I do remember reading that the screen introduced from the K10 onwards is made up of tiny pyramids that transmit the light better

Possibly as you use a smaller aperture when stopping down you are interfering with the light transmission proccess (ie. for each one stop of less light hitting the focus screen the transmission through it to the meter is reduced by more than one stop, and so on). The result is an overexposed picture.

I am not an optics expert, but my experience does show it to be the screen at the heart of this.

When I used my DS I had no exposure errors with "K" and M lenses using stop down metering. When I got my K10 I did.

When I put the DS focus screen into the K10... no exposure errors.

When I got my split prism focus screen the exposure errors returned, but not as bad as with the original K10 screen.

My experience leads me to believe it is ALL to do with the focus screen.
When the K10 first came out it was advertised as having a brighter viewfinder due to a new focus screen design. Whatever they did to allow more transmission of light through the screen has obviously affected the transmission when a lens is stopped down. The same "brighter" screen is clearly still the basis for the new camera models.

This does not affect a later lens with electrical contacts as these all meter wide open, and the camera knows what aperture to use when the shutter is triggered.

The main picture is a cracker Kris

Yep... me too. Zoom lens, what's that ?

Good luck with the M135/3.5 It is a great lens for the price.... so nice to use and excellent results.

I have had mine for 30 years !!

The answer with the "A" lens is to use it in the "A" setting

I use M lenses all the time and do not experience problems with the AWB white balance, althogh I rarely use AWB these days. it is also worth pointing out that an indoor shot such as your example is outside the range for AWB to work. Your camera may be different, but with the K10 AWB is for daylight shooting only.

What is your camera ?

Metering with M lenses is not straightforward. You can use them in Av but they will expose correctly BUT at max aperture only. In Manual exposure mode you use the green button (may be a different button on your camera)to "stop down" the lens momentarily and take a meter reading, this sets the shutter speed. You can also (on the K10) use the DOF preview (optical) to turn on the meter and adjust either shutter or aperture to centre the meter.

However you will find that either of the manual methods will progressively overexpose as you use a smaller aperture. So you will have to use the histogram check your results and adjust exposure accordingly.

Stefan

According to my K10 manual, an "A" lens with the aperture set to anything other than "A", will work in Av mode but any exposure will be wide open irrespective of the aperture setting on the lens.

You must use manual exposure to get the lens to stop down.

This may differ from the OP's camera of course.

I do recall that the advice when using an "A" lens is to leave the aperture on "A" and not try and select it manually.

It is a while since I used an "A" lens on my K10, but it may be that you will lose the aperture fuctionality...ie your shots will all be at wide open. I don't know what camera you are using, and it may well be that my advice is not appropriate , but it is worth you investigating

Lance , you are right. It is the only lens that the K10 does not like ! If I adjusted the K10 for the 31mm it would be off on my other lenses.

But as I use manual focus as a matter of preference with any lens it does not concern me.

My 31mm cannot auto-focus on my K10 either. When correctly (manualy) focussed it is very sharp at all apertures.