A different way of looking at the focus breathing of the 70-200mm f/2.8 VRII

Started Feb 3, 2013 | Discussions thread
Samuel Dilworth
Samuel Dilworth Regular Member • Posts: 249
Re: And a better way to look at it…

Leonard Shepherd wrote:

I disagree. The only lenses which you do not gain or lose some exposure relative to the marked aperture in the Nikon line up are the 50mm f1.8's and the 85 mm PC.

I said a stop, not ‘some exposure’. Obviously there are small differences in image brightness and depth of field caused by small differences in true-versus-reported f-number, t-stop, pupil factor, etc. as the lens is zoomed and focused. None of these explain the differences noticed by the thread-starter, which are caused by:

  • the roughly one stop difference in f-number between the f/2 prime and the f/2.8 zoom
  • big differences in vignetting at large f-numbers (affects off-axis image)
  • differences of field curvature and astigmatism (affects off-axis image)

The many other contributing factors might be interesting to some of us, but they’re not the cause of the thread-starter’s question.

I think you’re suggesting Nikon’s 50 mm f/1.8 and 85 mm tilt-shift lenses are perfectly symmetrical. They’re not quite symmetrical, but even if they were they would still project a dimmer image when focused more closely (per your later observation of a two-stop loss at unit magnification).

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