Full Frame vs. APS-C: sensor size and the consequences it has for you

You will probably have read here and there (or even in the comments made on this site) the obscure terms “Full Frame” or “APS-C”, which apparently refer to different sensor sizes. Let’s take a look together at what these terms mean exactly and why it is useful to understand them. You will discover that they are really simple to understand!

Before digital photography, light was captured by a photosensitive surface, which was commonly called photographic film. This could also be done with other types of surfaces, but let’s not get into all that, since what we are interested in is digital photography. On digital cameras, the film has been replace by an electronic sensor which is sensitive to light.

The different sensor sizes

Although other formats did exist (namely the medium size format), the “small format” 24×36 mm film (with a 3:2 ratio), also called “35 mm”, was the most popular and ended up becoming the film camera standard. Logically, digital cameras should have adopted this norm, but for both technical and economic reasons (a large sensor being more expensive than a small one), digital cameras have sensors of all different sizes. I will list these sizes so that you can understand what I am talking about and so that you will be able to determine what kind of sensor your camera uses:

  • full format” or “Full Frame” sensors are 24×36 mm. Equivalent in size to film, these sensors are present on certain high-end cameras (Canon EOS 5D and 1Ds, Nikon D3s and D700, and Sony a900)
  • APS sensors are rather large, but smaller than full format sensors; they are found on reflex cameras ranging from entry-level to high-end.
    1, The APS-H format is 1.3 times smaller than the full format. It is only used on the Canon EOS 1D.
    2, The APS-C format is 1.5 (Nikon, Sony, Pentax) to 1.6 times (Canon) smaller than the full format. Very common, it is used on all the digital reflex cameras that I have not yet named.
  • The micro 4/3 system is around 2 times smaller than a full format sensor, and with a proportion ration of 4:3 and not 3:2, it is generally found on compact cameras with interchangeable lenses as well as on Olympus E reflex cameras
  • A whole range of small 4/3 sensors (around 3 times smaller than a Full Frame sensor) are used on all conventional compact and bridge cameras

To give you an idea of the size of the different sensors, I simplified the chart taken from Wikipedia, and kept the dimensions of the “medium format” to give you an idea of the enormous difference is size (and to make you want to become indebted for life in order to buy one of these little marvels :P). The 4 little rectangles at the bottom left correspond to the different sensors used on existing compact and bridge cameras.

Consequences for the photographer

You will probably be saying to yourself: that’s nice, but I don’t really care! Except that the size of the sensor has interesting consequences that you are better off knowing about.

Image quality

I should instead be speaking about sensitivity or dynamic range, but these concepts are a little technical and it wouldn’t be useful to get into them at the moment. Let’s just have a quick look at how sensors work in order to better understand their characteristics. Basically, a sensor is made up of millions of photosites, each one of which has the ability to capture light – its red, green and blue spectral components – just like the human eye (yep, even digital cameras copied nature’s design ). These photosites are commonly referred to as “megapixels” for commercial purposes. 12 megapixels is equivalent to 12 million photosites on the sensor.

You are probably thinking that the more photosites the better and are congratulating yourself for having bought the 15 megapixel model instead of the 12. However, we haven’t yet taken the size of the sensor into consideration.

Imagine that you are trying to squeeze 12 million people into a rectangular enclosure. You’re in luck, you have a very large enclosure and everyone has enough room. Now, if you try to fit 15 million people in the same enclosure, each person will have less room. It’s the same thing for photosites: the bigger the sensor (like the enclosure), the more spread out they can be. Inversely, the more photosites there are, the less room there is for each one. For the same number of megapixels, a reflex camera’s photosites are in general 12 times larger than those of a compact camera! (since reflex cameras tend to have more megapixels, in reality, the photosites are anywhere from 8 to 12 times larger).

You might be thinking: “so what if they are more closely packed, they still capture light”. Yes, but… To make things easy to understand, the larger a photosite is, the more precisely it will be able to capture light. This means that there will be less noise and in general a better image quality.

You can see that the sensor’s size is important for the final image quality. That is why I recommend reflex cameras in order to obtain better quality images (in terms of technical characteristics and not artistic ones, obviously). Buying a full format reflex camera is often expensive, but an APS-C sensor will already be vastly superior to a compact or bridge camera’s sensor – which is ridiculously small.

The apparent focal length

A lens’s focal length does not really change depending on the camera it is mounted on. That having been said, the smaller the sensor, the greater the apparent focal length will be – that is to say, the more the subject will be enlarged relative to what is seen in reality. That is why reference is often made to “24×36 equivalence”when discussing lenses. In fact, the focal length indicated on a lens always corresponds to its equivalent focal length on a full format sensor camera. Does this sound like gibberish? Let’s look at an example:

If you mount a 50 mm lens on a camera with an APS-C sensor, you will have about the same angle of view in the viewfinder as you would if you mounted an 80 mm lens on a full frame sensor camera. In fact, on a camera which has a smaller than full format sensor (most cameras), the apparent focal length will be multiplied by the sensor’s multiplication coefficient.

The WHAT coefficient?

I fooled you! I placed this multiplication coefficient at the very beginning of the article without your even noticing it. For a Canon APS-C , this value is 1.6 for example! Let’s look at some examples to try to better understand:

Correspondence between apparent focal length and sensor format

Full Frame Canon APS-C Nikon, Sony, Pentax APS-C
10 mm 16 mm 15 mm
100 mm 160 mm 150 mm
18 mm 29 mm 27 mm
200 mm 320 mm 300 mm
28 mm 45 mm 42 mm
300 mm 480 mm 450 mm
35 mm 56 mm 52 mm
400 mm 640 mm 600 mm
55 mm 88 mm 82 mm
70 mm 112 mm 105 mm
85 mm 136 mm 127 mm

You will probably have understood by now that wide-angle lenses will have less of a wide angle (which can be a disadvantage for landscape photography for example), but zooms will have increased zooming power (which is an advantage for nature photography for example). This is not a negligible factor when deciding what camera to buy. But, don’t forget that a bigger sensor also means better image quality .

(FYI: this also works the other way around. With a medium format sensor, a 100 mm focal length will be shorter! )

Therefore, in reality, the focal length indicated on the lens is less important than the real angle of view that the lens will give you when mounted on your camera.

That’s it, I hope that this article will have helped you to understand that a 50 mm lens is better for portraiture when used with an APS-C sensor rather than a full format one, that an 18 mm lens mounted on your reflex camera is not as wide-angle and you thought it would be (except if you are lucky enough to have a Full Frame camera), that 21 megapixels is good, but if the sensor is the size of your little finger’s nail, it will be less good, etc…

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