Focussing blues

Recently I used the inch marking on a blue rotary guillotine to assess the accuracy of the AF on my K10D and all my lenses were a bit off and bock focussed.

In practice the 35mm for example focusses bang on most of the time. Would the very heavy colour cast affect the AF? Everything apart from the white lines was pure blue.

Thanks

Mike

You have a point about colour. Colour affects metering and may also affect AF. Then again, not all lenses focus all colurs of light at the same point.

If your pictures look sharp then I wouldn't worry too much. If you feel the need for further investigation then maybe a black and white target would be better.

Thanks John. I would guess that they wouldn't optimise for pure blue focussing but rather skin or green. As you say Good enough is good enough and I am resisting adjusting the K10!

I seem to have read in a similar thread here, that measurements shouldn't be done under tungsten lighting (hence blue-ish)
Don't ask me which thread
So you both have a point here, I'll reckon.

Is tungsten a continuous light or does it have flicker of sorts - I think it's continuous.

I'll try the B&W chart in good light. I took shots of my wife, one with the 70 at 2.4 and one with the 30 Sigma at 1.4 in sensible light and focussing on the bridge of her glasses. Both were off, the 70 barely but the 30 was quite pronounced. So that's a failure in a real world test. The 70 was usable and the 30 not. What I'm thinking is that I could adjust the K10 a little to get the 70 spot on and see if that draws the 30 into acceptable. Failing that I'll wait until I can get a K20 or K7.

Mike

The wife test isn't the best as people tend to move. The 30mm at f1.4 would have very little leeway, so it's not really conclusive.

Tripod, daylight and static subject is my suggestion. Dominoes in a staggered line would be a possibility.

Egad sir are you suggetsing my wife is unstable?

I agree and understand John but this simply reinforces other evidence. Today I have the time to get the charts out.

Mike

I thought tungsten was orange?

It's of no real relevance but I have adjusted the K10D using the debug mode and all my lenses now seem t focus better apart from one. I was fairly analytical taking focus chart at 45 degrees etc. Adjust then retake. Then I used on real life things and it all seemed good.

Today's shots seem OK too. But I have to sacrifice my Tamron 17-50 which is a pity.

Interestingly all the Pentax lenses seemed to be out by a similar amount.

Mike

The K-7 has a solution to this problem, it analyse the light that hits the AF sensor and adjust it so the camera should focus just as fine under tungsten as under day-light.

Maybe worth an upgrade Mike?

Offering to help me raise the funds Stefan? I would like a K7 from all that I hear it is smaller and has some features I'd like but the price will have to drop. The K10 is an excellent piece of equipment and I don't need anything more a apart from individual lens adjustments.

Mike