Boy oh boy! A lot of people really need to take Optics 101.
The focal length of a lens is a physical property of the lens, irrespective of the body with which it is used, the aspect ratio of the sensor or film in use, or even if the lens is attached to a camera at all. A 50mm lens is a 50mm lens if it is coupled to a full-frame camera, a crop camera, or a 120-film medium-format camera, if the aspect ratio is 3:2, 4:3, 5:4, 16:9, or 1:1, or if the lens is sitting by itself on a shelf gathering dust.
By definition, the lens' indicated focal length is equivalent to its physical focal length when the lens is focused at infinity. If a lens is focused closer than infinity, its physical focal length will always be less than its indicated focal length. This is especially apparent with macro lenses, where the difference can be enormous. There are no exceptions to this; it's a law of optics. Camera lenses are converging lenses, and that's the way converging lenses operate.
With multi-element lenses, the focal length has very little to do with the physical length of the lens. Normally focused lenses (those greater than 50mm) are almost always shorter than their focal length. Retrofocus lenses (those less than 40mm) are almost always longer than their focal length. Note—if you're going to start measuring your lenses, bear in mind that focal length is measured from the point of focus on the film/sensor plane, not the back of the lens. For Canon lenses, this is 44mm past the lens' mounting ring.
A prime lens is a lens with a single, fixed focal length, measured with the lens focused at infinity. A zoom lens is a lens with a focal length that varies continuously between a minimum focal length and a maximum focal length, also measured with the lens focused at infinity. All the rules of optics apply identically to prime lenses and zoom lenses. For example, a 70-200mm zoom lens set to 85mm behaves exactly like an 85mm prime lens. That is, if either were focused at 2m instead of infinity, its physical focal length would be less than 85mm. Note than the zoom lens would still be set to 85mm; its zoom setting would not have been changed.
Why are prime lens usually better than equivalent-quality zoom lenses encompassing the same focal length? Two reasons.
First, prime lenses typically have fewer elements than equivalent zoom lenses. (The don't have all those zoom elements.) The less glass the light has to pass through, the less distortion it acquires.
Second, the elements in prime lenses typically bend the light less than those in zoom lenses. Those zoom elements can really stress the light, introducing distortion. And the greater the zoom, the greater the stress. This is why superzoom lenses (10× or more) not only have lower I.Q. than equivalent prime lenses, they have lower I.Q. than equivalent non-super zooms. A 28-300mm zoom lens set to 135mm will have lower I.Q. than an equivalent 70-200mm zoom lens set to 135mm, which itself will have lower I.Q. than an equivalent 135mm prime lens.
You can't beat prime lenses for I.Q.