18-55mm lens what does it mean?

[analog.universe]

a 75mm lens has a way of rendering perspective and dof that a 50 will never emulate, even if you mount it to a crop sensor that gives you 1.5 times tighter field of view.

Could you clarify that, preferably with examples?

Let's say I have a tripod with a crop camera on it. I put a 50mm lens on that camera and take a picture. Then I put a full frame camera on that same tripod, put a 75mm lens on that camera and take a photo of the exact same thing, there really isn't going to be much difference between the two shots. The perspective certainly won't cahnge, because that depends entirely on the position of the camera.

In your example, provided both shots were taken at the same aperture, the full frame with a 75 will have a shorter depth of field, despite having the same perspective as the cropped 50. In practice it's helpful to associate "50 on crop means 75 on full frame" because people who've shot full frame have a feel for the field of view that a 75 gets you, but that doesn't address the dof issue. At equal apertures, equal subject distances, and focal length increased on full frame to match the field of view on crop, the full frame will have decreased dof.