Hello everyone,
I realise there are plenty of threads about this already but I hope you can bear with me on my lengthy thread anyway, it will be much appreciated
It's not long ago I bought my first Pentax lens, the DA* 50-135 as you might have guessed. Was excited to get it since it go t good reviews, however, right from the box it did seem to back focus a lot in the beginning. Went out to put it to the test with a mountain bike race and much to my dismay, it could not even focus on the well lit side of a stationary tree, it constantly threw focus in the background, zoomed in or not...
I had a protective filter on which I took off hoping that it was the culprit, sadly it was not, so in a frantic panic attack, I went into AF fine adjustment and put the lens to +10, suddenly the pictures seemed to be correct in focus! I thought to myself that it was a bit high in compensation but whatever it takes, right? Attended a few races more, the occasional back focus picture did appear every now and then, still the lens seemed to be working, the back focus could after all be due to riders coming towards me and me not pressing the button just fast enough for the rider to still be within the focus area.
Then the other day the lens appeared to go haywire with consistent back focusing at +10 AF adjustment, even with a flash on, broke my heart a little. Behaved that way through out the entire day, relentlessly, shade and direct sunlight alike.
Today when I got up, I saw a crow down on the lawn from my apartment. Took several shots and again focus was not right, however this time it was front focusing, ended up with a +5 on the lens in order to have focus in the middle of the focus point, at 135mm f/2.8.
Later, after I got home from exercising, I thought for the hell of it I'd try taking a picture of the wooden model sitting on the window sill and again back focusing reared it's ugly head, as I could see the dirt on the window a lot more sharply than the model I focused on. Several tries at 135mm and 50mm with f/2.8.
My initial take on it is that focus seem to lay very differently depending on light amount and the light source type, like sunlight vs artificial light.
Is this normal behaviour for a DA* lens at f/2.8?
I keep comparing it to my Sigma 70-200 HSM APO EX and it doesn't seem to suffer from the same focusing issues, I don't use AF fine adjustment with it.
As I'm writing this I realise I should go back and check the aperture of all the good race pictures I took previously to see if they were generally higher than f/2.8, sorry for not having done this already, still I'm think focus should be more consistent even at f/2.8.
I have already had my K-5 body replaced once due to focusing issues so but I'm sure it can't be the body once again.
Any thoughts?