Canon only had an APS-C (crop) lens mount availble from the Canon EOS 20D on, so all lens mounts previous to that were EF, rather than EF-S mounts, even though the sensors of some of those cameras were crop sensors.
It meant that the existing EF lenses would work on both FF and APS-C sensor bodies (as they still do). For example the first two fully-Canon production DSLR cameras (with APS-C CMOS sensors): the Canon EOS D30 (2000) and D60 (2002), had FF mounts but APS-C sensors. Thus, they could use only FF lenses, but the Field of View (or Angle of View) was cropped by the smaller sensor size. Conversely an EF-S lens is not designed to go on those bodies or any EF lens mount.
I should point out that the focal lengh(s) of a lens does not change by mounting it on a crop sensor body. It is a physical characteristic of the lens and it does not matter what body you connect a lens to. However with a crop sensor, what it views and records from what the lens delivers is smaller - this is the Field of View (FoV or AoV), which is the result of a combination of BOTH the lens and sensor.
In the illustration below light is transmitted through a lens that remains constant (so it models a single FL). The light delivered is constant, but the amount recorded is reduced from that of the FF sensor (shown dotted) by the crop sensor, (shown in solid shade).
The resultant images, if shown at the same dimension give the appearance of having a magnification, but you can see that the FoV is actually reduced.
A lot of confusion exists about the issue because the documentation, and often the discussion, is couched in terms of focal length, but as I pointed out that is only one part of the story. A much more accurate and appropriate term would be to relate to the Field of View. In an attempt to rationalize the two metrics, it is common to talk about an Equivalent Focal Length.
An Equivalent FL tries to give an idea of what the FoV is in FL terms, taking into account the smaller sensor.
In the table below I have listed the common sensors, crop factors & coverage areas relative to a FF -
The crop factor is a factor that describes what one would have to multiply the diagonal value of a crop sensor to get to that of a FF sensor and is often called the crop multiplier. So to get an EQUIVALENT Focal Length, one would multiply the ACTUAL focal length by the crop factor. Thus a 100mm FL lens on a FF sensor would have an EFL of 100mm, but on a crop Canon APS-C it would seem to have a EFL of 160mm. Again the lens hasn't changed in any way, but the resultant FoV has.
The areas given provide an idea of pixel density relative to cropping from a FF sensor to achieve the same reduced FoV. e.g. a 100MP sensor, cropped to the same FoV as a Canon (APS-C) sensor would have its effective pixels reduced to 39MP. It is for that reason that users of long lenses, who want the reduced FoV, prefer crop bodies because the pixel density is much higher than cropping a FF sensor with the same MP value.
Canon APS-H x1.3 59% Nikon APS-C(DX) x1.5 44%
Canon APS-C x1.6 39% Micro 4/3 x2.0 25%
1” (Nikon CX) x2.7 14% Phone 12/3 x5.6 3%
That is why a lot of long lens shooters, like myself, are waiting with bated breath for a Canon EOS R7 (pro APS-C sensor) body to be produced. If it has a 40MP sensor that would be Equivalent to cropping a FF sensor of 104MP to the same FoV, but should be much cheaper.
Also, by the way, there is a misconception that crop lenses (which don't fit FF bodies on Canon) are inferior. They often have the same quality optics but are cheaper to produce and are lighter than their FF cousins because of the engineering of optics specific to having the back elements closer to the sensor. Take the EF-S 17-55, awesome good lens! (Nod to Ernie!)
