Please tell me I am correct & the pro Is wrong???

quadrox wrote:

The error is not as bad as you make it out to be, but your confusion is very understandable.

Imagine you stand in the exact same spot and use the exact same lens to take a picture with a full-frame and a crop sensor. You get both images printed on e.g. a 3 yard x 2 yard canvas.

If you want to prove that the lens is the same for both cameras, you must let the full-frame image fill the entire canvas, and the crop camera only the center part of the canvas, to properly represent the difference in angle of view. The central part of the crop sensor image should look more or less identical to the central part of the full-frame sensor, only the crop-sensor image will have a large blank border.

Now, typically you would not print an image from a crop camera with such a large border, you would want to fill the entire canvas. If you do this however, you are now enlargin the image from the crop sensor camera, and suddenly it appears as if the image from the crop sensor camera was taken with a longer focal length than the image from the full-frame camera.

Of course the lens itself does not change when you put it on two different cameras, but it will look like it. Therefore you say that a given lens has an equivalent focal length on a crop camera compared to a full-frame camera, because the image will look like it had been taken with such a focal length.