I have a Pentax K100D. Came with a 18-50mm kit lens and I have some others that are for my DSLR (one Sigma and one Tamron). When I have my kit lens on my camera I can manually change the F-Stop, when I look through the viewfinder I can see the F-Stop and change it as well as the exposure(?)(the -.50/0/+.50). However when I have my other lenses on, I lose the capability to see the F-Stop or change it...It doesn't matter if the camera is in manual mode or TV or AV....I hope I'm asking this right? I don't know if this is a big deal or anything like that but I'm confused about why this does this? If anyone can shed any light on this, I'd appreciate it! Thanks so much!