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07-05-2013, 07:36 PM   #1
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Front Back Focus Issues?

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.

--------

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...




07-05-2013, 07:52 PM   #2
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QuoteOriginally posted by alamo5000 Quote
What causes these issues with lenses? Is it a manufacturing defect? Is it a calibration issue?
As I understand it both lens and camera are built to be within a certain specification. Not a precise specification to 100 decimals, just within a range determined to be 'good enough'. If both the camera and the lens are close to the center of that range then things work well. But in some cases the lens will be close to one side of the range and the camera close to the other, making the difference noticeable. Which is why camera makers have added AF fine tuning to some of the bodies.

Here is an article with lots of good information: Fixing Front and Back Focus - Introduction - PentaxForums.com
07-05-2013, 08:38 PM   #3
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Thanks jatrax! I read through the entire thing... wonderful information. I guess the distance to subject question is just simply relative to what you want to have happen and how you shoot and with what lens.

I think it's not a complex solution at all, but it would be nice if someone in the know can tell me (or us) if these issues go out exponentially as you get farther away etc etc. For example if you are a smidgin off at three feet, would this equate to being even more off at farther distances? That said the farther away it seems like it would be more forgiving because your in focus areas tend to get wider at the same apertures at greater distances to the subject.

I guess that information is more 'nice to know' stuff, but not essential... I didn't expect to get such a well written answer in such a short time LOL!
07-05-2013, 08:55 PM   #4
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Follow up:

I quickly went and did a test with my lens. I got a sturdy tape measure and propped it up at an angle like those charts...I did the whole test and I was off quite a bit. The tape had inches and centimeters on it and I was still off in my focus even at -3 as I mentioned above. I gradually increased until there was a 'centered' focus...but it was at the max on the body...-10...

I guess if that's all it takes that's all it takes. -10 or not...at that setting it was perfectly centered. I guess I might not need to send my lens off...

If I DO ultimately end up sending it off who can calibrate the lens for me? Where would I send it? And about how much would something like that cost? It's a sweet little lens and now that I see what it's capable of, I like it even MORE...

07-05-2013, 09:30 PM   #5
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QuoteOriginally posted by alamo5000 Quote
I guess if that's all it takes that's all it takes. -10 or not...at that setting it was perfectly centered.
If it's good at -10 then that's all that matters.

One point I neglected to mention is that the maximum aperture makes a big difference. So using your f/1.7 lens you can tell the AF is off. But using the kit lens for example you might never notice if it was off because of the greater depth of focus inherent in the slower lens. Maybe people use their camera quite happily until they get a 'fast' lens and then all of sudden things are back/front focusing. Not true, it's just that now you can see it.

I believe the only place in the US that does that work is CRIS Camera Repair by CRIS Camera Services: Digital Camera Repair: Canon, Fuji, Nikon, Pentax, Ricoh, Sigma they are the service center for Pentax. But seriously I think you are good, you can over think this stuff too much.
07-05-2013, 09:39 PM   #6
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All I know is so far based on initial tests the lens is working SUBSTANTIALLY better. Before wide open it was always just a little blurry which I wrote off as 'being soft wide open'.... yet at the same time so many people rave about how nice my particular lens model is...

NOW after I put in those adjustments I am very inclined to jump on the bandwagon and sing the praises of the lens I have

I simply just didn't know any better...I guess on one front I am experiencing the thrill of discovery...and on the other I am seeing exactly how much I've been missing out due to my own ignorance. I am excited now to know that a whole new realm of possibilities has opened up for me

I think if -10 fixes it, then no need to worry... but I will keep on doing a few tests (in better light for one) just to make sure I am on the right track...I want to test it under better lighting and make sure all the variables are good...

Dang! Just knowing now is kinda cool.
07-05-2013, 10:14 PM - 1 Like   #7
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QuoteOriginally posted by alamo5000 Quote
but I will keep on doing a few tests (in better light for one) just to make sure I am on the right track.
Just remember to do multiple tests, as the margin for error can be larger than the actual focus error unless the test is very carefully performed. AF fine tuning is a wonderful thing but without proper care it is easy to make things worse, not better. You sound like you are on the right track though.

