Well, not quite. The inputs to a depth of field calculation are
distance, aperture and focal length... plus a key ingredient you're
missing here which is resolution or that confusing term called "circle
of confusion". It's the number that accounts for print size and viewing
distance.
You are correct when you say that what gets captured on film or sensor
is the same for any given focal length regardless of format. But what
you're neglecting is what happens after the capture. In the end we have
to magnify the image on prints or on screens in order to view it. The
smaller the image format the greater the required magnification to make
a give print size.
The greater magnfication requires a higher resolution image to begin
with if it's going to look sharp on the final print. But, because of
the increased magnification or the "crop factor" between image formats
we don't use the same focal length lenses to frame the same shot and the
shorter focal length results in increased depth of field.
For example: A 35mm shot made with a 50mm lens at f/8 focused at 5 feet
and assuming 30 lines/mm resolution on the film yields a DOF of 4.3-5.9
feet. A 4/3 shot made with a 25mm lens at f/8 and focused at 5 feet but
now needing a resolution of 60 lines/mm for the same final print size
yields a DOF of 3.8-7.4 feet. Both of these resolution values should be
sufficient to make good 8x10's from either image but the 4/3 shot will
have more DOF because of the shorter focal length lens used for the
capture. And the same sort of analysis will hold true between 35mm and
medium format or medium format and large format.
Chuck Norcutt
Jim Couch wrote:
> As I understand it, depth of field is essentially for any given focal
> length, regardless of the format size. (DOF is the same for a 50m
> lens wether it is on an E1, an OM, or a medium format camera.)
> Because the E1 crops a section out of the middle of the lens
> (essentialy) it will have a SIMILAR PERSPECTIVE to a 100mm telephoto
> but the same DOF as the 50mm lens. To more clearly understand this,
> if you take the same photo, from the same place with a 50mm Zuiko on
> an OM and crop the resulting photo, it will be exactly the same as
> the photo taken with the E1 and the 50mm lens. (Allowing for the
> difference between film and digital.)
>
> I think the confusion becomes because of the tendency for folks to
> say that the E1 with it's 2x CROP factor will turn your 50mm lens
> into a 100mm lens. It does not, it is still a 50mm lens, and behaves
> as such, you are just cropping the picture down to the center
> portion. The result is that you get a picture that looks as if it was
> framed with a 100mm, but it still has the DOF of the 50mm.
>
> Jim Couch
>
> At 01:39 AM 12/2/2006, Moose wrote:
>
>
>>I don't remember all the details, but DOF with the smaller sensor is
>>generally greater than with FF film. With the 100/2 at f8 and the DZ at
>>f9, and assuming the coverage is about the same, the DZ image will have
>>more DOF.
>
>
>
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