Metering difficulties with manual lenses, can you help?

Using my G3 with manual lenses I noticed (in video and stills) that the metering changes depending on the lens round about f-4 to 4-5.6. it is fine below this, than it the pictures/ movies become darker after it.

In stils it is less of a problem, you can use the compensation (if your subject did not move...) after you have seen the first picture, but in movies it makes it impossible to change the aperture during a movie without affecting the exposure.

Any idea why this is and how I can solve/ circumvent this problem?

cheers

JL