Re: hyperfocal

["Eric Goldstein" <egoldstein@xxxxxxx>]

Tom Deering writes:

> At one time, I thought DOF was based on how good the glass was.  I
> assumed the reason one 50mm lens had a different DOF than another
> 50mm was due to some inherent property of the lenses.
>
> Then I found out that depth of field is based on a mathematical
> formula.


I don't want to get too esoteric here and of course Tom is really correct in
saying that in modern times, DOF does not vary with lens design (only
aperture; the common misconception that it varies with shooting distance is
also incorrect)...

But at an earlier time, some designs did seem to have more DOF than others,
largely due to uncorrected or undercorrected sherical abberation. Kingslake
makes reference to this. With a healthy does of such abberation in the image
even when "sharply" focused, the distinction between what was sharp and what
wasn't would not be quite as apparent, and so the viewer could tolerate more
DOF in this context. Lenses such as the Steinheil Unifokal (which was one of
the original optics available on the Heidoscop BTW) were marketed as having
greater than "normal" DOF.

This then leads to an essential point about DOF... it is not based upon
mathematics, it is expressed by mathematics. It is based upon perception,
viewing conditions and context. We have all had the experience of taking a
photograph which upon careful inspection has focus or DOF problems, but when
viewed normally "works." Conversely, there are some shots where even the
slightest out of focus areas are blaringly apparent and render the shot
unviewable given any viewing conditions.

The DOF equations are general guidelines and make specific assumptions about
viewing conditions which generally are too lax for the comfortable viewing
of stereo slide pairs, where the DOF requirements are typically (but not
always) greater.

Eric G.













  One of the variables is for a measurement of precision
> called the "circle of confusion".  You can think of it as a
> measurement of "acceptable fuzziness". Some lens manufacturers
> specify a larger COC, which gives their lens a greater DOF at the
> expense of softer focus within this range.  I'm guessing why the DOF
> table listed above was so, let's say, optimistic.
>
> I have a spreadsheet that calculates the DOF of the Sputnik (or any
> other camera.)  You can punch in the COC that you are comfortable
> with.  If anybody wants a copy, write to me off-list.
>
> Tom Deering