Does using a lens adapter change focal length?

Question

Does using a lens adapter change the field of view?

I recently bought a Sony NEX-5, an extra Minolta MD 50mm 1.7/f lens and a MD to E-mount adapter and I was wondering, as the adapter increases the distance between the sensor and the lens by ~10mm, does this effectively make my 50mm lens a ~60mm lens? Or am I just misunderstanding things?

Answer

It won't change the focal length of the lens, that's what a multiplier will do. You'll see these out there, often as a 1.4x or 2x multiplier with optical elements in them. These are designed to multiply the focal length of the lens.

In general, extension tubes, which is what is normally used to move a lens further away, are there to allow closer focus than a lens would normally have, thus giving them macro, or close to macro, ability. However, in doing so, they'll result in loss of infinity focus (which isn't an issue in macro photography). Nevertheless, they do not change the focal length of the lens.

In the case of your camera, however, the adapter is moving the lens further away from the sensor but it has to in order to continue to focus the same. The lens is designed to sit on an SLR and that means it's designed to focus onto a sensor (or film) that is further away from the rear element of the lens than your NEX-5.

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