f/1.8 on m4/3, ASP-C, FF; are they the same?

Started Nov 1, 2010 | Discussions thread
nugat
Contributing MemberPosts: 699
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DOF is subjective and conventional
In reply to digifan, Nov 3, 2010

DOF is a subjective phenomenon and depends solely on:
--eyesight
--watching distance
--print magnification
--allowed size of circle of confusion

Lens DOF markings (and strangely enough on-line calculators) date back to Barnack's 1912 assumptions based on 19th century CoC standards.

On a 10x8 inch contact print looked upon from one foot a circle of 1/100 inch was considered the "sharpness" limit. This roughly translates to today's 36x24mm frame CoC of 0.03mm magnified 8x to ca A4 print (0.03 mm x 8= 0.24 mm or ca 1/100 inch).

The 19th century assumptions of eyesight strength are rather inadequate, many 20/20 vision individuals (and correction glasses wearers) will see double or triple the detail. Computer pixelpeepers demand a "pixel-level sharpness", etc etc. Therefore many believers in zone DOF and relying on lens markings and DOF calculators are deeply disenchanted with results. In fact to get at the 20/20 vision level sharpness expectations those suggestions must be tightened 2-3 times and aperture closed 2-3 stops to make CoC that much smaller (eg. 0.01mm on full frame).

All that assuming that the lens resolution is good (40lp/mm), camera perfectly steady, focus ideal and diffraction not a factor.
Good reading:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

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