I studied up on this years ago when I got started in macro photography.
The Kodak Worships series book on close up photography has an excellent
chapter that describes the theory of dof and gives the formula to
calculate it. I think John Shaw covered it in earlier film era versions
of his "Close UPs In Nature"
Basically, in theory DOF is driven by the lens f stop setting and the
magnification of the subject. Focal length and format are not directly
involved in DOF but obviously longer focal length may mean greater
magnification and the magnification (reproduction ratio) of larger
formats is higher. If you fill the frame of a Q, a 35mm, and a 6x7 with
a 35mm wide subject the magnification ratios are roughly 1:5, 1:1, 2:1 -
ie. from 1/5th lifesized to 2x lifesized. Magnification goes up, DOF
gets more shallow. In theory, if you take a 1:1 magnification image with
a Q, a 35mm and a 6x7 the depth of field in each image is the same, but
obviously the crop of each image is completely different with the Q
showing something only about 5mm wide and the 6x7 showing something
about 7cm wide.
I tested this extensively with 35mm great and determined that at the
same magnification and f stop setting images taken with a 50mm, 100mm
and 200mm at the same f stop setting. Of course, in fact, the images
were basically identical but the distance between the camera and subject
varied considerably.
The one factor I am ignoring is the "circle of confusion" and effect
that enlarging the image itself has on the DOF of the final print....
and that is not something I have never been able to grok and so won't
comment on. Maybe someone else can.
Mark
On 3/7/2015 10:32 AM, Stanley Halpin wrote:
Bulent’s recent question about a possible foray into large-format film
photography, primarily for macro shots, yielded several comments about how one
gets a shallower depth of field with Large Format (LF). I seen that, heard that
before. One or more of the 645z reviews I’ve seen made a big deal about how the
medium format (MF) niche was crippled if not doomed by the inherently shallow
depth of field. I remember in the early days of APS-C and smaller sensors, some
were moaning about the difficulty of getting shallow enough depth of field.
OK, so it seems that the general consensus is that moving from LF to MF to FF
to APS-C to smaller sensor carries with it a corresponding increase in depth of
field.
But what does this mean? Really?
I understand that lenses with longer focal lengths have shallower depths of
field. So a 75mm lens will have shallower depth of field than a 50mm.
And I understand that a 75mm lens on a 645 format will have the same effective
field of view as a 50mm on a FF 35mm format. Is this why the larger formats are
considered to have shallower depth of field? Because I need to use a longer
lens to achieve the same field of view?
What if I had a 100mm macro lens that could be adapted to fit on the 645z and a
K-mount FF and a K-mount APS-C. Since the lens and its focal length are
constant, does the depth of field remain constant, with only the effective
field of view changing? Or does the depth of field also change?
In short, I think my question is whether the accepted relationship [LF & MF =
shallower depth of field] really has anything to do with the sensor format. I am
guessing that people are talking about a practical issue (need to use longer lenses
to get the same field of view on a LF) rather than a physical issue of the
interaction between sensor size and lens. But I can’t tell...
stan
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