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The first comment I can make is that the 24Mpx APS-C sensors used in recent model Pentax dSLRs have raised the bar in terms of technique. When the K-3 (my camera) first hit the market, complaints came in almost daily to this site from people unable to attain acceptably sharp photos even using the same lenses and techniques that worked quite well at 16Mpx or lower. The higher pixel density made obvious poor optical performance as well as lapses in technique. The standard answers were to be sure of focus, be attentive to steady hold when off-tripod and quality of support when on-tripod, and to not assign too much weight to the view when pixel peeping at 100%.
That being said, let's lead with one question and then consider what you have shared.
What lens does this happen with and at what focal lengths and shooting distances? Originally posted by Jnm I think about 20% are tack on, the rest slightly off perfect focus if zoomed.
By zoomed, do you mean 100% (~8X on replay view)? If anything higher than this, one can expect less than the actual sharpness. Even when evaluating at 100%, consistent sharpness when pixel-peeping, shot-to-shot, may be difficult to attain even with the best lenses and best focus systems. 20% less-than-sterling results may not be a cause for alarm. Without example images, it is hard to say.
Originally posted by Jnm I always use afs, back button focus, spot focus, prefer aperture priority usually 8 or 11.
Are you set up for AF-S,
shutter priority? By back button, have you changed configuration to allow shutter release with no focus attempt (AF button set to AF2)? If either, there is a higher chance of missed focus unless care is taken to confirm the green hexagon in the viewfinder before releasing the shutter. Back button focus requires quick reflexes for the capture, particularly for moving subjects.
As for aperture used...when "zoomed" to 100%, the normal advantage for DOF at f/8 or f/11 no longer applies. At 100% one is evaluating whether the subject was at the plane of focus. Anything less will be less sharp.
Be aware that even with center spot AF, the area being evaluated may be larger than one thinks and the camera may signal focus for something other than the intended subject or portion of the subject.
Originally posted by Jnm If low light use tripod, Manual mode, live view, 2 sec timer, often high iso as well. I have even resorted to mirror block to avoid vibrations but no matter what I do the focus is off.
In low light, there are too many variables to address here without example images. I'm not sure that live view is a good idea unless you are using electronic shutter. If using AF, I have found the AF assist light to work very well and the PDAF system (optical viewfinder) will offer better performance than the contrast detect system in live view.
Manual focus in live view in dim light is a mixed bag, even if magnified, due to graininess (sensor noise).
Originally posted by Jnm It happens even in bright light situations, although higher percentage are good. It doesn't focus on the wrong area like background for instance, it just doesn't focus perfectly.
A common suggestion on this site is to try AF fine adjustment. Some members here will go as far as stating that fine adjustment should be done for
all lenses to assure adequate performance. I fall somewhat shy of that advice and suggest that unless one has firm evidence that the camera's best efforts are consistently behind (back focus) or in front of (front focus) the intended point of focus, there is little to be gained by doing the procedure.
Doing a fine tune and doing it right is a lot of work and it is good to rule out other causes before going down that road.
The suggestion above regarding example images and full-resolution (100%) crop to show blur detail would be very helpful. If you need advice on how to do a full-resolution crop, just ask and we will supply detailed instructions.
Originally posted by Jnm Manual focus is not an option because my eyesight is simply not acute enough to identify perfect focus.
Sorry to read this. Your challenge is shared by several other members on this site and you are in good company.
Steve