I listened to a podcast over the weekend in which the question was asked "does perspective change with focal length?" The answer given was very long winded and concluded no it doesn’t. The reasoning given was if you took a picture of something with a 300mm and a 35mm then cropped the picture taken with the 35mm until it showed the same field of view as the 300mm, the perspective would be the same, so no – perspective doesn’t change with focal length. Duh!
That podcast put me in mind of the linked to thread about depth of field and focal length comparisons between FX and DX and made me wonder why people can’t answer questions simply. Written in the thread linked to are such gems as “50mm is 50mm whether used on DX or FX” and “the focal length doesn’t change it is the crop factor of the sensor that changes” also “ the magnification doesn’t change between DX and FX, it is the angle of view” – all being true but not helping the OP. IMHO I think we ought to think about the question AND who is asking it before we go down a path which causes confusion for the OP and not clarification.
Let’s start with the perspective point: Who the heck thinks like that? It’s retarded! Obviously people mean when the subject is similarly framed with say, a 35mm and 300mm lens does the perspective change? Answer? Of course it does. You have to change your shooting position to get the same image size on the sensor (picture), but yes – the perspective does change with focal length.
Now DoF and DX/FX: Does the DoF Change between DX and FX? Yes. Let’s not start by trying to make ourselves look clever by making the explanation as technical as possible, in most cases the OP doesn’t need or want that. Suffice it to say that for a given image size on the sensor (picture), FX has a shallower DoF than DX.
Finally focal length: Does a given focal length on a DX magnify an image more than it would on a FX? Yes it does. Often OP’s just want that and find trying to get to the bottom of technically accurate and complete explanations of the reasons behind the answer given just OTT.
We all should at least start off with the simple answer. Most people who want to know more will either ask the question an a way that makes it obvious that they want/need to know more, or come back after the simple answer and ask why. Much is made of ‘it’s the artist and not the gear’, so why not keep it simple as a lot of artistic types I know are not interested in getting too deep into technicalities?
To this OP's question does the f-stop change between DX & FX, that actually is simple – no it doesn’t.