auto focus mf camera

Hi,

Recently, Bob Monaghan posted:

Why bother, some ask? Because it would be a lot cheaper than
investing in an AF kit in medium format just 'cuz our eyes are
getting older and less flexible ;-)

I must admit to eagerly awaiting the justification for such a device!
;-)

None the less, I'm curious as to how you'll couple it to the focus
mechanism in a meaningful way. I suspect that when the built-in loupe
fails me, it's time to sell. 8-)

Neil

my goal isn't to create an autofocus system which somehow focuses manual
lenses; I can do a good job of moving the lens focus ring myself, but it
would be nice to have an "in-focus" indicator, esp. with some wide angle
lenses which are harder to focus by eye. I use diopters now, including the
adjustable ones in some prisms and chimney finders, but there are other
issues (astig. etc.). A really accurate in-focus indicator with a good
null indicator would be quite useful (more so than the In-focus lights
in some some systems which are on over a very wide range of distances ;-)

according to one source, the AF module splits light at two angles onto two
contrast sensors which go to a comparator. As you bring the image into
focus by manually focusing the lens, the differences become smaller until
they are minimal and you get an in-focus indication. You could go to an
LED set to detect the null point of "in-focus", or use a small analog
meter or whatever you prefer ;-) Depending on the sensor size and design,
you could mount it at/under the ground glass screen or prism and supply
power and wiring to the indicator setup. They did something like this to
the pentax ME-F. The trick is going to be getting info on the AF sensor
and a repair parts source for it ;-)

regards bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

"Bob Monaghan" wrote in message
...
according to one source, the AF module splits light at two angles onto two
contrast sensors which go to a comparator. As you bring the image into
focus by manually focusing the lens, the differences become smaller until
they are minimal and you get an in-focus indication. You could go to an
LED set to detect the null point of "in-focus", or use a small analog
meter or whatever you prefer ;-) Depending on the sensor size and design,
you could mount it at/under the ground glass screen or prism and supply
power and wiring to the indicator setup. They did something like this to
the pentax ME-F. The trick is going to be getting info on the AF sensor
and a repair parts source for it ;-)

That would be pretty cool. I suppose an earphone jack would retrofit better
than a meter. If you can express contrast as a frequency, the beat tones
would probably be the most direct feedback you can get. Moving scenery would
make pretty interesting music while you twiddle. :-)

Recently, Bob Monaghan posted:

according to one source, the AF module splits light at two angles
onto two contrast sensors which go to a comparator. As you bring the
image into focus by manually focusing the lens, the differences
become smaller until they are minimal and you get an in-focus
indication. You could go to an LED set to detect the null point of
"in-focus", or use a small analog meter or whatever you prefer ;-)
Depending on the sensor size and design, you could mount it at/under
the ground glass screen or prism and supply power and wiring to the
indicator setup. They did something like this to the pentax ME-F. The
trick is going to be getting info on the AF sensor and a repair parts
source for it ;-)

This project would present some interesting challenges, anyway. I can
imagine a finder with a hinged, angled sensor platform that would move in
a similar way to the eye-level sports finder on Rollei TLRs.

It would probably be cheaper to buy an AF camera than try to make this
work right, but what fun would that be? ;-)

Neil

Neil Gould wrote:

Recently, Bob Monaghan posted:

according to one source, the AF module splits light at two angles
onto two contrast sensors which go to a comparator. As you bring the
image into focus by manually focusing the lens, the differences
become smaller until they are minimal and you get an in-focus
indication. You could go to an LED set to detect the null point of
"in-focus", or use a small analog meter or whatever you prefer ;-)
Depending on the sensor size and design, you could mount it at/under
the ground glass screen or prism and supply power and wiring to the
indicator setup. They did something like this to the pentax ME-F. The
trick is going to be getting info on the AF sensor and a repair parts
source for it ;-)

This project would present some interesting challenges, anyway. I can
imagine a finder with a hinged, angled sensor platform that would move in
a similar way to the eye-level sports finder on Rollei TLRs.

