The sensor has nothing to do with DOF except insofar as the size of the
sensor factors into final image magnification. Also "crispness" and
"sharpness" are really different things from DOF. But ignore all that
for a moment because I'm getting ahead of the story.
A short lens and DOF lesson
Consider a simple lens of 100mm focal length forming the image of a
star. The light rays that will form the image emanate from all parts of
the lens and converge at the focal point to form the image of the star.
Consider that the light rays form a cone which is wide at the lens
and tapers to a point at the image. But what happens to the light rays
if there is no film or sensor at the point of focus? The rays cross
over at the focal point and begin to diverge and form an expanding cone
on the other side of the focal point. Consider that the path of the
light rays looks like two dunce caps or ice cream cones stuck together
at their tips.
Now what happens if the focal point (where the dunce caps meet) doesn't
exactly coincide with the film or sensor, ie; it's slightly out of
perfect focus. If it gets moved off to one side or the other the image
will not be a sharply defined point. Instead, the star will assume the
size and shape of the cone wherever it intersects the film or sensor. It
becomes an ever larger circle the further away from the true focal point.
But now we have to ask; "What is a sharply defined point." The answer
is; one that is small enough that your eyes can't tell the difference.
It doesn't matter whether the star image is a little bit bloated from
sitting a little bit ahead of or behind the true focus point... as long
as your eyes can't see the difference in the final print. That's where
CoC (Circle of Confusion) comes in. CoC measures the diameters of those
cones ahead of behind the true focal point and says: If that little
starlight blob isn't any bigger than X then your eyes won't be able to
tell the difference between that and perfect focus... as long as the
print isn't magnified any more than Y. It's rarely stated in
discussions of DOF but Coc and DOF tables are actually based on human
visual acuity... how small an object can you see at typical reading
distance.
Now let's back up a bit. Consider those dunce cap cones again. What
happens as the aperture closes down? The big end of the dunce cap gets
smaller and the angle to the tip of the cap gets shallower. Therefore,
as the focal ratio gets larger the diameter of those cones as you move
away from the true focal point is changing more slowly. Therefore it
takes a longer excursion away from true focus for the diameter of the
star image to exceed the CoC.
This whole discussion is based on geometry and has nothing to do with
film or digital. Assuming that your sensor is capable of resolving the
detail that the lens is delivering your sensor has no part in
determining DOF.
Chuck Norcutt
Ali Shah wrote:
> If the focal length, conditions, distance, and
> aperture were identical. The Olympus sensor in
> comparison to the larger sensors in N*kon and C*non
> (not full frame) dslr means that the photo produced by
> the Olympus may not be as crisp or sharp and perhaps
> would not have the same depth??!
>
>
>
>
> --- Winsor Crosby <wincros@xxxxxxxxxxx> wrote:
>
>> It is not the quality of the sensor. It is the size
>> of the sensor and
>> the resultant focal length and distance from the
>> subject. Here is a
>> page with a short discussion and a calculator.
>>
>>
> http://www.dpreview.com/learn/?/Glossary/Optical/Depth_of_Field_01.htm
>>
>
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