Thanks for the info, John. I'm not sure we're quite on the same wave
length here. I was talking about DOF in the final image, let's say an
8x10 print. You say under b. "if maximum allowable circle of confusion
diameter remains unchanged". Since I am positing the same allowable CoC
diameter for identical size prints from different size negatives (this
is the same for slides, just change the names), I do allow a larger COC
on the larger neg, as it will be enlarged less to produce the final image.
Another interesting thing about DOF that hasn't been raised in recent
discussions is the underlying assumptions. 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. ( I know you DOF mavens know this, but perhaps others
have not thought it out.) 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.
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.
DOF tables and marks on lenses are useful. They are even more useful if
their underlying assumptions, actual meaning and limitations are understood.
Moose
John A. Lind wrote:
At 09:43 1/13/02, Moose wrote:
I didn't say better or worse DOF, just different. I skimmed through
all that DOF thread and saved some for later study and elucidation. I
do think that, for the same final image size, the MF confused circles
( ;-) ) are magnified less than the circling confusion of 35mm,
leaving the final confusing circles about the same size (on the final
viewed image, not the film) for equivalent coverage lenses on both
formats. Maybe a DOF maven will confirm or deny - treatise not
necessary, but possibly unavoidable .
For the same field of view DoF will change if you change the film
format size:
a. Focal length must increase linearly as size of film increases to
maintain same field of view (linearly means f^1).
b. As focal length increases, DoF decreases at the square of the
focal length (f^2) if maximum allowable circle of confusion diameter
remains unchanged.
c. Maximum allowable circle of confusion diameter increases linearly
as film size increases (c^1). This increases the DoF linearly *if*
focal length remains unchanged.
Final result of using a larger film format (size) with same field of
view:
a. Take one step forward with increase in DoF from linear increase in
max CoC diameter allowed.
b. Focal length must increase linearly to maintain same field of
view. Take two steps backward with decrease in DoF at a rate of the
square of the focal length change.
To maintain the same DoF between my OM-1n with a 50mm lens and my M645
with an 80mm lens, I must stop down the 80mm lens on the M645 by about
a half stop more than the 50mm lens on the OM-1n. About the same on a
6x6. IIRC, a 6x7 format is about a full stop more and a 6x9 is about
1-1/3 stop more.
-- John
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