Large back focus on a new lens

adwb · 11-02-2011, 09:24 AM

I bought a new Pentax smc DA 35mm F2.4 AL Lens to put on my K-r to use as a walk around lens as the combination is small and light.
When the lens arrived in the post I grabbed the nearest body and tried it out.

This was my K5 and it was immediately obvious in the LCD replay there was a huge focus point issue. when zoomed in a massive back focus resulting in a full 10 adjustment to the right to get the image in focus correctly.

O bother [ or words like that I said] and decided to try it on the KR anyway.
To my surprise it was spot on and trying to adjust the focus point made it worse.

So I started testing my other lenses on the k5 and with the exception of a Sigma 24-60 which has a slight 2 points to the right back focus all the other are fine.

How can a lens be so far out on one body but not another? Given the results of the other lenses it is not the K5 body either.

I tried the lens on A k10d and it is pretty close to being correct there as well.

Thing is, do I send it back? After all it is fine on the K-r which is what I got it for?

Thoughts / explanations any one??

GDRoth · 11-02-2011, 09:37 AM

Sorry for the dumb question, but how do you go about making these tests.......is it simply a visual (eye ball) test or some more structured way.....is there an explaination anywhere on the web that might instruct one on how to do this?

adwb · 11-02-2011, 09:56 AM

in this case the problem was very obvious as Ii have two hanging baskets with flowers and by chance when I took the initial shots they were behind one another since I aimed the red centre dot at the flowers in the front edge of the front basket and they were blurred but the back basket was sharp it was obvious there was a problem. I used that rough test to check out the lens on the KR and K10 at at the same ISO/ speed and at f8 .
once I found the problem as above I used a test sheet from Tim Jackson which I use to see what is the best aperture for a lens to sort out the adjustments.

11-02-2011, 09:24 AM	#1
adwb Veteran Member Join Date: Sep 2009 Location: Bristol UK Photos: Gallery Posts: 1,636	Large back focus on a new lens I bought a new Pentax smc DA 35mm F2.4 AL Lens to put on my K-r to use as a walk around lens as the combination is small and light. When the lens arrived in the post I grabbed the nearest body and tried it out. This was my K5 and it was immediately obvious in the LCD replay there was a huge focus point issue. when zoomed in a massive back focus resulting in a full 10 adjustment to the right to get the image in focus correctly. O bother [ or words like that I said] and decided to try it on the KR anyway. To my surprise it was spot on and trying to adjust the focus point made it worse. So I started testing my other lenses on the k5 and with the exception of a Sigma 24-60 which has a slight 2 points to the right back focus all the other are fine. How can a lens be so far out on one body but not another? Given the results of the other lenses it is not the K5 body either. I tried the lens on A k10d and it is pretty close to being correct there as well. Thing is, do I send it back? After all it is fine on the K-r which is what I got it for? Thoughts / explanations any one??

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11-02-2011, 09:37 AM	#2
GDRoth Veteran Member Join Date: Sep 2010 Location: S.E. Michigan, USA Photos: Gallery \| Albums Posts: 830	Sorry for the dumb question, but how do you go about making these tests.......is it simply a visual (eye ball) test or some more structured way.....is there an explaination anywhere on the web that might instruct one on how to do this?

11-02-2011, 09:56 AM	#3
adwb Veteran Member Join Date: Sep 2009 Location: Bristol UK Photos: Gallery Posts: 1,636 Original Poster	in this case the problem was very obvious as Ii have two hanging baskets with flowers and by chance when I took the initial shots they were behind one another since I aimed the red centre dot at the flowers in the front edge of the front basket and they were blurred but the back basket was sharp it was obvious there was a problem. I used that rough test to check out the lens on the KR and K10 at at the same ISO/ speed and at f8 . once I found the problem as above I used a test sheet from Tim Jackson which I use to see what is the best aperture for a lens to sort out the adjustments.