Friday, November 6, 2015

Hyperfocal distance with cropped image cheatsheet

TLDR: Hyperfocal distance is proportional to enlargement.

What happens to the hyperfocal distance if you intend to print a crop at the same size for the same viewing distance?


Back to basic assumptions:
  • Visual acuity of 5lp/mm at 25cm viewing distance. This is equivalent to a final print circle of confusion of 0.2mm.
  • Print size is 8x12. Enlargement = 12"/36mm = 8.47. Circle of confusion = 0.2mm/8.47 = 0.0236mm.
  • If you crop and print at the same size, you increase enlargement and decrease the circle of confusion.

Hyperfocal distance is inversely proportional to circle of confusion and is proportional to enlargement.

Example:

35mm full frame fixed focal length camera
f8.0 aperture
8x12 print size

Hyperfocal distance = 35 * 35 / 8 / 0.0236 = 6.5m.
With 1.5x crop (simulated 52mm), hyperfocal distance = 9.7m.
With 2x crop (simulated 70mm), hyperfocal distance = 13m.

Caveat:

Viewing distance is traditionally supposed to depend on the focal length (to get the same perspective), but these days, everything is viewed at phone distance :)

2 comments:

  1. Does anybody print any more? Or are you trying to compute hyperfocal distance for a cropped instagram image viewed on an app? In that case isn't phone resolution the limiting factor?

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  2. We still print photo books. I wanted to know how aggressively I can crop and what happens to the out-of-focus areas. Wikipedia etc's explanations are all hard to understand and the number do not work out. I wanted to figure this out from basics.

    It is getting really hard to justify carrying a zoom lens. Full frame fixed 35mm or 28mm with great high ISO seems the way to go :)

    "Retina" phone displays have higher resolution than what your eyes can differentiate at normal viewing distances. But I argue for phone viewing, this is totally irrelevant. You need pixel-peeping sharpness. Everybody seems to pan and zoom at 1-to-1 scale anyway. So, downsize until your target sharpness at pixel scale before sending. Or just don't bother. Nobody cares.

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