Demystifying “Crop Factor”

By: Keith Mullin

What Is It?

“Crop Factor” is the idea that imagers smaller than a full 35mm stills frame effectively increase the focal length of any given lens.  Essentially they “crop in” on the image being transmitted from the lens onto the focal plane.  For example, the most common crop factor talked about is that of APS-C or Super35 sensors, which have crop factors of 1.5x or 1.6x depending on manufacturer.  This results in a 24mm lens on a APS-C/Super35 camera behaving like a 35mm lens would on a full frame camera.

There are many other sensor sizes out there, each with a different crop factor.  The most commonly referenced sensor size after APS-C/Super35 in terms of crop factor is the Four Thirds sensor.  This sensor is most often found in cameras like the Panasonic GH4 and GH5.  The crop factor of a Four Thirds sensor is roughly 2.0x when compared to a full frame camera.  So on a Four Thirds camera, our 24mm lens would behave like a 48mm lens on a full frame camera.  Most sensors of other sizes (1″, 2/3″, 1/3″, etc) are found in fixed lens cameras or broadcast cameras and don’t get brought up in discussions of crop factor too often.

But What About Crop Sensor Lenses?

There are tons of lenses that are designed to be used exclusively with smaller sensor cameras.  Sigma’s 18-35mm and 50-100mm f/1.8 Art lenses are a great example.  Surely the design of these lenses eliminates the crop factor of the smaller sensor?  So a 24mm crop sensor lens on a crop sensor camera will behave like a 24mm full frame lens on a full frame camera, right?

I hear this argument frequently, but it is incorrect.  Focal length is an expression of a physical measurement between the lens and the image sensor and does not change no matter what size image sensor the lens is designed for.  24mm is 24mm is 24mm no matter who made the lens and what camera or sensor its going to be paired with.  So what then is the difference between a crop sensor lens and a full frame lens?

The difference between full frame and crop sensor lenses is the size of the image circle created by the lens.  Lenses designed for smaller sensor cameras make smaller image circles than ones designed for full frame cameras.

What the crop sensor lens is projecting onto the crop sensor has the same scale as the full frame lens, but the extra “wasted” space of a larger image circle isn’t being transmitted.  They do this to make the lenses more affordable and lighter.  A smaller image circle means less glass, which means less materials, which means it’s cheaper and lighter.

Why I Think Crop Factor is a Mostly Useless Term

I hate it when people start talking about crop factor.  There, I said it.  Why?  Because it is a relative term that is a comparison of one camera with one type of sensor to another camera with another type of sensor.  It doesn’t tell you anything concrete about the camera that is right in front of you.  The information provided by the comparison is really only useful if you are familiar with the characteristics of one of the cameras.  For example, knowing that a 24mm lens on a crop sensor camera behaves like a 35mm lens on a full frame sensor doesn’t do you any good unless you know how a 35mm lens behaves on a full frame camera.  Personally I have spent much more time with crop sensor DSLR’s and Super35 camcorders than full frame cameras, so knowing the crop factor never really helped me out.  I can’t picture in my head what any given focal length looked like on a full frame camera because I don’t have the experience with that size of a sensor.

Why Angle of View is Better

Angle of view is an expression of how wide or narrow a field of view a given sensor has with a given lens.

There is an equation that is used to find the angle of view.

AoV = 2(ARCTAN(Sensor Width in mm/(2*Focal Length in mm)))

It’s not the easiest equation, at least for those of us that don’t do a lot of math all the time.  For example, based on my calculations, a 24mm lens on a Sony FS7 has a horizontal angle of view of 52.4 degrees.  And there it is.  I now know something about my camera and lens pair that is independent of any other camera system or lens.

But the good news is that once you figure out the angle of view for a camera, that’s it.  Or you could do what I did and make an Excel spreadsheet that has the equation programmed in so you can change focal lengths and sensor sizes as models change.  I also keep a copy on my Google drive so I can access it from anywhere.

The row in blue is the width of the sensors in mm, and the yellow column is a selection of different lens focal lengths.

In the chart you can see the difference between the sensor sizes and the affect they have on the angle of view.  You will also note that different camera manufacturers have slightly different specifications of Super35, with Canon being just slightly larger than Sony.

The beauty of angle of view is that once you know the angle for the camera and lens set you’re using, you can do away with a lot of the guess work around camera placement and lens choices.  You will be able to frame things up reasonably well without even turning the camera on.  For me this makes the process of choosing a camera placement and lens needed to create the shot I want much quicker and easier and I very rarely pick a lens and then decide I need to change it, because I am able to use the angle of view to get a good approximation of the shot ahead of time.

Don’t Completely Forget About Crop Factor

Crop factor CAN be useful information to have in some situations.  It will come in exceedingly handy when trying to match cameras with different sensor sizes.  For example, trying to match a Sony a7Sii with a Sony FS7.  Crop factor actually goes way beyond just affecting the focal length, it also applies to aperture, and can be used to set ISO numbers to achieve similar noise profiles.  It’s a quite complex subject, and I’ll let Chris Rule from ShinyEye Studio explain it in this video.  Jump back to the beginning if you want a really great explanation of depth of field, or watch from the 20 minute mark to see how to match cameras.