[Leica] RE: DOF

[Jim Brick <jim_brick@agilent.com>]

>
>> Depth of field is a factor of image size and relative aperture. The film
>> format/ lens combination affects the image size. A 45mm lens in on a 4x5
>> camera will record an object "smaller" in the final print than a 45mm on a
>> 35mm camera. You could enlarge the 4x5 negative more than normally is done
>> and print an identical image size (assuming you shot both cameras at the
>> same subject distance) and achieve identical depth of field (and COC) but
>> this is not normal practice.
>

Here is a good DOF site. After hitting PLOT, don't be impatient as it takes
 some time to calculate.

http://www.dof.pcraft.com/dof-frames.cgi

Set up 75mm lens, f/1.4, .5 feet, and you will see that at f/1.4, 2', DOF
is minuscule. 1/4" at best. The thickness of a tulip petal.

For a given COC, a 50mm lens 10 feet away from the subject has the EXACT
SAME DOF as a 100mm lens 20 feet away from the subject. A 50mm lens at
f/1.4 focused at 10 feet has from 9'7" to 10'5" in focus. A 100mm lens at
f/1.4 focused at 20 feet (exact same image size on the film as the 50mm
example), the DOF is from 19'7" to 20'5". The depth of field is EXACTLY the
same. EXACTLY 10 INCHES IN BOTH CASES for the given COC.

DOF has EVERYTHING to do with IMAGE SIZE, which is a combination of the
focal length you are using AND how far away you are from the subject.

If the "image size" on the film is the same, the DOF is the same for a
given f/stop, REGARDLESS OF FOCAL LENGTH and REGARDLESS OF THE FILM SIZE.

This is a simple fact.

*** MORE ***

Depth Of Field is a function of f/stop and image size. An 80mm lens on a
4X5 has precisely the same DOF as an 80mm lens on a Hasselblad,
which has exactly the same DOF as an 80mm lens on a Leica, in every situation.
One does not give less or more DOF than the other.

Take two Hasselblads, one with an 80mm lens, the other with a 160mm lens.
Take the 160mm Hasselblad exactly twice the distance from the subject as
the 80mm Hasselblad is (this will make the photographed image - its size -
on the film exactly the same for both cameras) and the DOF in the image in
both cameras, at exactly the same f/stops, will be exactly the same.

Conversely, if you photograph with the 80 and 160 from exactly the same
distance and f/stop, the DOF in the 160 image will be considerable less
than the DOF in the 80 image. The 160 image size will be twice as large as
the 80 image as well.

One manufacturers 80mm lens has the same optical characteristics as any other
manufacturers 80mm lens. Or 100mm lenses, or 150mm lenses. It's not a
function of manufacturer or film size, it is the function of image size on
the film and f/stop. It is the law of optical physics.

An 80mm lens on a 35mm camera has the same DOF as an 80mm lens on a 6x6
camera which has the same DOF as an 80mm lens on a 4x5 camera.
The 6x6 and 4x5 80mm has more "coverage" than the 35mm format, but the
image size (the size of what it is you are photographing, as recorded on
the film) is identical, therefore the DOF is identical.

What f/stop you use and where you focus in your scene will determine how
much is in focus. From the exact point of focus, DOF extends 1/3 forward
(toward the camera) and 2/3 back (away from the camera). If you photograph
a persons face, at wide open (f/2.8 or f/3.5) and focus on their eyes,
there is a good chance that the end of their nose will be out of focus. If,
however, you focus on the tip of their nose, their eyes will be in focus.
Except for perhaps Pinocchio. As you stop down, more comes into acceptible
focus, increasing 1/3 front and 2/3 back.

Jim