So is it the case that matrix metering doesn't give the correct exposures for B&W and a red filter gives an even more incorrect exposure than no filter at all i.e. compounds the problem or is it simply that whatever the exposure is shown to be without the filter then adding the filter simply adds say 2/3 stops to the in-camera exposure as a red filter will do.

I realise that a red will add extra exposure and none,some or all in-camera exposure systems will underestimate the correct exposure with a red so some manual compensation may have to be made.

To check by how much, if any, that a camera underestimates the extra exposure for a red surely all the F6 user has to do is check what exposure the filter factor gives compared to the TTL meter and then compensate for the difference?

I fear we may be putting the OP off buying a F6 for B&W film and red filters when any problem if there is one is far from insurmountable.

pentaxuser