This is a simple issue. Bob is speaking of perceived depth of filed on
a viewed print. JCO is speaking of critical depth of field in respect
to the ability of a given lens to resolve detail. Both are correct,
but each is discussing an entirely different matter.
Let it go.
On Apr 7, 2009, at 4:02 AM, Bob W wrote:
Instead of making an unsupported assertion why don't you provide a
formula
for calculating depth of field? Then we will all be able to test your
assertion.
I have provided a formula which shows quite clearly that you are
wrong. The
calculation uses coc as a factor. The formula for coc depends on
viewing
distance and print size, therefore by changing either of these the coc
changes. If the coc changes, the depth of field changes. You can't
argue
with the numbers.
Give us a formula which shows you are right, then people might start
to take
you seriously.
Jose
NO WAY JOSE. you can never change the dof
after the shot, DOF is an "in camera" thingy...
you have to change the in camera image magnification or
f-stop to change the image DOF.
JC O'Connell
[email protected]
Coc is always a factor.
You can change the viewing distance or the print size, and
the depth of
field changes.
Bob
The question was regarding relative DOF, COC is not
a factor. The only way to increase DOF from whatever
your reference is, is to decrease IN CAMERA magnification
or increase f-stop number. All that other stuff is moot.
You cant change the relative DOF of an image after you
shoot it.
JC O'Connell
[email protected]
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of
Larry Colen
Sent: Monday, April 06, 2009 5:47 PM
To: Pentax-Discuss Mail List
Subject: Re: Trading resolution for depth of field
On Mon, Apr 06, 2009 at 10:32:37PM +0100, Bob W wrote:
So, if I'm willing to trade resolution for depth of field, am I
better off using a wider angle lens and cropping (my
intuition says
yes), or do I get the same benefit by just combining
pixels (which
would also reduce noise) for a larger circle of confusion?
To calculate the nearest (dn) and furthest (df) points in
focus use
the following formulae:
dn = U * F^2 / [F^2 + (U * c * f)]
df = U * F^2 / [F^2 - (U * c * f)]
Ah. Thanks. Focal length is second order factor, circle of
confusion is
first order, so focal length has a greater effect on DOF, than CoC
(pixel size).
where
c = circle of confusion
U = subject distance
F = focal length
f = f-number
To calculate the circle of confusion
c = (v * D) / (1000 * S)
where
v = film format / image size
D = viewing distance
S = print size
Source: The Professional Guide to Photo Data, 3rd edition,
by Richard
Platt.
Very easy with a spreadsheet.
Bob
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