It would probably be cheaper to buy an AF camera than try to make this
work right, but what fun would that be? ;-)

Neil

How about an accessory rangefinder?
http://www.photoethnography.com/Clas...s/finders.html
http://www.erikfiss.com/foto/cams/voigtrf/e.html

Maybe the easiest way to get one would be to rip it off an old Polaroid
pack camera, but then the scale would have to be calibrated somehow.

-- Lassi

Recently, Lassi Hippeläinen
posted:

Neil Gould wrote:

Recently, Bob Monaghan posted:

according to one source, the AF module splits light at two angles
onto two contrast sensors which go to a comparator. As you bring the
image into focus by manually focusing the lens, the differences
become smaller until they are minimal and you get an in-focus
indication. You could go to an LED set to detect the null point of
"in-focus", or use a small analog meter or whatever you prefer ;-)
Depending on the sensor size and design, you could mount it at/under
the ground glass screen or prism and supply power and wiring to the
indicator setup. They did something like this to the pentax ME-F.
The trick is going to be getting info on the AF sensor and a repair
parts source for it ;-)

This project would present some interesting challenges, anyway. I can
imagine a finder with a hinged, angled sensor platform that would
move in a similar way to the eye-level sports finder on Rollei TLRs.

It would probably be cheaper to buy an AF camera than try to make
this work right, but what fun would that be? ;-)

Neil

How about an accessory rangefinder?
http://www.photoethnography.com/Clas...s/finders.html
http://www.erikfiss.com/foto/cams/voigtrf/e.html

Maybe the easiest way to get one would be to rip it off an old
Polaroid pack camera, but then the scale would have to be calibrated
somehow.

The problem that needs to be solved with that approach is that it is
uncoupled from the focusing mechanism of the camera. You don't wind up
with "focus confirmation", but with "focus information" that has to be
manually transferred to the lens via rather ambiguous distance scales. It
would make getting eyes in focus a matter of chance!

Neil

yes, that's a neat trick too ;-) You can use the null indicator output
voltage to control a Voltage to Frequency converter chip and listen for
the lowest frequency as the precise in-focus point ;-) Much more accurate
than the go/no-go LED in-focus indicators in most cameras ;-)

You can also make use of stereo headphones by using bipolar +/- output, so
as you focus from far to in-focus to nearer points, the tone goes from hi
to very low in say left headphone, then starts up from low to high on the
right side headphone, so you always know which way to turn the focus. When
the focus is exactly right, the frequency would be equally low in both
ears ;-)

Null devices are fun (I'm resurrecting an old heathkit LCR bridge using
such a setup now in my garage, nifty ;-)...

grins bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

yes, that's very true. The reason for an in-focus indicator is that it
eliminates the issues with transferring distance info to the lens. I did
find some articles on a stand alone sound based distance measuring device
run thru a PIC microcontroller in a Nuts and Volts magazine article from a
few years back. But the problem is the range is limited and you still
can't be sure that you have the lens properly focused or that the signal
is bouncing off the desired subject point. Hence a thru the lens in-focus
indicator is needed, which suggests a contrast based AF module grafted
onto the finder (i.e., where a split rangefinder screen might be placed?).

And most short range active IR AF units are limited to about 20 feet or
so, not really useful with a long telephoto lens ;-)

I was looking at a junk captiva polaroid SLR last night in the junque box;
I picked it up as 6x9cm coverage AF setup (also possible polaroid hacked
back for koni omega per one past poster..) with 107mm slow (f11-ish?) lens
but it is AF, autoexposure, and an SLR. Hacking it onto a rollfilm back
would provide a rather modern (if fixed lens) AF unit (use ND filters or
hack the autoexposure module to vary ISO film speeds etc.).

But in our case, this is a unit with an AF module that sells for under $50
with the whole camera (mine was $1 at a yard sale). Now if I could just
get detailed info on some of these AF modules, this project might be a lot
easier ;-) In the meantime, lots of other projects to do too ;-)

grins bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************