# Talk:Field of View

in the formula

$FoV = 2 * atan \frac{35}{2 * Focal Length}$

The "35" should be some "length" because it is divided by a length, and should become unitless. Of course the length is the size of a standard negative: 35mm. If you want to "ignore units" as most americans like to do, it should note that "Focal length" is in mm. But more accurate is:

$FoV = 2 * atan \frac{35 mm}{2 * Focal Length}$

Should I just edit this into the main page, or should some discussion be held first? -- REW.

Just edit. Actually a "35mm" negative is 36mmx24mm in size. 35mm is the width of the entire film, including perforation etc. Since the formula is universal, I changed it to "size". (This error was pretty old, thanks for pointing it out) --Erik Krause 10:59, 11 June 2009 (UTC)

## close focusing increases FoV

I believe this phenomenon is known (in cinematography at least) as "focus breathing". In my own limited experience, the FoV is slightly larger when close-focusing compared to infinity focus. Presumably it is more pronounced in some lens designs than others. If I understand correctly, the lens' specified focal length (eg nikon 50mm) will be correct for infinity focus, so at its minimum focusing distance the lens' FoV would be better represented by a focal length number of -say- 49.1mm ? For the purposes of lens correction calibration, would it be appropriate to compensate for close focusing by reducing the focal length variable ? or would that throw some other parameter(s) out of whack ? --Beeswax 13:57, 27 January 2011 (UTC)

Exact FoV does depend on even more factors. Details are explained in Rik Littlefield's paper about the no-parallax-point --Erik Krause 17:48, 28 January 2011 (UTC)
Thanks Erik, you've made me realise I failed to ask my question properly! I'm a Hugin newbie, and the "focal length value" I mean is that in "Hugin / Camera & Lens tab / Geometric / focal length" input ... so the question is really about how Hugin interprets this value. Is it used only to determine FOV or does it affect other calculations too ? If I tweek it to achieve a better FOV fit, will I upset something else ? My apologies for being so clumsy! Beeswax 15:27, 30 January 2011 (UTC)