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Re: [OM] 35mm vs. 6x9

Subject: Re: [OM] 35mm vs. 6x9
From: dreammoose <dreammoose@xxxxxxxxx>
Date: Sun, 13 Jan 2002 15:16:06 -0800
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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