Re: [photo-3d] DOF & Rangefinder

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

Chuck writes:

> I really don't know what is meant by "I don't use a rangefinder, I use
> maximum DOF."  All I can assume is that they use their tightest F stop and
> focus for about 15 feet.  The problems with this are the tightest F stop
> rarely produces the sharpest photos and many stereo cameras vignette badly
> at F22.

Chuck, do you know what the hyperfocal distance is?  I don't think
you do.  So let me tell you:

For every f-stop there is a distance associated with it,
called the hyperfocal distance.  This gives the maximum DOF for
this given f-stop.  If you focus at the hyperfocal distance for the
given f-stop then your Depth of Field (DOF) extends from half
that distance to infinity.  People who focus at the hyperfocal
distance do not necessarily use the "tightest" i.e. smallest f-stop.
But they want to have infinity in focus and as much in the foreground
in focus as possible.

>I use a rangefinder AND DOF.  With the rangefinder, I measure the distance
>to the nearest object I want in focus.  I then measure the distance to the
>farthest object, then set the camera focus between the near and far distance
>with the two distances at similar F stop markings on the DOF scale. This
>gives the same sharpness at near and far with everything in between sharper.

Fair enough.  Does it matter what is this "similar F stop markings on
the DOF scale"?

>I then select the F stop based on light, lens sharpness, and DOF.  Mostly I
>use F11 or F16.

You went to this trouble to use f11 or f16?  If you look up the
hyperfocal distances for f11 and f16 for the Realist, etc., you 
will see that focusing at the hyperfocal distance for either of
these f-stops will give you plenty of DOF to have everything
sharp in the picture.

>It is easier if I have infinity or very far objects in the picture.  I use
>the rangefinder to measure the distance to the nearest object and then set
>the focus to twice that range.  This is the principal behind the hyperfocal
>table in the older Realists.

No, it's not.  If you use a relatively wide aperture and you have
to have infinity in focus then focusing at twice the distances of
the nearest object will not necessarily give you a sharp infinity.

>In any case, if you are going to adjust the focus, you should know the
>distance to the objects you are photographing and the handiest way is with a
>rangefinder.

If you are going to use f11 and f16 in the Stereo Realist then
you don't really need to know the distances of the near and
far objects with any degree of accuracy.

The bottom line is this:  Your method is good but for the
majority of the photographic situations with the Stereo Realist
you do not need to know the distances of the near and far
objects with high degree of accuracy.  Knowing what the
hyperfocal distance is and how it works, will simplify
life for many individuals in many situations.

George Themelis