EF vs EF-S

I have always been taught to account for crop factor on a crop sensor when using a lens not designed specifically for the crop. 
I've never questioned this until I tested the 50mm EF 1.8 vs the 55mm-250mm EF-S on my 700D, and the images (although with a difference of 5mm focal length) were roughly the same FOV. That would suggest that the 50mm focal length is true even on an APS-C sensor.
Can anyone provide insight here?

Lens focal lengths are rarely exactly what they claim to be. The 50mm may be 53mm rounded down to 50mm and the 55mm may be 53mm rounded up to 55mm.

Focal length also changes with focus distance.

Both EF and EF-S lenses show their focal length in terms of full-frame 35mm, not "effective" or "crop factor" terms.

AlephNull wrote:

Both EF and EF-S lenses show their focal length in terms of full-frame 35mm, not "effective" or "crop factor" terms.

not quite. Focal length is a property of the lens. It has nothing to do with the sensor. 50mm simply means an infinity light source will focus at 50mm behind the lens (simplistically speaking - but think magnifying lens, trying to light a piece of paper).  Anyway, the sensor size behind the lens then dictates the field of view/angle of view that you get. For historical reasons, the 35mm FF format is taken to compare field of view/angle of view.   Having said that, lenses are of course designed to cover a certain image circle. The EF-S lenses are designed to cover only APSC, EF cover FF, the TS lenses have an even larger image circle, so probably could be used on medium format (that's why they are TS, so the image circle can be shifted off the optical axis and still cover the sensor).

MasonHoy wrote:

I have always been taught to account for crop factor on a crop sensor when using a lens not designed specifically for the crop.
I've never questioned this until I tested the 50mm EF 1.8 vs the 55mm-250mm EF-S on my 700D, and the images (although with a difference of 5mm focal length) were roughly the same FOV. That would suggest that the 50mm focal length is true even on an APS-C sensor.
Can anyone provide insight here?

As others have said... all lens markings are the same.  50mm is the same if it's EF or EF-S.  So, you are right on there.  EF-S lenses are desgined for crop sensors, so they will not project light on the entire full frame sensor, but the focal lengths are all the same as EF lenses.

What you need to be aware of is that the focal lengths will mean different things for crop sensors and full frame sensors.  This becomes important as you read reviews and think about purchasing lenses and whatnot.

For example, you may read about people suggesting a 24-70mm lens as a great standard zoom.  Most of the time people suggesting this are doing it for Full frame users where 24mm is wideangle and 70mm is just past standard:  a very nice general photography range.  For crop users however, this lens would not be very wide at the wide end and would be slightly into the telephoto range on the long end.  A very different view through a crop sensor camera in other words.

Thanks all!