Hi,
I noticed the same thing today. Few days ago I bought M42 > Kmount
adapter to enable the recently acquired screw mount SMC Takumar
3,5/28. Just today I met with a friend who owns Carl Zeiss Jena
135/3.5 lens (which was made for Practica cameras and has M42 mount)
with intention to compare his CZJ 135/3.5 with M135/3.5. To make a
long story short, when I was metering with the ME Super the same
scene with both lenses, I noticed that at the same aperture, the
shutter speed chosen by the camera was different with each lens. For
example, at F3,5, it was metered at 1/125 with M135, however with
Carl Zeiss Jena it was about 1/2 stop lower, between 1/60 and 1/125.
I see two explanations: first, because of better SMC coating on the
Pentax there was less light loss or, second, that true wide open F
stop of the CZJ is slower than wide open F stop of M 135/3,5.
Unfortunately, I didn't test them at other apertures.
Matjaz
> Shel wrote:
> I can't really respond that except to share an example of some
> interesting metering behavior, which, in a convoluted fashion, may
> provide some insight if not an actual answer. As you know, I have a
> few Pentax lenses and when I meter with some k-mount lenses, using the
> meter in ~all~ my LX bodies, I find that the meter readings are different
> than with other lenses at the same aperture. I've made the readings from
> the same patch of open sky, so things like a different FOV, or factors
> such as colors, shouldn't enter the picture. The k-mount lenses in
> question all seem to be off by about one stop as reported by the LX meter
> readout. However, shooting at the metered setting results in properly
> exposed negatives.
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