What’s a “crop factor”?

Crop factor (along with “cropped sensor”) is a term sure to trip up digital photography newbies.  It’s certainly not one of the first things you need to learn about, but you should probably have a handle on it before you start amassing too much equipment, because it has to do with the physical size of the stuff in your camera, and it matters for a whole bunch of reasons.  If you’re ready to dive in, lets’s take a crack at this.

First, let’s get retro for a minute.  Remember film?  If you do, you almost certainly remember 35mm film canisters, but how about 110 canisters?  Kodak instant cameras?  The old bellows cameras cameras you find in antique stores today?  All of these were different types and formats of film, and just as those formats varied from one film camera to the next, today’s digital cameras also have different types and sizes of image sensors.

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Wikipedia

Digital image sensors are typically sized relative to a 35mm film exposure, so a digital sensor the same size as an exposure of 35mm film is said to be a “full frame” sensor.  Although we’re seeing more and more true full-frame cameras these days, they’ve historically been few in number and quite pricey, since the sensors get expensive to produce in large sizes.  In any event, most cameras have sensors about two thirds the size of a full-frame sensor, and these are typically known as cropped sensors.

Sensors certainly come smaller than typical “cropped” size — like the 4/3 size commonly used in compact mirrorless cameras or even smaller sizes used in point & shoot cameras and cell phones.  There are also much larger digital sensors, too.  Medium format sensors are once again as big as a full-frame sensor.  So, why does sensor size matter?

First and foremost, sensor size often determines lens compatibility.  Most medium format cameras will only work with lenses made specifically for those bodies, and (adapters notwithstanding), the same can be said for 4/3 cameras.  For camera owners in the middle, though, it’s possible to mix & match cameras & lenses, so a little more discussion will be needed for most of you.  For Canon, Nikon, and Sony owners (and maybe more), you may have a choice to buy lenses made specifically for full-frame or cropped-sensor bodies, although the lenses & cameras share the same mount, and therein begins the confusion.

Same mount means the lenses are (mostly) interchangeable, so why do they say one’s made for full-frame and one for cropped sensors?  Here’s the long and short of it.

  • Generally, full-frame lenses work on cropped-sensor bodies, but the reverse isn’t true.  Depending on brand, crop-sensor lenses are prevented from mounting on full-frame bodies because their rear elements can actually interfere with mirror operation; in other cases, crop-sensor lenses will mount, but may appear with a vignette in the viewfinder and in photos.
  • Actual lens compatibility / incompatibility is a factor only for wide-angle lenses.  By the time focal lengths reach about 40mm, interference and/or vignetting are no longer an issue.
  • A lens with a given focal length will appear to be longer on a cropped-sensor camera.  This is known as the crop factor, and it’s typically 1.5 – 1.6x.  In other words, a 100mm lens will appear to be about a 160mm lens on a camera with a 1.6 crop factor.  The notion that the lens is longer is really an illusion, but we can save that for another time.

Most of the time, this will be of little consequence to you, but it’s worth keeping in mind when comparing cameras and lenses to make sure you’re comparing apples to apples, so to speak, and it’ll certainly be important when you’re planning your equipment purchases.

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