The "standard" circle of confusion, for any format was calculated based
on "acceptable sharpness" results for a given print size, (which is kind
of arbitrary), for a print at standard viewing distance, but these
values are published.
Standard viewing distance is defined by focal length times enlargement.
This assumes a perspective-correct viewing distance, by design, and
builds the capture format size into the calculation.
Let's assume you want to make a standard size final enlargement that
uses the minimum cropping, I'm using 11x17 inch.
Let's assume standard size CoC for each format.
Let's compare three hypothetical sensors of 24mp to eliminate resolution
differences.
APS-C, Standard CoC = 0.019mm,
FF, Standard CoC 0.029,
The unicorn 6x9 sensor, CoC 0.067mm.
I chose these because they're all 2:3 aspect ratio and will crop roughly
the same way, for a final display size I'm using 11x17 since a print of
this format has become "standard" in the inkjet era, and using almost
all of the image is possible with minimal cropping, (about 1/2 inch off
the short print dimension in case you're wondering).
Assume standard viewing distance for a normal lens, (defined as the
focal length = the diagonal of the format).
Focal length is rounded to the closest mm.
Magnification is rounded to the nearest whole number.
APS-C 16x24mm normal focal length 28mm.
FF 24x36mm normal focal length 43mm.
The unicorn 6x9 56x84mm normal focal length 101mm.
Standard viewing distance for APS-C = 18 x 28mm = 504mm or about 19.8
inches.
Standard viewing distance for FF = 12 x 43mm 515mm or about 20 inches
Standard viewing distance for 6x9 = 5 x 101mm = 505 or about 19.8 inches.
So the standard viewing distance is for all intents and purposes the same.
What does this mean for circle of confusion.
Lets look at the math, simple though it is.
The CoC on our 11x17 print for the acceptable CoC for each capture
format will be
APS-C 18 x 0.019=0.342mm
FF 12 x 0.029=0.348mm
6x9 5 x 0.067=0.335mm
The difference in final CoC for each format on the display image, in
this case print, are pretty much accounted for the rounding errors to
get to the nearest mm in focal length, and nearest whole number in
magnification.
Having controlled for higher resolution sensors at the beginning, I
think I can safely say that if each image is made under optimal
conditions there will be no way to tell the prints apart at standard
viewing distance, because it's designed to work that way.
Given the base assumptions, no math driven application will show any
difference, because once you take into account the way CoC is designed
to be used, there can't be any difference, in the point you arrive at on
the curve.
On 9/15/2019 8:49 PM, Larry Colen wrote:
On Sep 15, 2019, at 5:42 PM, P. J. Alling <webstertwenty...@gmail.com> wrote:
You also have to take into account viewing distance, and the resolution of the
viewing device. The higher the resolution of the viewing device the more
accurate you assessment of DOF will be when printed.
I’m not so interested in absolute depth of field as the shape of the way the
level of focus changes with distance. To get the absolute DoF you’d take a
slice at the final effective resolution. Based on Bruce’s comments, I suspect
that a larger format ends up being at a different point on the curve.
On 9/15/2019 3:23 PM, Larry Colen wrote:
Bruce Walker commented on facebook about how with medium format you get a
particular transition from in focus to out of focus which finally made
something click for me. Everybody talks about depth of field as if it is a
sharp transition, but it isn't, and the same image could have different depths
of field at different sizes and resolutions. How many photos look just fine as
thumbnails/contact prints, but are out of focus, or mis-focused at a larger
size/higher resolution? What I would love to see are graphs of the size of the
circle of confusion, preferably a 3-d graph that would show it based both on
focal length and distance. you could also show a slice at a particular pixel
size which would give you the nominal depth of field. Does anyone know of
someone having done graphs, or better yet a program where you can enter those
values and see them change for yourself?
--
Larry Colen
l...@red4est.com
--
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America was founded so we could all be anything we damn well please.
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--
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- P.J. O'Rourke
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