epozar@xxxxxxxxx wrote:
> > 2. Spot or cw-mode - another but only technical aspect: My OM Ti
> > announces usually 1/3 stop lower in spot-mode than in CW-mode (in Auto as
> > well as manual), of course metering an absolutely even lighted object
> > (wall f.e.).
>
> Hi Hans,
>
> On Spot, only a central, *the
> lightest* part of the image is measured. This value is OK for both open
> and closed/working aperture.
>
> On Average the value presented to the photographer is OK for the open
> aperture but is usually one or even two thirds of an f-stop less than a
> true speed for the closed/working aperture. This is because the system
> measures (via a secondary mirror) both a central and peripheral parts of
> an image - and the later are usually darker at the open aperture -
> inflicting a "drop" you noticed.
Hi Emil,
thank you very much for your explanation. I will try the test you
suggested straight this evening. Your notice about the low center
weighing of the average-mode covers exactly my suggestions (and explains
to me the tendency to underexposer in average-mode which my OM 4 Ti
showes in cases of high contrast with even small but bright zones at the
edges of the image; but that is of course not a problem).
There is one thing which keeps me still wondering:
At the moment (it is early morning here in Germany and I did not have my
usual tea yet :-); and my brain is pretty much the contrary of a
scientifically trained one...) I find your explanation absolutely logic
in the case that spot-mode gave HIGHER values (indicating shorter speed)
for measuring an area with no lens-caused fall-off. An average
measurement including seemingly "darker" zones because of lens-caused
fall-off should indicate indeed a less average light-value. But I get
instead indications for lower speed, means less light at spot...
Well, if the penny might fall (as we say in Germany for getting the
point, pretty unusually lately but successfully) I will send you a note
:-). Or - last edge of hope for me - my description has been mistakable?
Very best regards
Hans-Juergen Hertz-Eichenrode
c/o European University Viadrina Frankfurt (Oder)
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