Wayne.Harridge@xxxxxxxxxx commenting on the somewhat kludgy seeming method of
limiting long exposures using an LED writes:
<< Thanks Tim, it's an interesting way of handling that situation, I would
guess that it is not the first idea someone would come up with to save
battery life. Why wouldn't you just use a timer to close the shutter
directly after 2 min, or feed a small voltage directly into the integrator,
or put a high value resistor across the light cell ? >>
Wayne, a similar thought crossed my mind. Here are some speculations:
1) It is such a neat (weird) idea! (Engineers sometimes like the "creative"
solution).
2) A late engineering change (fix) as you also suggested, which does not
require additional chip resources or capacitors.
3) In the OM2N they did add a seperate timer it just does not close the
shutter directly but feeds a relatively larger light level into the cell so
that the ASA dependent time is a much shorter part of the total timing limit.
4) If they did as you suggested and the timer switched a resistor to generate
current into the integrator the so called "noise gain" of the circuit would
generally be increased. With increased noise gain the integrator is more
suseptible to voltage offset drift which is relatively poor for the Mosfets
used here. Any added parts on the integrator input increase the chance of
leakage which is already a problem with the circuit values chosen.
5) Maybe they designed in the ASA dependent cut off time as a feature but
reconsidered when users found it a pain when the camera was tripped in the
camera bag and the battery life was shortened.
If I were to pick I would think 4 was the reason especially as the circuit
actualy does two seperate timings for different switch settings (see comment
at end)
John Hermanson wrote:
<<
The 2N LED flashes at the same intensity regardless of ASA setting whereas
the 2s LED brightness is based on ASA. Low ASA = low LED light, high ASA =
bright. So, low asa low light photography gives you a much longer auto
shutter speed>>
I agree with the part about how the time varys with ASA but the OM2 does not
vary the intensity of the LED with ASA. You can see this by checking the
circuit diagrams: there is no circuit adjusting the LED drive current
dependent on the ASA resistors. In the OM2 the LED is in series with a fixed
resistor (resistor selected for the particular camera at the factory). The
varying time just comes from the fact the integrator needs to "count" more
photons when the ASA is at a low setting to get the required number of
photons on the film. It does not know that the photons it is counting are
coming from the limiter LED instead of reflected off the film. The timing
diagrams in the OM2N service manual also illustrate how this works for both
cameras. For comparison the LED is connected across the shutter magnet and in
series with a timer ic. in the case of the OM2N.
The ASA dial just sets the comparator driving the curtain magnet to trip at a
higher integrator voltage level for low ASA than it does for high ASA
settings.
An interesting aspect of the LED limiter that is not well known is that it is
also what limits the maximum exposure time when the camera is in the off
position and is then triggered. In this case a different resistor ( higher
current) is used to feed current through the LED making it brighter. In
contrast in the 2N the timer circuit ic again does most of the timing rather
than the LED light integration. This has an interesting subtle effect but
this post is already too long and technocratic.
Regards,
Tim Hughes
Hi100@xxxxxxx
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