[Leica] Re: Re: Focus thread

[Jim Brick <jim@brick.org>]

Human vision and tonal difference detection is far more acute than what can
be "visualized" by a math algorithm reading electronic bits either from a
focus sensor array or a piece of the actual photo array in the case of a
digital camera.

Humans can focus accurately on items that will cause AF to hunt endlessly.
Recently I took the new Contax MF AF camera and pointed it at a Plexiglas
magazine (View Camera magazine) rack on the camera store counter, at an
oblique angle. It was super easy to focus manually on any part of the rack,
anywhere along the entire length because my brain could see what it was and
could see when there was a sharp definition at any point along the
Plexiglass rack. When switched to AF, the lens hunted in and out forever,
never stopping as it could not find sufficient data to generate histogram
spike differentials as the lens moved from one plane to another.

On a target that has sufficient contrast lines to enable AF to quickly
converge, it still cannot converge to the Nth degree that the human
eye/brain can. The human focus mechanism can detect minute differences and
adjust the focus accordingly. The same movement in a convergence algorithm
will show no difference therefore, AF cannot focus as accurately as a human
and therefore gets lower lp/mm marks.

The Asian algorithms (AF cameras - perhaps only digital - intended for the
Asian market only) are exposure/focus related and tend to use wider
apertures for more background blur than non-Asian algorithms. Non-Asian
algorithms tend to want everything in focus and a cooler color balance.
Non-Asian algorithms will sacrifice shutter speed for a smaller f/stop and
have a warmer color balance. Vice versa for Asian algorithms.

Hey... I just work here. I don't make this stuff up!!! But I have to deal
with it daily.

And don't get me wrong... I think AF is great for the type of photography
that can use it effectively. And, of course, for those with failing
eyesight. As of now, I just happen to have no use for it.

Jim


At 04:08 PM 11/5/00 -0500, Martin Howard wrote:
>
>I agree that systems are dumb and photographers provide the intelligence,
>but isn't contrast differences what all focus systems use?  Ground glass,
>rangefinder patch, and autofocus?  If we look at something that doesn't have
>sufficient contrast difference, we cannot tell if it is in focus or not.
>
>If you get a higher lp/mm because the human eye is better at detecting
>correct focus than the autofocus algorithms, then that STILL doesn't mean
>that knowledge of the subject has anything to do with it.  How would knowing
>that you're taking a picture of a car or your aunt Agda make a difference to
>determining whether the hood ornament or Agda's eyes were in focus or not?
>
>Why on earth would you have different focus algorithms for Asians and
>non-Asians???  Are Asians' eye-sight better than ours??
>
>Martin Howard