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04-12-2009, 05:44 AM   #46
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QuoteOriginally posted by nanok Quote
i think maybe you meant "lower magnification and larger circle of confusion", typo maybe? (that is, unless by magnification you mean ratio between recorded image and "real life size")
No typo.
By magnification, I meant 'm'.
m is a very small number for ordinary photography and approaches 1 for macro photography.
At constant FoV, m is larger for the larger recording medium (as is the CoC c).
DoF decreases with increasing m (btw, as you said).


BTW...
In "Wheatfield's" formula, the term
f' = f (m + 1) / m
may be rewritten as f' where f' is the effective focal length determining FoV, as the effective focal length increases with increasing magnification. f' = 2 f for 1:1 macro photography. Which is why a DA35mm becomes effectively 70mm (~105mm on 135 film) or why a wide angle lens still delivers tele perspective for macro shots.
"Wheatfield's" formula may then be written like:
DoF ~= 2 f' c / (m a) = 2 F' c / m
where a is the physical aperture [in mm] and F' is the effective f-stop number. Maybe, this formula is easier to deal with.

Example: at m=1 (F' = 2F) and c = 0.017mm (APS-C), we get: DoF ~= 4 F c = 0.07mm F or ~= 1mm at F=f/16.


Last edited by falconeye; 04-12-2009 at 06:05 AM.
04-12-2009, 01:29 PM   #47
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QuoteOriginally posted by falconeye Quote
No typo.
By magnification, I meant 'm'.
m is a very small number for ordinary photography and approaches 1 for macro photography.
At constant FoV, m is larger for the larger recording medium (as is the CoC c).
DoF decreases with increasing m (btw, as you said).
right. i realized it while i was typing, as is obvious; i decided to keep my initial confusion in there as i think others might have been confused (namely, some might have thought of magnification from recorded image to print instead, if not following the discussion patiently, which works out to the reverse).

QuoteQuote:
BTW...
In "Wheatfield's" formula, the term
f' = f (m + 1) / m
may be rewritten as f' where f' is the effective focal length determining FoV, as the effective focal length increases with increasing magnification. f' = 2 f for 1:1 macro photography. Which is why a DA35mm becomes effectively 70mm (~105mm on 135 film) or why a wide angle lens still delivers tele perspective for macro shots.
now that you point it out, this might be interesting for some practical purposes: for instance, trying to set the focal length for the SR for a manual macro lens (for macro shooting, of course) to 2xf (has anybody tried this? does it seem to work? i will and get back here)


i find the formulas so far actually pretty easy to take in (especially compared to the wikipedia detailed ones ), but as i said, i think it is good to try and "word them out" for people who happen to hate math/not be technically inclined. it's not easy, though, and it always feels, looking back, that the formula is still much more telling and easier to understand for me compared to my own words
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