All Dof calculations are based on an assumption about what will appear
fuzzy to a viewer. That,in turn depends on the magnification of the
image and the distance at which it is viewed. If you place the 4x6 and a
20x30 print of the same image at different distances from the eye so
that they are of the same apparent size, they will both have the same
DOF as viewed. Put a 4x6 and an 8x12 at the same distance from the eye
and what appears sharp on the smaller print may appear unsharp in the
larger print. The DOF tables published with lenses and the DOF markings
on them have to make some assumptions about what will be sharp enough
for the average user.
An image does not suddenly go from sharp to unsharp at some magic
distance from the focal plane. Unsharpness or 'out of focus' increases
continuously with distance from the focal plane.Thus an assumption is
made about the point in this continuous process where the average eye
looking at a certain size image at a certain viewing distance will
perceive an image to be out of focus. The allowable CoC diameter is then
derived from this kind of subjective data and all kinds of wonderful
calculations are made and tables published. All very well and
wonderfully useful to the practical photographer. For the thoughtful
photographer involved in all aspects of the dance with light and
technology that creates the final image, it is useful to remember this
is just a guideline that doesn't necessarily work for the particular
purpose at hand.
DOF can even be different for different viewers. A simple example: My
right eye, while quite nearsighted, has a visual acuity (resolving
power) well above average. When corrected with glasses, my vision in
that eye is about 20/7 (I can easily read the 20/10 line and make out
some of the 20/5 line.) That is, I can resolve detail at 20 feet that
the average eye cannot resolve without being within seven or eight feet
of the subject. Thus I may look at your prize flower picture at the same
distance you do and say it would be just perfect if the detail in the
stamens projecting forward were actually in focus (Not that I would say
it that way!). My acceptable CoC diameter may be different than someone
else's.
There is no standard for DOF, so published DOF tables na lens markings
from different manufacturers may be different for lenses that have the
same imaging characteristics. DOF tables from the same manufacturer for
lenses of the same focal length and speed in professional and amateur
lines may have different DOF tables on the assumption that the pro
requires higher standards of sharpness.
Working only in small output format, as you have discovered, does not
prepare one very well for larger output format.
Moose
Albert wrote:
I went through in detail over my 8x10. First, this is what I found out:
3 subjects, my 3 cousins, two females on each side, the taller male in
the middle. I took the picture with them lined up about 45 degrees
slant, not parallel to me.
OK, I focused on the wrong person, I focused on the rear person, and
so the rear and the middle were sharp, but the front was a bit blurry...
My "sharp" areas were very sharp, but what looked like not so sharp
areas were actually just that, out of focus. Also, I always thought I
had good selection of DoF, but I guess not.. everything looked sharp
and in focus at 4x6, but not so at 8x10... and it's not the developers
fault.. The picture is well developed..
Hmm.. I might have to print more 8x10's, this one has taught me a lot.
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