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-   -   Strange AF focus behaviour (

rhermans Jun 7, 2008 7:44 AM


a few months ago I noticed that the af worked better around f7 than other settings, so 2 months ago I started printing focus charts and tried to figure out what was going on.

Now I'm quite sure about is.
using the K10d + Sigma 100-300 at 300mm

AF - F4 - serious backfocus
AF - F7.1 - spot on
turn AF off, set to F4 - spot on

MF - F4 - (focus till I get confirmation of focus) - serious backfocus
MF - F7.1 - (idem) - spot on
set to F4 - spot on

(didn't try this only with focus charts)

I'm not able to see the difference in the viewfinder between the focus at f4 and f7.1, my eyes aren't that good.

The same problem happens with my other lenses but is most pronounced with this one. Because the others have this to, it's a camera problem and I should sent the cam in - but haven't done this (and propably won't before fall).

I've searched the web for something alike but didn't find anything.

The only thing that I don't understand is that there seems to be a relation between the f/b - af and the apperture.

Just currious what you all think about this,


bluwing Jun 7, 2008 8:10 AM

Hi Ronny,

It is a strange one for sure.. Just a wild thought is that the Focus circuit baord might have a marginal component. Just a thought..

How bad is it with Pentax lenses? Do you have any 300mm lenses to check it out ?


bluwing Jun 7, 2008 8:15 AM


Another thought would be to try Manual focusing to where you just see the Confirmation light come on. And then try it focusing to where the Light goes off.

This would give you an Idea of how far the confirmation light is and if you need to send it in for service. But I would not wait until fall


Wingman Jun 7, 2008 8:17 AM

Ronny, the following could be at play:

1. When you use the auto focus, it may be focusing on something other than your target (depends on where the focus sensor is pointing). On Manual focus you could be focusing on exactly what you want. I found this to be the issue when I was testing my K20D earlier this week. I resent my focus sensor from automatic to center focus and thereby solved that issue.

2. At F/4 the DoF is much shallower; therefore at F/7.1, the greater DoF could be making up for the back focus problem.

Just my 2 cents!


rhermans Jun 7, 2008 8:40 AM



sorry to say that I don't have any Pentax 300's, the only Pentax I have is a 50mm, but it has the same problem, less pronounced because of the larger dof, but still has it. I have to send the camera in, but because this being my only body find it very hard to do. :roll:


I'm using center focus, and the thing is that when focused with f7.1, and then changed to f4 (without refocussing) the object stays in focus, the other way round it doesn't even get into focus. The strange thing is that when using a focuschart (printed one a bit larger A3 size) is that when focusing on the line you seen the backfocus at f4 slowly getting into focus at f7 and then going more towards a frontfocus above F11 but because of the large dof it keeps the focus.

mtngal Jun 7, 2008 3:36 PM

That's interesting. I did notice more backfocusing with larger apertures with my old K10, but just assumed that had more to do with the DOF, rather than a change in focusing behavior. But mine would "beep" at the correct focus when I was manually focusing, which (if I understood your post correctly) is not what's happening to your camera.

It does sound like a trip to the repair shop. The good thing is that it sounds like they'll replace the AF mechanism and your camera will be better than ever when you get it back.

Or you could always just do what I did - spend a whole lot of extra money and buy a K20.

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