It's the camera that applies the crop factor, the lens just projects whatever image it's going to project. Let's look at it two different ways: first, "equivalent" focal lengths.
Let's use a "normal" prime for comparison (and by the way, "normal" refers to the amount of perspective vs. what one would observe with the naked eye, not the field of view). On 35mm film, or a full frame digital camera 43mm is the "normal" focal length, although you will hear lenses as long as 55mm referred to as "normal". You can calculate it by taking the square root of the diagonal of the image area (24^2+36^2=5625, sqrt(5625)=43). The FA 43 limited was designed to be a "true normal" lens, but remember, it was designed for film. Put the FA 43 on a cropped-sensor body, and it becomes a short telephoto lens. The focal length is still 43mm, but now it gives the field of view equivalent to what a ~65mm lens would on a 35mm film camera. Now this is the part that's hard for some folks to grasp. Although the crop of the camera does not change the contents of the image at the center of the frame (i.e. the perspective), it does cause the information at the edge of the frame to be discarded. In order to regain that information, one must move the camera back, the act of which, in practice, changes the perspective of the lens. Perspective is 100%, completely and totally a function of camera position relative to the subject within a given format. To capture roughly the same image (i.e. a normal perspective) on a cropped-sensor camera, you would need a lens that is 33% shorter. So a 28mm lens functions as a normal lens on a cropped body. This would allow you to keep the camera in the same position.
Now let's look at it another way. Let's take the same focal length from three different formats and look at it on the same body. We'll use the DA* 55mm, the K series 55mm, and the 645 D FA 55mm. The lenses were respectively designed for cropped-sensor digital, 35mm film, and cropped-sensor medium format. Put all three lenses on a cropped-sensor camera like the new K-3 and you will get the exact same image (with the possible exception of less vignetting and more corner sharpness from the 645 lens), although one lens was designed as a portrait length, one as a "long normal", and one as a "true normal" for their respective formats. Now let's put all three lenses on a larger format, like the 645D. With the DA* 55 and the K 55, you are likely to get heavy, if not total vignetting in the corners, but still the same image from all three lenses. The 645 lens is a larger diameter and projects a larger image circle, but a 55mm lens is a 55mm lens.
So the crop factor is completely a function of the camera, and lens focal lengths are absolute, not relative measurement. Terms like "normal", "telephoto", "portrait", and "wide" however are relative and their definitions change with the format. Hope that helps, though others have done a good job of explaining too. Sometimes equivalency is easier to understand in the terms that make the most sense to the individual.
Last edited by maxfield_photo; 10-09-2013 at 12:35 AM.
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