A while back I bought a Pentax F50 f1.7 that is mounted on my K-5. I honestly have kind of sat on the lens a while and didn't really try to figure the thing out...
Yes, I did take some shots with it here or there... but when I opened it up all the way my photos seemed a bit 'soft'...I chalked it up to, well yet another lens being soft wide open...
Then I got to playing around the other evening with the lens and I almost by accident think I discovered the lens has front/back focus issues. I went to the Custom Menu #26 and under "apply one" I did a "-3"...
Then, low and behold, holy smokes batman! That lens is one sharp little beast! For all this time my shots were technically out of the in focus area...then I took some more shots at a family reunion and again I was blown away by a few of them. (not because I am a spectacular shooter, but because the lens was sharp!)
I was walking around with the lens wide open all day...and I think I stumbled upon something merely by accident and a feature of my camera body that I didn't even know existed...(read your manual people! LOL!) I guess I have man-itis LOL!
Now that that is out there... I think my lens was focusing on a point on the far side of the in focus area...for example if you did single point focus and put the point on someone's eye, the tip of their nose would be more in focus, but the rest of their face slightly blurry...but when I put in the "-3" value my focus point was more inside the in focus area, so if you focus on someone's eye their face was generally enveloped in the in focus area as well. By moving that little focal point around it made WORLDS of difference.
Now for the questions.
1. What causes these issues with lenses? Is it a manufacturing defect? Is it a calibration issue?
2. I described what I did to fix said problem above, but I did it sort of on the fly and by hand holding a few shots.
What other tests should I use to see if the right value is put in there? Is there a specific set of tests I should do? The manual says to use a tripod...which I didn't do (yet)... but what distance is optimal for fine tuning the focus point? 3 feet to the subject? 5 feet? 10 feet?
3. Each of those "-1" points moves the focus approximately how much? Knowing this might help me determine exactly where it needs to be. Just based on my random experiments somewhere between -3 and -4 are 'probably' in the right ballpark...but I want to find out for sure by doing some controlled testing.
Any other tips or useful information will be extremely helpful so please bring me some knowledge folks! Share away...and thank you all for being so awesome. I love visiting this board...I learn a lot here.
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The following shot was taken wide open with the F50 @ f1.7... (I cropped some and didn't really do much PP, but you get the idea...) This shot is substantially better than what I have previously been getting with this lens...