As David has already stated . The Dof is dependant on
1 Focal length
3 Subject Distance
4 Circle of confusion
It also depends on viewing distance of the image but lets assume that the viewing distance is a constant.
In a particular format the circle of confusion is the same so for a particual format DOF is only dependant on the other three and the viewing distancehttp://en.wikipedia..../Depth_of_field
Here you will find the relevant formulas as im unable to type them clearly not having an equation editor software. With the formulas you will clearly see what depth of field is a function of; and the variables involved; and how changing the variables changes depth of field.
One common myth is that depth of field changes with focal length. Yes it does but not if the image size is kept the same.
Dof field doesnt change if the Image size is kept the same in the frame.
That means that if I change my lens from a wide angle to a telephoto but i keep my object size the same my DOF doesnt change because the subject distance also changes which is another variable in the Dof equation.
There are exceptions to the this when the subject distance tends to equal the Hyperfocal distance as well as the Focal length of the lens ( macro basically)
As David has pointed out that Dof has to many parameters that have to be kept constant while changing formats and comparing but a general rule of thumb is
"All things being equal depth of field is inversely proportional to film format size"
This because the circle of confusion is a variable in the Dof equation and as other other variables are kept constant the Dof changes in relation to CoC
The key phrase is "All things being equal."
So if you compared equal sizes of projected 16 mm and 35 mm images same subject to camera distance same field of view(not same focal length)..viewed from the same distance etc then 16 would have more Depth of field as compared to 35mm (when the subject distance is less than the hyperfocal distance.)
Here are some sites explaing the above but with comparisons from still photography formatshttp://www.photo.net...ics/dofdigital/http://en.wikipedia..../Depth_of_field
Edited by Manu Anand, 29 July 2005 - 05:20 AM.