I keep reading on the internet, including here, that a a 1.6 crop factor Canon 7D (say) has a "longer reach" than a full frame 6D or 5D Mk III. Hence a 300mm tele becomes 480. I was even told this in a respected cameral shop.
Surely this is nonsense? It seems mathematically illogical to me.
The whole thing about a crop sensor is surely that it is a cropped part of a full frame (the central bit). Hence, if I took exactly the same shot on a full frame using the same lens, and then cropped the shot on my computer, it would give exactly the same "zooming effect". (Leaving aside IQ, noise etc issues etc between different sized sensors).
It seems to me that if I am right, there is no real advantage in having a crop body for so called "longer reach". You might as well take the same shot with a full frame and then crop it in processing.
I do realise that crop cameras are often used with smaller and lighter glass, so there are carry around factors to consider, as well as cost. But I am looking purely at the so called advantage of the crop sensor.
Have I misunderstood?
It seems to my that if I have a FF, then whatever the widest end of my lens is, then the camera will deliver that. But that same lens on a crop camera, will not us the full capacity of the lens: hence clear disadvantage. The converse is surely not true at the other end of the lens as the full frame will still be able to resolve the same image as the crop, with processing on the computer?