M3 DOF marks on RF patch - how to use ?

[alexz]

Enjoying immensely shooting with my beater M3, I still cannot grasp the idea of those DOF marks that appear on the top and botton edges of its RF patch.
As far as I remember, the upper (wider) one refers to DOF at f/16, while the lower one (narrower) reefrs fo f/5.6. But how to actually use these ? Any hints ?
Did you find them useful indeed ?

 

[laptoprob]

If perfectly in focus, the image in the patch coincides 100% with the surrounding area. At 5,6 and still in focus, the edge of an object in sharpness is shifted to max. at the edge of the small DOF mark. At f16 at the edge of the big DOF mark.

Simple but brilliant!

 

[ferider]

Like Rob said.

Very useful, also to predict a Sonnar's focus shift - half the lower
mark corresponds to f2.8.

See also:

http://www.rangefinderforum.com/forums/showthread.php?t=41774

and (from the original manual)

Best,

Roland.

 

[alexz]

OK thanks, sounds to be a bit clearer. I realize it is mostly applicable when the plane of focus is angled to the film plane, right ?
I'm playing with mine now, xmm, still somewhat fussy...while lens DOF sacle hints for DOF, say from 3-4 meter to nearly infinity, the upper mark (f/16) of tht patch occupies only a smalll portion of that DOF zone (i.e. suggesting for considerably narrower DOF area)....

 

[ferider]

It only works for 50mm, Alex, and close up better than far away.

Roland.