In article , Richard Schtzl <Richard.Schaetzl@xxxxxxxxxxxxxxxxxxxx>
writes
>But still is my question: Does the bar of my camera meter reacting not
>to fast and sensible? At least the bulbs consists of glowing wire and
>the efect of changing voltage (50Hz in Europe) couldnt be big.
>
The flicker present from light bulbs is actually double the mains
frequency, so in Europe they flicker at 100Hz, whilst in the US they go
at 120Hz.
Are you seeing this effect on tungsten lamps? With these the amount of
flicker is usually quite limited due to the thermal capacity of the
filament - which depends on the bulb wattage - so you get a DC
illumination with a small amoount of 100-120Hz flicker on top. Under
this illumination source you ought to be able to get the OM-4Ti meter to
stabilise on a single setting - although you will get two 'dots'
illuminating on 'boundary' conditions between the two stable levels.
With a fluorescent lamp the level of illumination has a much higher AC
content depending on the actual phosphor on the tube and varies from
manufacturer to manufacturer. It is quite likely that the level of AC
present under fluorescent illumination would cause a problem with the
OM-4Ti meter which could prevent you stabilising the meter reading on
one precise level. You can see this effect clearly if you run a US
video camera under European lighting or vice versa - tungsten light
flickers much less than fluorescent.
The fact that you are getting stable readings under sunlit conditions
indicates that the meter is working correctly. Remember that the meter
on the OM-4Ti has to have a fast response because this is the same meter
that controls the TTL flash and OTF metering functions as well! There
is a filter in the circuit which smooths the meter output before it gets
to the display, however it is a simple analogue filter and these are
notoriously variable in knee frequency. Since all OM-4 cameras indicate
a significantly variable reading when viewing a 50-60Hz source such as a
TV screen it is clear that the filter 'kicks in' above that frequency.
It could be that your camera has a filter that has its knee point
towards the upper point of the design specification due to the tolerance
of the analogue components used - capacitors are widely toleranced
components, with +/-10% being about as good as it gets, and +/-50ot
unheard of for the surface mount components used in the circuit.
Given that you are only seeing a change between two positions, which are
1/3 of a stop apart, the filter must be operating fine, but obviously
not filtering out all of the 100Hz mains signal.
I can just manage to get a stable signal on mine under normal tungsten
light, but not at all under fluorescent light. Its not a problem with
the camera, just one of the limitations of analogue filter circuits.
Hope this puts your mind at rest.
I just considered another possible cause - your mains might be asymetric
due to the operation of some components in your home - such as the
fridge, air conditioner etc. or even in the supply itself. This would
give rise to a 50Hz component which would be present even on tungsten
lamps but which you would not see. To check this you really need access
to an oscilloscope or an accurate analogue DC voltmeter. If there is
any DC reading at all on your mains (in the UK there is a specification
as to how much it can be on the delivered supply, but without looking up
the regulations I can't rememeber how much it is) then it could give
rise to this effect, since the 50Hz is below the knee of the filter.
Either way, it doesn't change the situation - if there is a problem with
your camera, its not in the meter. :-)
--
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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