Originally posted by Stone G. Apart from Lowell Goudge's (main) explanation above it should also be noted that aperture alone doesn't tell, how much light actually reaches the sensor. There are transmission lossess and it is no coincidence that lenses in the professional movie business are characterized by T(ransmission)-stops rather than by F(ocal ratio)-stops.
Transmission losses with old MF lenses may be somewhat brand specific due to glass and coating types used as well as design specific due to number of elemets used, losses due to scatter and internal reflexions. My own experience is that Pentax MF lenses tend to underexpose a bit while Tamron Adaptall-2 lenses tend to do the opposite. Also, colours differ due to the different coatings used and Pentax colours from that time seem to be more saturated.
Finally the (epoxy) bonding between glass elements may change somewhat over time. A well documented example is that of old Takumars that use very slightly radioactive (VERY slightly - no danger here!) thorium glass. Over many years that very sligth radiation will actually make the expoxy cement turn darker and more yellowish.
while your explanation is relevent to why the camera esposure does not match an independent light meter, it is not relevent to TTL metering,
The issue at hand is that the TTL metering and the true exposure on the sensor do not match with a particular lens.
this means that for some reason, the light hitting the metering sensor appears to be higher intensity than the light hitting the image sensor. THis is the only explanation.
We have found that with many old lenses, the metering can be quite variable (i will not say eratic because we have through multiple members and different lens / camera combinations demonstrated the behavior is repeatable)
the only point I can really stress is that part of being a competent photographer is understanding how each lens works on the camera. That means a simple investment of perhaps 10 minutes per lens and body to map the exposure.
Note that this is not just for legacy lenses, but
ALL lenses. I cannot stress this point enough. My Tamron 28-75/2.8 for example, even in A mode shows an exposure error, starting at no error wide open, with a gradual but linear drift upwards of about 1/2 to 1 stop by F32. As a conseqquence, I generally set EV comp to -1/2 stops to minimize the average error. My sigma 70-200/2.8 has perfect exposure on my K10 alone, but add a 1.4x TC and I need to set EV comp to -.7 and with a 2x TC I need -1.3 stops of EV comp. this is all because the TC does not correct the aperture, and the K10D metering needs to absolutely know the true aperture to correct for the non linearities in the metering.
part of your job is to understand how your kit works, especially when using legacy lenses, third party equipment etc... It is not pentax's job to make the camera work perfect for every permitation and combination of lens and attachment you can dream up. If they tried they would go bankrupt. It would be nice if things were better, but I also have to point out that pentax is far better than canikon already in this regard.