On 12/30/01 1:43 PM, "John Robison" <omrobison@xxxxxxxxx> wrote:
> Hi Tom (and John Lind) Please pardon my 4AM reply, I
> woke up at 2:30 and coulden't get back to sleep. Went
> back to bed, caught a few more Z's and now have my 2nd
> cup of coffee in hand(in a Olympus mug,no less). What
> I tried to say was that if you set up your OM to
> photograph say a head and shoulders portrait with a
> leafy background and as an expirement you first
> mounted your 24mm, set apature at f4 and placed camera
> 2 and 1/2 feet from subject, exposed frame 1, now
> mount the 50mm at f4 at 5ft from subject, frame 2, now
> mount your 100mm at f4 at 10ft from subject. frame 3,
> woulden't all 3 frames have about the same DOF and the
> background have about the same degree of fuzzyness? Or
> am I all wet? About the math, I am a MMM that Math
> Mental Miget, I can visualize the why of DOF but when
> you guys start talking log and cos and things like
> that your are way over my head<G>. And John L., love
> your site, it's a standard place of reference I go to
> often. John Robison
Yes, within the same format (this gets important later), any focal length
lens will have the same DOF as any other focal length lens if the both have
the same framing and aperture setting. This is because while the longer lens
has a shorter depth of field, you have to move it further away from the
subject to get the same framing because of the narrower angle of view, which
increases the DOF. A shorter lens has a greater DOF but is used much closer
to get the same framing, which reduces the DOF.
Now where this changes is when you change formats. If you took 35mm, 6x6,
and 4x5 and used the normal lens with each body and the same aperture, the
35mm shots would exhibit the greatest DOF. This is because the normal lens
for a 35mm is 50mm (actually it's 45mm isn't it?), for 6x6 it's 80mm, and
for 4x5 it's 150mm. So for each of these shots, you'd have the camera set at
the same spot and get the same framing. But the DOF is dependant on the
focal length of the lens, not the angle of view, so the longer lenses on the
larger formats cause a reduction in DOF.
What I THINK the Hasselblad user might have meant would be to take the 'blad
with an 80mm lens and shoot from further away (getting greater DOF) and then
enlarge the section of film that you want. But then you're losing all the
advantages of the larger film size (although the extra DOF might make up for
in in this case).
--
Andrew "Frugal" Dacey
frugal@xxxxxxxxxxxxxxx
http://www.tildefrugal.net/
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