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Forum: Pentax DSLR Discussion
10-30-2009, 08:03 AM
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Nope - that is not FF/BF issue.
I am sure about the focus point - I always use the center AF point for focusing. I repeated this for many more pictures with the same conclusion.
If I shot on apertures between 1.4 and 2.5 I get crisp and sharp pictures - the phenomenon start when I choose f/2.8 on the camera :]
chrisg had the same problem - I wonder where it is programmed to do that (lens or camera)
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Forum: Pentax DSLR Discussion
10-30-2009, 06:24 AM
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brecklundin,
Its not so easy to make the lens refocus - sometimes I point into object which are 0,5 - 1m distant from each other and the camera is not refocusing.
But you're right - seems that camera moved on those two. But this was just a clear example - I have the same consistent results no matter if I use tripod, ir pilot and 2s mirror lockup.
I can give you another set of pictures:
Tripod used, a wire to shot remotely and probably a 2s mirror lockup.
Both are f/3.5 - however the first one was shot with the focus locked after shoting @f/1.4 - I put f/1.4 on camera - made the shot - and then switched focus into manual and then changed f/1.4 into f/3.5.
The second was shot normally - I turned AF on and made the foto with a wire pilot. You can easily see that something is wrong.
It looks now that it is programmed to change focusing plane over f/2.8 towards back in close distance focusing, and to shift focus plane toward the front when focusing on further objects.
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Forum: Pentax DSLR Discussion
10-30-2009, 02:26 AM
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My lens is reporting a proper focal lenght. It is however reported by some software as "Sigma 70-200 2.8".
My lens has already been calibrated by the Sigma service. So either they are bold enough to send back a bad lens to me or ignorant not to know that the lens is inacceptable for normal use.
I see chrisg was having the same problem as me. However I still have another problem with the lens.
Not only it changes focus different way with different apertures (there's a shift between f/2.5 and f/2.8 - which I also dont know if it can be attributed to camera or lens), but also is can have front focus on close distances and be ok on far away shots (apertures below 2.8) or... the other way around - back focus at close distances at f/2.8 (or higher values) and front focus on far away shots.
What a piece of crap :) I am giving it back to Sigma as soon as I can.
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Forum: Pentax DSLR Discussion
10-29-2009, 04:55 AM
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Hi chrisg,
I have the same problem with my Sigma on K200D rig.
I made some tests on close and far distances.
If I shot at f/1.4 - f/2.5 I get sharp and crisp pictures,
however when changing the aperture on the camera to 2.8 or higher (value) makes the camera refocus the lens differently.
In case you dont understand what I mean...Put the camera on tripod, point it towards a wall or whatever's contrasty enought for af system. Then halfpress the button on f/1.4 - you'll get the lens to focus somewhere and lock the focus(no matter if its going to be a sharp picture or FF, or BF). Then change the aperture to f/1.7 or f/2 - halfpress the button - you should just get a bip without any refocusing - cause the object didt move. However...
If I change aperture to 2.8 (or higher f/3.5;f/4 etc.)on the camera and then halfpress the shutter realise button - it refocuses the lens like the object had moved !
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