First time FF, lost in lenses

Started Apr 7, 2012 | Discussions thread
Henrik Herranen Senior Member • Posts: 1,722
Re: First time FF, lost in lenses

Y0GI wrote:

While I understand what you are saying, I'm still not sure if the figures support it. Going back to your original statement:

  • better DoF control 2: almost a stop larger maximum aperture at longest focal length for shallowest portrait DoF (105mm f/4 = 26.3 mm vs 55mm f/2.8 = 19.6 mm)

How do you arrive at a "almost a stop larger" advantage by comparing the pupil sizes?

As you probably know, one stop is the diameter ratio of sqrt(2). Now, 26.3 mm / 19.6 is almost sqrt(2). Admittedly, if the field of view of the systems that are being compared are not similar, this oversimplification is technically incorrect: the apparently longer system gets more blurring than numbers alone would suggest because it "magnifies" out of focus details more.

Nevertheless, when DoF + blurring tables are not available, and when apparent focal lengths aren't too far apart, I've found out that comparing physical aperture values give a surprisingly good "blurring estimate". But there's no deeper science to this, I'm afraid.

Kind regards,

  • Henrik

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