Originally posted by IsaacSteiner Recently i've been using an old Pentax-M 40mm 1:2.8 lens on my Pentax K110D, and today I put my Pentax DA 18-55mm lens on, and most of the time I shoot AV mode, so like usual, I put it to AV mode
Ahh, I think I see what the problem is. You've been using the M 40mm in Av mode, haven't you?
When using lenses from before the A series (e.g. M and K series), the camera cannot tell which aperture you have set with the lenses aperture ring.
What you will see when you do this is that the camera will meter with the lens wide open (f2.8), and then when it goes to take the picture, it will stop the lens down to the aperture you have selected with the lenses aperture ring.
So, say, if you had the aperture ring on the lens at f5.6, then the photo will be underexposed (darker) by two stops. This is because the camera metered for f2.8, but with the lens actually set at f5.6, you've lost two stops of light.
With more modern lenses (e.g. DA series, FA series, F series, A series), the camera can sense the aperture the lens is set to and thus meter correctly.
So, what you're seeing is not anything wrong with your kit lens, it's actually a reverse expectation caused by using an old manual lens in the wrong mode. The only mode which works properly for older manual lenses is Manual mode, or Av mode, provided the lens is set to maximum aperture (f2.8 in this case).
There are some instructions on how to use an older manual lens here:
https://www.pentaxforums.com/forums/pentax-photography-knowledge-base/3800-ca...unt-d-slr.html
Ignore step 3, as that is specific to screwmount lenses. Aside from that, the process is the same.