Why is my non-DX Nikon 50mm f/1.8D lens actually 50mm in a DX camera?

I have a Nikon D80 DSLR camera which is not full format (FX) but rather DX, and I'm using the 18-135 mm DX kit lens. I also own a Nikon 50mm f/1.8D lens, which is supposed to be 50mm for FX cameras, as it is not specifically for DX cameras (there's a separate DX lens of the same variety).

The lens I own is exactly this one: http://www.amazon.com/...00005LEN4/

The reviews also say that this lens effectively is 75mm for use in DX cameras. I had always assumed that my lens was effectively 75mm while taking pictures. However yesterday I decided to compare my two lenses: I shot the same scene with both 50 and 75mm focal lengths with my kit lens, then shot with my 50mm prime lens. To my surprise, the picture from the 50mm lens was exactly the same with the 50mm on my kit lens, not the 75mm.

So I'm completely dumbfounded, why is this so?

Your 75mm lens is actually about 112.5 mm on a DX body.
It's not compared to another lens on your camera, it's compared to using the lens on a full frame camera.
Full frame cameras are the true mm. Crop sensors are roughly 1.5x to 1.6x the mm listed on the lens.
EVERY lens at 50mm will be roughly 75mm. Every lens set at 75mm will be roughly 112.5mm on your camera.
It's not the lens that is different, it's your camera's sensor that is a different size from a FX camera sensor.

Your results are what is expected.

A 50mm focal length is 50mm on all cameras regardless of sensor size. Focal length is an optical specification.

What you are thinking about is the field of view which takes into consideration focal length and sensor size.

On a FX camera (sensor the same area as a 35mm film frame which is a crop factor of 1.0), a 50mm lens has a horizontal field of view around 40 degrees.

On a DX camera (crop factor = 1.5x), a 50mm lens has a horizontal field of view around 27 degrees, or the EQUIVALENT of what a 75mm lens will see on a FX camera. The key word here is "equivalent".It still has a focal length of 50mm, it's just that the image projected on the sensor is cropped compared to a full size sensor.

Perhaps these articles will clear up what I'm spewing…
http://www.tutorial9.net/tutorials/photography-tutorials/crop-factor/
http://en.wikipedia.org/...rop_factor

You can use all FX lenses on DX cameras without problem. Using a DX lens on a FX camera will give you vignetting, however, Nikon cameras will sense this and use a reduced area on the sensor equivalent to a DX sensor size. Pretty tricky, these Nikon folks. The difference between DX and FX lenses is the projection area.

Using your FX rated 50mm prime on a DX body will give you the sweeter portion of the lens (central area) which means less vignetting and less color aberration.

50 mm is still a 50 mm on yours, DX, or a FX camera.

Think of it this way. You're inside a room, and looking out a window. A FX is wide open window. A DX view is 2/3 open window.