Optical bench measurements

[Peter Forsell]

Hi,
call me Curious George, but I had an opportunity to have three of my lenses measured in an optical bench by a colleague/friend. Do these numbers seem reasonable to you, are these accurate enough for government use?

AI 50mm f/1.8 - f=51.61mm, epd=28.66mm, N=1.80
AI 50mm f/1.2 - f=50.14mm, epd=41.66mm, N=1.20
AI 55mm f/1.2 - f=55.48mm, epd=45.38mm, N=1.22

(legends: f=focal length, epd= entrance pupil diameter, N=relative aperture)

The accuracy of these measurements determines whether I'll get him as thank you a bottle of Chardonnay or a bottle of vinegar.  He is not an optics pro, but I believe he knows his stuff.

[Bjørn Rørslett]

Buy him the Chardonnay.

[Roland Vink]

The focal lengths measured seem a bit short to me. Most (all?) "50mm" Nikkors have a focal length of 51.6mm, see:
  http://imaging.nikon.com/history/story/0049/index.htm (VII. 50mm, but Is It Actually 51.6mm?)

And the 55/1.2 has a focal length of 55.5mm, see:
  https://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/6070nikkor/standard/55mmf12.pdf

[Akira]

You may want to look for a nice wine vinegar...

[Peter Forsell]

Okay, thanks everyone. My friend was puzzled, but at the same time kind of relieved that all the focal lengths are off in the same direction. He says, that the measurement is mostly automatic, so there is probably a calibration issue. He will confer with an optics tech and redo the lenses next week.

Thanks, and yes I got him the Chardonnay. 

[Roland Vink]

The focal length of the lens is measured with the lens focused at infinity. These are all unit-focusing lenses so the focal length does not change as you focus closer.

I'm also guessing that the focal length is measured at the centre of image. For example, compare the AI 50/1.8 with the 50/1.2. Assuming they have the same focal length, the magnification (image size) will be identical. However the AI 50/1.8 has very low geometric distortion, while the 50/1.2 has some barrel distortion. So while the centre of the image will be identical between the two lenses, barrel distortion means the 50/1.2 has a wider field of view to the edges, which means that overall, the focal length appears to be a bit shorter ... I'm not sure how the optical bench measures the focal length and whether this could be a factor? That could explain why the measurements for the AI 50/1.2 and 55/1.2 (with greater barrel distortion) are relatively further out than the AI 50/1.8.