[Brightcolours]

[quote name='nuke12' timestamp='1359307670' post='21557']

I'm not big on word play. Call it effective focal length, interpreted focal length, apparent focal length, whatever? It comes from this formula;



D = sensor size horizontal width in millimetres (36mm used)

f = effective focal length in millimetres

a = horizontal angle of view



f = D/(2*(TAN(a/2)))



That is this formula transposed.



a = 2*ARCTAN(D/(2*f))



The first formula on the wiki page for angle of view.

http://en.wikipedia.org/wiki/Angle_of_view



EDIT: Typo. Changed 35mm to 36mm for the 36x24 full frame format.

[/quote]

That is an incorrect way to calculate a lens' focal length, no wonder you come to 280mm. A correct way is described here:

http://www.giangrandi.ch/optics/lenses/focalcalc.html



The problem is to know what the front nodal point of the lens is. This front nodal point also "unlinks" the focal length with FOV, by the way...



Using that focal length calculator, and were I to disregard the distance to the front nodal point and instead take the distance to the sensor instead, I would come to a focal length at MFD of 215.7mm for the Nikon 70-200mm f4 VR.

Often, when the front nodal point is not known, people simply use the middle of the front element as front nodal point. Do not ask me why, as I am not that much into optics to understand that.



Now I estimate the front element of that lens to be about 22cm from the sensor/film plane... Which then makes the object distance at 1m (MFD) 78cm.

The focal length will then be 168mm at MFD for the Nikon AF-S 70-200mm f4 VR.



So... The focal length shrinks towards MFD, but the FOV gets noticably wider.