[Leica] Depth of field... (more than you wanted to know)

[Jim Brick <jimbrick@photoaccess.com>]

I wrote this a while back for the HUG. I was answering a statement
(included) made by a contributor. Even though I use the word Hasselblad, it
is applicable to Leica, and most other cameras. The Holga is in a category
of its own! A jar of Vaseline can turn any camera into a Holga.
___________________________________________

There seems to be some confusion about Depth Of Field and lenses because At
11:10 AM 8/14/99 -0400, Someone wrote:

>>I can certainly empathize with anyone who experienced focus-shock coming
>>from modern SLRs.... even my old YashicaMat 124G was easier to focus.  With
>>the new camera [Hasselblad] the ugly surprise wasn't missing focus, but
shallow focus --
>>subjects with one eye in and one eye out, and such.  I just wasn't prepared
>>for the skimpy depth of field.

The statement is incorrect. Depth Of Field (DOF) is a function of f/stop,
image size, and acceptable Circle Of Confusion (COC). An 80mm lens on a
YashicaMat has precisely the same DOF as an 80mm lens on a Hasselblad, in
every situation. One does not give less or more DOF than the other. Set
them side by side, use the same f/stop, and the DOF will be identical.

There are, of course, optical formulations that affect DOF, therefore it is
possible that two 80mm lenses can have slightly different DOF
characteristics. For all intent and purposes, this is a moot point.

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 DOF 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, f/stop, and acceptable COC. It is the laws of optical physics.

An 80mm lens on a 35mm camera has the same DOF as an 80mm lens on a 6x6
camera. The 6x6 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 tight face composition, 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 Pinnoccio.

As you stop down, more comes into acceptable focus, increasing 1/3 in front
of, and 2/3 in back of, the critically focused subject plane.

A mediocre lens will "appear" to have more DOF than an first class lens.
This is because those things that are in focus (at the plane of actual
focus) will appear to be extra sharp and then fall off to not so critically
sharp (acceptable COC) within the wanted DOF range. While a mediocre lens
will not have that extra critical sharp area, it will blend better with the
acceptable DOF range. There is no abrupt transition. So the better your
lenses, the more visible the transition between the very narrow critical
sharp plane of actual focus, and the rest of the photograph. 

The bottom line is that there is only a "very very narrow" plane of precise
focus. Everything outside of that plane will fall under acceptable blur
(COC) and extends farther behind the critical focus plane than in front of
that plane. And beyond the acceptable COC blur, is unacceptable blur,
eventually becoming Bokeh.

In order for your photographs to turn out as your mind's eye sees them when
you are taking them, you must understand DOF, how it works, its
limitations, and how to get around the limitations.

Leica and Hasselblad lenses have DOF information inscribed on them. You can
use this information to zone focus your lens for generally "acceptable" DOF
focus. For more critical work, use the scales as if you were opened up one
stop more than you really are. For "drop dead" DOF focus, use two stops.

Jim