film. Film strip from Depositphotos

What is Crop Factor?

Here is the simple definition of crop factor: Crop factor is the ratio of the 35mm full-frame diagonal to the diagonal of a smaller crop sensor.

A 35mm full-frame sensor has a diagonal that measures 43.3mm. Once the digital revolution happened, the camera industry was no longer constrained to the sizes of popular film formats, and manufacturers released digital cameras with a wide range of image sensor sizes — some cameras are full-frame, many have sensors smaller than full-frame , and a handful are larger than full-frame.

This difference produced is known as the “crop factor” (sometimes referred to as the “format factor” or “focal length multiplier”), which expresses this difference mathematically — it’s a number that tells us how a lens behaves in relation to the size of the image sensor.

It’s important to understand that the crop factor doesn’t affect your lens focal length or its speed, based on the size of the lens aperture. It just allows a photographer to understand what the 35mm equivalent is as a baseline or base reference for comparing various lenses. In the photography world, that baseline is still 35mm film.

35mm, or 135mm film as it is known in photography, has a film image area of ​​24 x 36mm (2:3). Notice that it is longer than 35mm. In photography, the film is placed along the horizontal plane, giving the photographer a longer value in width.

By contrast, in motion picture photography, the 35mm film is usually placed vertically, and as such, those values ​​are flipped, making it 3:2, or roughly 25.1×16.7 mm. Of course, that’s a smaller image area. But the ratio of the distance remains the same, only flipped. This same ratio was carried down to the popular APS-C image sensor, which is roughly based on the size of the Advanced Photo System film negative in its C (“Classic”) format, which measures 25.1×16.7mm.

A look at the relative sizes of an APS-C sensor (left) and a full-frame sensor (right) in a Canon Rebel XT and a Canon 5D Mark II (respectively). Photo by Autopilot and licensed under CC BY-SA 3.0.

Digitally, there is no film to place horizontally or vertically, but those same dimensions have carried over and are used to the crop factors between the two. So, if a photographer shoots with both APS-C and full-frame, they can quickly do math in their head to understand how each lens functions between the two.

Common Sensor Sizes and Crop Factors

Camera manufacturers use different branding designs for their crop sensor format cameras and lenses.

  • Nikon DX generally has a crop factor of 1.5x.
  • Sony E (non-FE) has a crop factor of 1.5x
  • Fujifilm X has a crop factor of 1.5x
  • Pentax DA has a crop factor of 1.5x
  • Samsung NX has a crop factor of 1.5x
  • Canon EF-S/EF-M has a crop factor of 1.6x
  • Sigma Foveon has a crop factor of 1.7x
  • Micro Four Thirds has a crop factor of 2.0x

Sigma DC, Tamron Di-II, and Tokina DX are abbreviations used by the third-party lens manufacturers for the APS-C image sensor formats of multiple camera manufacturers, so the lenses may vary in crop factor.

Here is an illustration showing the sizes of popular smaller image sensor formats relative to 35mm full-frame:

This illustration shows common sizes stacked to show their relative sizes more clearly:

Common crop sensor sizes overlaid on 35mm full-frame. Illustration by Moxfyre and licensed under CC BY-SA 3.0.

Equivalent Focal Length

The field of view changes between a full-frame sensor and a crop sensor, and with that change in the field of view comes a crop factor in order to understand what would be the full-frame 35mm equivalent. The crop factor is meant to inform the photographer how a lens would behave in its field of view.

A smaller sensor captures a smaller area of ​​a lens’s image circle, resulting in a narrower angle of view compared to what is captured by a full-frame sensor. Illustration by KymFarnik and licensed under GFDL-1.2-or-later.

For example, using a 35mm lens on an APS-C camera will usually have a crop factor of around 1.5x or 1.6x. Doing the math (35 x 1.5 = 52.5mm) that same 35mm lens has a field of view that is the 35mm equivalent of 50mm. So while a 35mm lens is considered to be a wide angle lens on a full-frame camera, it is basically the equivalent of a normal 50mm lens when used on an APS-C crop sensor camera.

Here is an illustration of this principle:

An illustration of equivalent focal lengths when shooting a portrait with two different lenses and two different sensor sizes. Illustration by Rama and licensed under CC BY-SA 2.0 FR.

