Full Frame vs. APS-C and MFT: Crop factor explained

by | Aug 21, 2013

What is a sensor?

In short, it’s the thing that collects the light; the digital equivalent of film. The sensor is composed of millions of pixels (1 megapixel equals 1 million pixels). The number of megapixels used to be the most marketable and identifiable metric of image quality, but today, the overall physical size of the sensor is 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. There are a plethora of sensor sizes on the market 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 of 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, respectively. 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.

Isn’t this interesting? Are you excited yet? 😉

Olympus Micro Four Thirds sensor

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,” (roughly 3:1), 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 2012, Canon also made digital SLRs with APS-H sensors, perhaps named after the larger “High Definition” APS frame size, although Canon stuck to the standard 3:2 ratio.

(Note: APS “High Definition” has nothing to do with what high definition means today. Also, nobody has yet made an APS-P sensor, which is a good thing, because of the aforementioned 7″ of negative space issue.)

What you do with this knowledge is up to you.

So, 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”).

Larger still is the aptly named large format, which has yet to truly transition to digital due to the ridiculously high cost of making a sensor that big. Although, some digital solutions do exist. Since both medium and large formats are not as popular as the much smaller 35mm format, 35mm digital is now referred to as “full frame.”

The more you know…

MFT, APS-C, and Full Frame cameras

I was told my 50mm lens isn’t actually a 50mm lens on my APS-C 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 the 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.

Using this standard, an APS-C sensor (Fujifilm, Sony E, 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. Canon APS-C has a 1.6x crop factor, while Micro Four Thirds has a 2x crop factor.

The reason this number is used, instead of stating the actual difference in 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 field of view equivalent to a 75mm lens on a full frame camera (50mm x 1.5 = 75mm).

The assumption here is that photographers are already familiar with how a lens looks on a full frame camera. That may have been the case in the early days of digital photography, when everyone was transitioning from 35mm film and professionals, especially, needed to know how their lenses would change on new digital bodies. 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 “50mm looks more like a 75mm” probably means nothing to someone who didn’t grow up shooting 35mm film or who at least studied photography in school. 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.

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 more light you have to work with, 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 work better in low light settings, and produce less noise (the digital equivalent of film grain) than smaller sensors. However, technology has advanced to the point where APS-C and even 4/3 sensors are pretty dang good, and both will easily meet the needs of most photographers.

Smaller sensors also mean smaller cameras and lenses, and are usually 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.

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 super-sized smartphones.

Sensor sizes, compared

But if I want a shallow depth of field, I need a big sensor, right?

Here you go: with equal framing at equal focal length and equal aperture, depth of field will decrease as sensor size increases. That doesn’t mean you can’t achieve a shallow depth of field on a Micro Four Thirds camera, you’ll just need a lens with a wider aperture than what would be required to achieve the same depth of field on a full frame camera.

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. Yes, sensor size does affect depth of field, but it actually does so indirectly.

Stay with me: 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, in order to get the same framing. These two things directly increase depth of field. Equivalent aperture is a term that’s been coined to describe this effect, and is mainly used by camera reviewers as a way to explain the depth of field difference between crop and full frame cameras.

You can get your equivalent aperture just as you would your equivalent focal length, by multiplying the set aperture by the crop factor. So, a 35mm f/1.8 lens on an APS-C sensor is roughly equivalent to a 50mm f/2.8 lens 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 get completely different framing from each sensor. For example, the full frame picture might be a full-body portrait; an APS-C sensor would make, say, 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. Or something like that.

What can I take away from all of this?

The most important thing, again, is that what you see is what you get. If you’re shooting a 50mm lens on an APS-C camera, you don’t need to be aware that it’s behaving like a 75mm lens—you just need to frame your shot.

If you commonly shoot in dim lighting, like music concerts or wedding receptions, you will benefit most from getting a full frame sensor. Likewise, if you really can’t do with anything but the shallowest of depths of field, then the larger the better.

If, however, you’re a casual shooter who wants a camera to take vacation or family photos, I would recommend sticking with something smaller. A smaller camera is more likely to travel with you and, therefore, you’re more likely to get good pictures with it. After all, there’s a reason so many people take pictures on lower-quality phone cameras, or listen to compressed music on YouTube, or eat out instead of cook at home: convenience. So if you’re in the market for a new camera, do yourself a favor and buy one you’ll actually want to use.

Oh, would you look at that? It’s five o’clock somewhere.

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