I wonder how many of the reviews in the review section that say "soft wide open" are just because the lens was not fine tuned. I always run a new lens through a set of tests and calibrate the AF for it as soon as I get it.
07-05-2013, 10:23 PM   #8
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QuoteOriginally posted by jatrax Quote
I wonder how many of the reviews in the review section that say "soft wide open" are just because the lens was not fine tuned.
Exactly!


QuoteOriginally posted by jatrax Quote
I always run a new lens through a set of tests and calibrate the AF for it as soon as I get it.
I didn't even know you could calibrate one until just a day or so ago... I have all DA Limited lenses that I use (except for the 50mm)... They all seem extremely sharp even wide open... I haven't had any problems with them... but at least now I know how to check...I don't know that I need to at present though. All seem to perform up to my expectations...

My other 50mm is a manual focus f1.4... it's pretty darn good in my opinion...it's not a Pentax brand... I just don't manual focus that much for the most part. I like it though... I will use it from time to time depending on what I am doing...

As for the F50 I am completely impressed... WOW.

I have just experienced the thrill of discovery!


Last edited by alamo5000; 07-05-2013 at 10:31 PM.
07-06-2013, 12:42 AM   #9
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QuoteOriginally posted by alamo5000 Quote
I got a sturdy tape measure and propped it up at an angle like those charts...
I hope you had an unambiguous target in addition to the tape measure.

You cannot perform AF adjustments with an angled tape measure alone.

You may find my AF adjustment hints useful.
07-06-2013, 03:11 AM   #10
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af adjustment might be perfect at one distance but differ at other or infinity and than come zoom lenses with different adjustments based on focal lengths. there is no one prefect setting
07-06-2013, 06:28 AM   #11
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Glad you found how to adjust your lens. For me it is kind of a slippery slope since now I have to have all my lenses adjusted to their optimum. I firmly believe in doing this in a controlled fashion, so I ended up buying Focus Tune. This gave me the most consistency in testing to get my lenses dialed in perfectly.
07-07-2013, 10:14 PM   #12
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A follow up question....I use a lot of manual focus lenses, but at times i do rely on the focus confirmation function (beep) on my camera. I have noticed at times though my focus will be off. Most clearly, a recent set of pics with my Vivitar 200/3.5 of some wildlife showed clear front focusing, as the foliage a couple feet in front were sharp as a tack, while the deer i was shooting at were a bit blurry when i looked at the photos in LR. So i guess i have two questions. First, can the focus confirmation function also be subject to front/back focus issues (and is that also correctable)? My second question is if i do the adjustment as mentioned above, will the camera automatically apply the setting i input when i put on a particular lens, or do i have to remember the setting for each lens and apply it manually each time i use that lens? Thank you.
07-07-2013, 10:41 PM - 1 Like   #13
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QuoteOriginally posted by toukan Quote
First, can the focus confirmation function also be subject to front/back focus issues (and is that also correctable)?
Yes, sort of. The focus confirmation is not absolutely precise especially for fast lenses and is more of a range than a precise spot. That said, yes it can be adjusted, whether it will help or not is another question. Try focusing front to back and then back to front, sometimes a lens will be in focus at one edge or another of the 'focus confirmation' range.

QuoteOriginally posted by toukan Quote
My second question is if i do the adjustment as mentioned above, will the camera automatically apply the setting i input when i put on a particular lens,
On the k-5 the camera will remember a number of different lenses (20 if I remember right) but only for lenses with an ID chip. So manual lenses, in general, no. You could do a global adjustment but that would affect all lenses.

I would recommend not jumping in to adjusting the focus fine tuning without a lot of carefully measured testing. Taking a shot of a deer in the woods that looks like it front focused is NOT a precise test. Set up on a tripod, with a well lit fixed target and run multiple tests, 10 plus trials and see if the results are consistent. AF fine tuning is a wonderful thing but it is also an easy way to mess things up quickly. Not something you should casually do without a lot of testing.
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