A 50mm lens produces two different field of views when used on full-frame and APS-C crop sensor cameras. The narrower field of view captured with a 50mm lens on an APS-C camera is the equivalent of what you get when using a 70mm lens on a full-frame camera.

Here is a chart of common focal lengths and how the 35mm equivalents of those focal lengths on three common crop factors:

The issue of equivalent focal lengths also comes into play due to the fact that APS-C sensor cameras may support both APS-C lenses designed for that sensor as well as full-frame lenses designed for full-frame cameras (eg Canon’s crop sensor DSLRs support both full-frame EF as well as crop sensor EF-S lenses). Understanding crop factor will help a photographer understand why the same lens produces different photos when used on full-frame and crop-sensor cameras.

If you own both a full-frame Canon 5D Mark IV and an APS-C Canon 90D, a 35mm lens will be wide angle on the former and a normal lens on the latter, and a 50mm lens will be a normal lens on the former and a telephoto lens on the latter.

A Common Baseline for Discussing Focal Lengths

Before the early days of the digital revolution, nobody really cared about crop factors since the lion’s share of photography was shot on 35mm film. Sure, there were smaller film sizes like 110 for cheap Instamatic cameras, but those shutterbugs didn’t know or even care about the crop factor that the cameras had. They just pointed and shot.

But when the art moved from analog film to digital cameras, the most common sensors weren’t full-frame — they were much smaller. Being physically smaller, these sensors capture a smaller area of ​​the projected image, resulting in a narrower angle of view.

Suddenly, those 35mm film lenses didn’t have the focal length visually that they have with film cameras. The focal length of the lens was the same, mind you, but they showed a narrower view, to begin with, due to the sensor size and the image circle that the sensor could see.

The crop factor of each digital camera system became an important common baseline for discussing and considering lens focal lengths. With the crop factor, shooters could calculate the field of view of their lenses and know going in that a 50mm full-frame lens would only have a narrower field of view of 75mm on an APS-C camera (or Super 35mm, which is the reference for the video world).

The resulting photos captured using the same focal length on full-frame (red), an APS-C sensor with a 1.5x crop (blue), and a Four Thirds sensor with a 2.0x crop (green). Illustration by Compumundo and licensed under CC BY-SA 3.0.

So the problem is one of comparison. If the same lens can produce different images on different cameras, how can a photographer predict what field of view they’ll cover on those cameras? Thus, the crop factor became apparent. Eventually, Canon developed the EF-S mount, which provided APS-C sized lenses, and thus, the crop factor was negated. But it’s still a concept that is equally important today.

Just as using a full-frame lens on a crop sensor will produce a narrower, “zoomed-in” field of view, using a crop sensor lens on a full-frame camera (with an adapter) will produce the opposite effect — a” zoomed out” result. Things get wider, so wide, in fact, that the image will likely become vignetted as the image size projected by the lens is not large enough to properly cover the imaging area of ​​the sensor.

If you only shoot a full frame camera with full frame lenses, the subject of crop factor may not matter too much for you in your photography. But many photographers use cameras with Micro Four Thirds sensors, Nikon’s DX sensors, Foveon sensors, and more, all of which are smaller, and then there are the larger medium format and large format sensors. Each one has its own dimensions, and as such, its own crop factors.

If you change lenses and move from one to the other, then understanding the interplay of crop factors can make a photographer’s life easier.

Apps for Calculating Crop Factor

So when planning a shoot and keeping all those crop factors and lenses straight, what can one do to easily see crop factors at work? As the old saying goes,” there’s an app for that,” and a good aid in managing crop factors while planning a photo shoot is LensKit (available on iOS). This app has a built-in simulator that does the crop factor math for you and simulates what the field of vision will look like. The photographer can input the camera being used, along with the lens, and the crop factor is applied, showing the result.

Another helpful app you may want to look into is mmCalc, available on the web, iOS and Android.

Conclusion

At the end of the day, understanding the crop factor is critical for considering which lenses to use on a shoot based on the sensor size of the camera. The easiest thing to remember is that the smaller the sensor, the larger the crop factor, and the narrower the field of view of the lens.

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