"Higgins, Michael (Ex AS01)" <MHiggins@xxxxxxxxxxxxxxxx> moved upon the face of
the 'Net and spake thusly:
> Sorry for asking a fairly general lens question on the Oly list, but you all
> seem like such a nice bunch I didn't think you'd mind. And know there are a
> lot of experts out there, so I'm sure to get some valuable answers.
>
> I understand that depth of field is the range of distances that are in focus
> for a lens / aperture combination, but don't understand the nature of how
> focus varies in this range and the range extending of either side. Is there
> only one distance from the lens that is EXACTLY in focus with degrees of
> being out of focus on either side? and that at a particular, arbitrary point
> this degree of out-of-focuses is considered unacceptable? and this is
> defined as the boundary of the dept of field? And this limit is rather
> arbitrary? - who decides it?
You've got it.
It generally goes
"The human eye can distinguish so many lines/mm on film. Given that
we are going to enlarge by so many times, what is the largest "blur"
on film that will look like a "point" when viewed on the print from
the intended viewing distance".
So "depth of field" is HIGHLY subjective and variable.
However, most lens manufacturers (and the rest of us) punt the
question by assuming "average degree of enlargement and average
viewing distance" (a hand-held 10x15cm print?).
As to *measuring* DoF:
The image of a point in the subject should be a point on the film. If
the image is out-of-focus, it will be a fuzzy circle, known as a
"circle of confusion". The more out of focus the larger the circle.
You have to choose the size-limit on your circle of confusion, and
*define* any CoC smaller than that limit as being "in focus".
1/30 millimetre (0.000033 metre) is a common figure used for 35mm film.
focal-length-divided-by-1000 is another rule-of-thumb CoC limit.
See here for the math: http://photo.net/photo/optics/lensFAQ.html
> Or is there some characteristic of lenses that starts to put objects badly
> out of focus at these well defined points, while between these points the
> focus remains very good?
There is one plane of "exact focus", and images of points either side
of that plane get more and more out of focus.
At subject distances much less than hyperfocal distance, two lenses of
different focal length will exhibit the same depth of field when using
the same aperture and magnification.
However, the circle-of-confusion cast by a point at infinity will be
much larger for the longer lens. That is, the long lens goes "fuzzy"
more rapidly as you move away from the plane of focus.
> Also, is depth of field a constant for a particular focal length, aperture
> combination regardless of lens design?
I beleive so, modulo lenses with fancy "defocus" controls, which I
beleive is actually controlled spherical aberration (asbestos undies
on here).
> this feature ever tested in lens tests? (previous discussions of boche have
> led me to believe that some out-of-focus blurs are better than other
> out-of-focus blurs)
Indeed---a non-circular aperture will give a non-circular CoC.
> Or maybe I should just go find a stuffed bird and decide for myself the
> answers to these questions. [almost OM content]
>
Yep. When it comes down to it, one person's "tack sharp" is another's
"throw that blur in the bin".
I've got lots of shots I thought were perfect until I tried to enlarge
them to 20x30cm.
cjb.
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