Originally posted by troglodyte Does fov change when you reverse mount or only minimum focus distance get closer?
I think this a question about macros, not widening angles.
First part: I *think* (but I'm not sure) the FOV remains the same -- that's a function of the sensor/frame size and the lens' focal length. Is there an optics designer here who can answer that?
Second part: it's not so simple. Any lens that is NOT reversed cannot be focused closer than its focal length. Put a 50mm lens on tubes or bellows, with 50mm or 100mm or 150mm or whatever of extension -- and the closest focus is still 50mm. AFAIK the only way to focus closer is to change the lens' focal length.
When a lens is reversed, it's working distance is the register of that lens type -- with Pentax, Nikon, Sony, that's about 45mm. Think about it: an un-reversed lens gathers in light from a distant view and focuses it onto the frame, about 45mm from the back of the lens mount. Reverse the lens, and light bouncing off a subject that's ~45mm from the lens back is gathered and projected onto the frame.
How this affects the minimum focus distance (MFD) varies among lenses. Reverse a 20mm lens, and your MFD stretches from 20mm to 45mm. Reverse a 100mm lens, and the MFD shrinks from 100mm to 45mm. It doesn't matter whether the reversed lens is stacked onto a primary, or sitting at the end of huge extension -- your MFD is either the lens' focal length (straight) or its register (reversed).
And now we return to working GOATSE upon a lens. Stretch away...