Archived posting to the Leica Users Group, 2000/11/05

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Subject: [Leica] Re: Focus thread
From: Jim Brick <jim@brick.org>
Date: Sun, 05 Nov 2000 09:29:02 -0800
References: <01C046B3.F7A945D0@user-2ive2pg.dialup.mindspring.com>

At 11:59 PM 11/4/00 -0500, Martin Howard wrote:
>
>Nevertheless, none of the above has anything to do with Jim's assertion that
>autofocus is no good because the programmer who wrote the code for the chip
>doesn't know what you're going to take a picture of.  Again, it has about as
>much relevance as the statement that the guy who paints the shutter speed
>numbers on your Leica doesn't know if it's a sunny day or raining when
>you're taking pictures at your daughter's graduation.
>
>Martin Howard                     |

Actually, it does have much relevance. I just finished both autofocus and
autoexposure microcode that will be running in a high-end Agilent/RCA
camera. Personally knowing the mathematics involved in making a lens move,
gathering data and processing it at a high rate of speed, generating
histograms of each "focus" capture and correlating it to the previous
capture, and using proportional/integral algorithms to speed-up and slow
down the lens as it moves to/away from optimum mathematical convergence, I
can tell you that the whole process is prone to subject failure. AF can
only look at contrast differences. The algorithms implemented will be
different for each programmer. Like color balance. In digital cameras we
have Asian and non-Asian color balances. We also have Asian and non-Asian
focus/exposure algorithms.

So, it does indeed make a lot of difference who is implementing the
exposure/focus/color balance and it is true that these algorithms are
purely mathematical. Which is why you can get AF to hunt endlessly if
pointed at the wrong subject.

High end cameras such as the D1, F5, EOS1, etc, have many years of
experience behind them and do a reasonably good job. you can still make
them hunt and still get their exposure algorithms to fail. But it still
boils down to that you are the photographer and the camera has no clue what
your photographic vision is. It is subject blind and relies on contrast
differentials to focus. Exposure gives weights to various matrix
configurations (adjustable from nine to 36 in our camera) but still boils
the total accumulation of these weighted sectors, to yes... roughly 18% gray.

These systems are simply dumb. Photographers give them too much
intelligence. Even those using fuzzy logic and AI use math to interpret
contrast ratios and gray densities. Humans are much smarter than this. This
is why the same lens touts a higher lp/mm resolution if hand focused vs if
auto focused.

Jim

Replies: Reply from Jim Brick <jim@brick.org> ([Leica] Re: Re: Focus thread)
In reply to: Message from Austin Franklin <austin@darkroom.com> (RE: [Leica] Re: Greg Bicket's focus thread)