Full Frame vs. APS-C and MFT: Crop factor explained
Written by Daven Mathies
What's a sensor?
The sensor is the most important part of your camera; it's the thing that collects the light, the digital equivalent of film. Resolution—basically, the number of pixels—used to be the main defining metric of image sensors, but physical size is actually more important. As smartphones continue to take over the entry-level photography market, more and more attention is being given to sensor size in how cameras are marketed today. There are a plethora of sensor sizes and no real standard for describing their size. Some state what looks like the actual measured size, such as 1/1.7", 2/3", or 1". Panasonic and Olympus' Micro Four Thirds cameras use the enigmatic and improper fraction 4/3. These numbers do not measure active imaging area, but are related to the size of the sensor (and, at least in the case of 4/3, are throwbacks to old video tube designs). Nikon uses the completely made-up designations of CX, DX, and FX, to refer to 1", APS-C, and full frame, while Canon generally sticks to the terms full frame and APS-C—even though their version of APS-C is slightly smaller than the standard APS-C size used by Nikon, Sony, and Fujifilm. Are you excited yet?
APS-who?
Gesundheit! APS-C takes its name from the failed late-nineties film format called Advanced Photo System, which offered photographers three frame sizes in one: "High Definition," a 16:9 ratio; "Panoramic," a roughly 3:1 ratio; and "Classic," the standard 3:2 ratio. This system was fantastic, because it allowed your mom to accidentally shoot an entire role of film in Panoramic mode and not realize it until the 4x11" prints came back from the lab with everyone's heads cut off and seven inches of negative space surrounding them. Anyway, the C in APS-C is for "Classic", and digital APS-C cameras offer roughly the same frame size as APS film shot in this mode. Until last year, Canon also made digital SLRs with APS-H sensors, presumably named after the larger "High Definition" APS frame size option, although Canon stuck to the standard 3:2 ratio. (That "High Definition" has nothing to do with what high definition means today.) Nobody has yet made an APS-P sensor, which is a good thing. What you do with this knowledge is up to you.
What's the deal with "full frame?"
Full frame simply means the digital sensor offers the same surface area as a frame of 35mm film, and it has become somewhat synonymous with "professional" in photography jargon. It is not, however, the largest frame size: there are various medium format cameras that offer significantly larger sensors, despite "medium" making one think of something less than "full." Bigger still is large format, which has yet to truly transition to digital due to the ridiculously high cost of making a sensor that huge, although some digital solutions do exist. But since medium and large formats were not as popular as the much smaller 35mm format, 35mm digital is now referred to as "full frame." The more you know...
I was told my 50mm lens isn't actually a 50mm lens on this camera. What's up with that?
To make things more confusing, sensor sizes are often differentiated by a crop factor that uses full frame as a benchmark. A camera is assigned a crop factor based on the difference in diagonal size (not surface area) between its sensor and a full frame sensor. A standard APS-C sensor (Fuji, Sony, Nikon DX) has a 1.5x crop factor, meaning if you divide the diagonal length of a full frame sensor by that of an APS-C sensor, you get about 1.5 (Micro Four Thirds has a 2x crop factor). The reason this number is used, instead of stating the difference in actual surface area, is that it allows you to easily compare how your lenses will look on a smaller sensor camera as opposed to on a full frame camera. A lens with a focal length of 50mm mounted on an APS-C camera, for example, has a similar field of view to a 75mm lens on a full frame camera (50 x 1.5 = 75). This was important in the early days of digital photography, when photographers switching from film needed a quick way to know what to expect when using their lenses on digital cameras. Nowadays, however, the reliance on crop factor is simply a way to confuse consumers looking to buy their first DSLR. After all, despite its direct relationship to it, focal length never referred to angle of view anyway, so knowing that a "50 looks more like a 75" probably means nothing to someone who didn't grow up shooting 35mm film. Your 50mm lens is still a 50mm lens, it just doesn't yield the same field of view as it would on a full frame camera. The only thing you really need to know is this: what you see is what you get. Yes, it took 600 words to get to that staggeringly simple conclusion, but now we're there. You're welcome.
From left to Right: Olympus E-P5 with 17mm (35mm equivalent), Nikon D7000 with 50mm (75mm equivalent), and Canon 6D with 40mm (full frame—no crop factor).[/caption]
Ah, so I don't have to worry about any of that technical nonsense?
But wait, there's more! It's easy to make the connection that bigger is better when it comes to sensor sizes and, in general, you would be right. Larger sensors "see" more light, and the greater the light, the better the image quality (well, I mean, with regards to a sensor being able to produce a clean image, that is. A bathroom-mirror selfie with a 36MP Nikon D810 is still a bathroom-mirror selfie). Full frame sensors will have less noise ("grain") than smaller sensors and work better in low-light situations. However, technology has advanced to the point where APS-C and even 4/3 sensors are so good that they will easily meet the needs of most photographers. Smaller sensors also mean smaller cameras and lenses, and they can be significantly less expensive. I would almost never recommend a full frame camera to the casual photographer for this reason. But, for the concerned professional or advanced amateur who demands superlative resolution and low-light performance, full frame is still the way to go. And it is becoming easier to get there, thanks to cameras like Nikon's D610, Canon's 6D, and Sony's A7-series mirrorless cameras—all of which offer high-resolution, full frame sensors for at or below $2,000 (without a lens). If all you are looking for is a smartphone-destroying camera, though, keep in mind that even the Micro Four Thirds system or cameras using 1" sensors, like Sony's RX100 III or Nikon's 1 series, have sensors much larger than the one that resides in your phone. Furthermore, these smaller cameras aren't much bigger than the current crop of smartphones. 
Sensor sizes compared to full frame (35mm). Oooh, ahhh![/caption]
That's all there is to it, then?
Yes, there you have it. Except that somebody is undoubtedly expecting me to mention something about depth of field. So here you go: with equal framing at equal focal length and equal aperture, depth of field will decrease as sensor size increases. So you could say, depth of field is inversely proportional to sensor size—given all of the above variables remain, err, constant. Of course, if you say that, as I just did, you will probably confuse the heck out of everyone—including me. But yes, sensor size does affect depth of field, but in truth, it does so indirectly. The crop factor of a smaller sensor generally means you will be shooting with a shorter focal length lens or from a greater distance to your subject, and these two things directly increase depth of field. Equivalent aperture is a term that's become popular among camera reviewers recently as a way to describe the depth of field difference between crop and full frame cameras. Simply multiply your aperture, just as you would your focal length, by your crop factor to find the equivalent full frame aperture. So, a 35mm f/1.8 on APS-C is roughly equivalent to a 50mm f/2.8 on a full-frame camera. You could, however, shoot with the same lens, at the same aperture, from the same distance, and get the same depth of field regardless of sensor size—but you would have completely different framing from each sensor. The full frame picture might be a full-body portrait; an APS-C sensor, a three-quarters portrait; a 1" sensor would give you a headshot; and a 1/2.5" point-and-shoot sensor would produce a close-up of an eye.
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