RE: [Leica] Re: DOF -Optical vs Apparent

["Austin Franklin" <austin@darkroom.com>]

> Yes.  Since you are, to my knowledge, the only person ever to
> have claimed
> that depth-of-field can be measured, as opposed to
> calculated, I would like
> you to state how one goes about conducting this measurement,
> particularly if
> it is that simple.

DOF is basically a limit of resolution.  Whether it is lense resolution,
film resolution, paper resolution or the resolution of the eye.

If your image reaches the resolution of one of those, you will basically
have a 'range' of image depth where the focus in that range will be the
'same' and can not be any sharper...basically a plateau of sharpness.

One way to measure the DOF, is to take an image of a (very fine) grid on a
slant.  I would believe you need to choose the grid based on which
resolution you are trying to measure, film, paper (which is only one form of
replication that one can choose), or eye.  Better yet, possibly sets of
decreasing distance (between the sets) parallel lines might be better since
it also shows that sharpness isn't increasing/decreasing, which would be
more difficult to see with an equal distance pattern.

I would suggest using a very coarse grain film for this example, since you
will easily reach the limits of the film with most any Leica lense...and it
will make the 'measurement' easier.  The finer grained the film, the less
the DOF will be on the film.

To determine the DOF of the film, simply view the film and see where the
grid runs together.  The point furthest from the camera that is discernable
to the point closest to the camera that is discernable is your film DOF.
Because these discernable points are the limits of the resolution of the
film, there will be nothing between the two points that is 'more' in focus,
and the fall off will be quite dramatic.

To measure DOF of the viewed image, print the image...place it at viewing
distance and have the viewer tell you the two discernable points.  Remember,
that's how your eye doctor 'measures', not calculates, your vision...by your
feedback.

You can, of course, calculate this too, but the calculations (WRT mostly the
eye) are based on assumptions that don't necessarily follow (the COC
specifically) through as matching the measurements you will get if you
actually take the measurements.