A's vs. DA's

I'd say it's a near-certainty that the coatings have changed over the years. Pentax must surely improve them with every new lens design, considering the competition they face.

My guess is that the AWB setting is also designed to take the modern coatings into account which is why the white balance between K series and DA series lenses can be very different.

Best wishes, Kris.

But white balance is metered through the lens so why should a coating effect that?
Well it does but since it's meter through the coatings it should correct itself.

If the coating gives a slight cast (e.g., slightly magenta, just for the sake of argument) and the AWB is programmed to remove that cast, when you use a lens that doesn't have that slight cast it will "correct" it giving a different value for the AWB. Or that is my take on it anyway!

I am just guessing wildly really but there is no doubt that certain K series lenses (esp the 135mm f/2.5) always give quite different AWB values compared to more modern lenses.

K.

Indeed Kris, I thought you meant you saw a white balance different on your photos.

AWB is just a dumb device that simply wants to make a white paper appears white, no matter what lens you use and yes you can get different values because of colour cast that way.

I used to have an A50 2.8 macro and 'traded up' to a A100 2.8 macro. There was some fall off of colour definition on both lenses at extreme distances but down in the macrobial world wherefor these lenses were built they come right into their element (no pun intended) - the colour (and mono) rendition of close-ups taken with these lenses is utterly superb.