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Re: [OM] Re: OM4-Ti meter

Subject: Re: [OM] Re: OM4-Ti meter
From: Kennedy <rkm@xxxxxxxxxxxx>
Date: Sat, 4 Jul 1998 14:16:32 +0100
In article , C.H.Ling <chling@xxxxxxxxxxxxxx> wrote
>This is only the thing that you see, not necessary what the computer inside
> OM4Ti is doing. 
>
>I have just performed an experiment, with a Zuiko 100/2 and OM4Ti mount on
>tripod, focus to plain wall. Without inserting film, in auto mode, adjust
>the aperture to let the reading goes to 60s+1/3 stop, turn memory on,
>trigger the shutter, the memory read 60s+1/3.
>
>Clear the memory, adjust the aperture again to let the reading goes to
>60s-1/3 stop, turn memory on, trigger the shutter and read 8s+1/3.
>
Unfortunately, my Black OM-4Ti is in for service at the moment, but I
have tried this on two other cameras - a Champagne OM-4Ti and a basic
OM-4 (note the OM-4 appears to have a very low drain current, as posted
on another thread, so this may have an updated circuit in it).

On both cameras the results were exactly the same - indicated and
memorised at 1/60 +/- 1/3 stop.  In fact, I thought there was a problem
since I expected the speed to be much slower without film.  The meter of
both cameras were out relative to each other by 2/3 stop, but the
indicated and actual shutter speed stayed almost unchanged without film
in either camera.

I then extended your experiment and placed a sheet of white laser
printer paper in the film plane and then a sheet of black paper (ie.
fully black toner printed) and checked the indicated and actual
exposures for a range of shutter speeds.  The entire experiment took
around half an hour with a couple of exposures at each speed to check
for accuracy.
_________________________________________________________________
Champagne OM-4Ti Ser. No. 11559xx
With BLACK paper in the focal plane :

Indicated Speed    Actual Memorised Speed   Difference (in stops)
    2000                    2000                   0
      |
     250                     250                   0 
     125                     125-1/3             +1/3 
      60                      30                  +1 
      30                      15-1/3             +1 1/3
      15                       4+1/3             +1 2/3
       8                       2                   +2
       4                       1                   +2
       2                      ~3sec (by watch)   +2 1/2
_________________________________________________________________
Black OM-4 Ser. No. 10398xx
With BLACK paper in the focal plane :

Indicated Speed    Actual Memorised Speed   Difference (in stops)
    2000                    2000                   0
      |
     125                     125                   0
      60                     30+1/3               +2/3 
      30                      15                  +1
      15                       4                  +2
       8                       2                  +2
       4                       1                  +2
       2                      ~3sec (by watch)   +2 1/2
_________________________________________________________________
Champagne OM-4Ti Ser. No. 11559xx
With WHITE paper in the focal plane :

Indicated Speed    Actual Memorised Speed   Difference (in stops)
    2000                    2000                   0
      |
     250                     250                   0 
     125                     125+1/3             -1/3 
      60                     125-1/3             -2/3 
      30                      60                  -1
      15                      30+1/3            -1 1/3
       8                      30-1/3            -1 2/3
       4                      15-1/3            -1 2/3
       2                       8-1/3            -1 2/3
       1                       4                  -2
_________________________________________________________________
Black OM-4 Ser. No. 10398xx
With WHITE paper in the focal plane :

Indicated Speed    Actual Memorised Speed   Difference (in stops)
    2000                    2000                   0
      |
     250                     250                   0 
     125                     250-1/3             -2/3 
      60                     125                   1 
      30                      60+1/3            -1 1/3
      15                      60-1/3            -1 2/3
       8                      30                   2
       4                      15                   2
       2                       8+1/3            -2 1/3
       1                       4+1/3            -2 1/3
_________________________________________________________________

As you can see, neither of these cameras exhibited the sudden transition
at 1/60 you are experiencing.  

Both of these cameras seem to follow the expected rule for exposure in
auto mode by integrating the light reflected from a combination of the
shutter blind and the film plane.  I expect that the figures around 1/60
with nothing in the film plane are the same because of the glossy
reflection of the back plate - it changed by 2/3 to 1 stop when a matt
black surface was put in the film plane.

I tried the same experiment with a piece of Kodak Gold 100ASA film in
the plane, but the differences were never greater than 1/3 of a stop -
indicating that this film at least has a similar reflectivity to the
shutter blind.

I do no think that the Champagne OM-4Ti should exhibit any difference
from the Black OM-4Ti since this model was only introduced for a short
period due to fashion - what is inside is exactly the same.  There
should be far more difference between the 4 and the 4Ti than there is
between Black and Champagne finishes of the Ti.

Clearly the reflection of the film will cause a difference which is
proportional to the shutter speed selected - as the results of the above
experiments show.  Others have posted large differences with Fuji Velvia
and Sensia at slow speeds, which is a consequence of this mismatch
between the shutter blind reflectivity and the film reflectivity.  I
have never used Velvia, but I would guess from the indicated exposures
of around 1stop over the indicated exposure, that the film is darker
than average and reflecting less light at long exposures.  (This assumes
that the quoted figures are the exposure difference, if they mean the
shutter speed is about a stop faster then the film is lighter than
average films).

If you really are getting such a sudden transition at 1/60 (and if
anyone else is experiencing the same) I would suggest returning your
camera to the centre at which it was serviced and asking them to fix it
properly - it neither behaves as theory nor practice indicates it
should.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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