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03-12-2018, 04:12 PM   #1
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Can anyone help me calibrate my lenses with the KP

I am completely lost on how to calibrate my lenses. Page 63 of the manual explains it, but I am not sure I understand how to adjust my lenses. It says "adjust the value", but in relationship to what exactly? I just want the darn pictures to be in focus.

Can a KP owner shed any light on this?

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03-12-2018, 04:22 PM   #2
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You need to set your camera on a tripod and focus wide open on a test chart. Once you have established if you have a FF/BF issue you can then adjust either per lens, or globally for all lenses. I suggest you use the former.

edit... yes no need to buy a testing device. Print out the chart from the end of this article http://www.kscameraclub.org/docs/pdfs/focus_test_chart_edited.pdf

Use natural light if possible and widest possible aperture. Centre AF point only. Tripod/SR off. Do 10 tests for each setting, making the lens focus from either infinity or closest focus distance each time. Go with the majority result. Don't expect 10/10 to be correct.

Last edited by pschlute; 03-12-2018 at 04:29 PM.
03-12-2018, 04:23 PM   #3
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If you search there are plenty of other threads that go into detail in methods for how to perfectly fine tune your lens, but basically it's because light travels two different paths to get to the sensor and the AF module. If there is a slight misalignment the paths will be different lengths and cause focus error. The fine adjust setting allows you to input an adjustment factor to correct this, either per lens or for all lenses. If your camera and lens are perfect examples you would just leave it at zero, but most tend to be just slightly off, and at 24MP it's easier to notice missed focus. Positive adjustment brings the plane of focus towards the camera, negative adjusts the plane is focus farther away.

Here's a good place to get started learning how to adjust your lenses
https://www.pentaxforums.com/reviews/lens-align-front-back-focus/the-remedy.html

If you don't want to spend money on a focus chart you can try building your own. There are other methods too, so search around the forum a little if you're interested.

---------- Post added 03-12-18 at 06:30 PM ----------

Another thing to mention is you shouldn't base your decision on only one shot since there's always some variability. Take multiple pictures and un-focus between each one to give the AF system another chance.
03-12-2018, 04:37 PM   #4
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QuoteOriginally posted by TXPentaxK50 Quote
I am completely lost on how to calibrate my lenses. Page 63 of the manual explains it, but I am not sure I understand how to adjust my lenses. It says "adjust the value", but in relationship to what exactly? I just want the darn pictures to be in focus.

Can a KP owner shed any light on this?
Definitely read the guide (also linked above):

Fixing Front and Back Focus - Introduction - In-Depth Articles

If you don't have a proper test chart and don't want to get one, there are some good printable ones online. Just be sure the test setup is well-controlled, as described in the article. Good luck!


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03-12-2018, 04:37 PM   #5
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So are most lenses slightly out of focus and they do in fact need adjusting?
03-12-2018, 04:46 PM   #6
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QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
Technically yes, but many are off so little you wouldn't know if you didn't look for it. Sometimes you have to pixel peep to see it too. It makes the most difference with wide aperture lenses that have narrow depth of focus, or with very large prints.

If you'd never heard of fine adjustment you might never even realize a lens is slightly off. If it isn't bothering you you don't need to adjust anything. Some of us just like to be as accurate as possible.

---------- Post added 03-12-18 at 06:48 PM ----------

Think of it this way... Do you need your ruler to be accurate to 1/64 inch or are you happy with 1/16 inch? Different strokes for different folks!
03-12-2018, 05:43 PM   #7
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QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
In film days we didn't notice unless we printed large. In early days of digital it was not so noticeable. Now we are using 24 or 36MP sensors on high resolution screens. Manufacturing tolerances for cameras and lenses have no doubt improved over the years but not enough to meet the resolving ability of our kit.

Far better that a camera can be fine tuned with different lenses to give perfect focus than not to have that option at all.

it is the combination of lens/camera that is the key factor here. A lens may be manufactured to perfect tolerance but can still back focus on one camera and front focus on another.
03-12-2018, 06:34 PM   #8
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QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
Many lenses may benefit from AF fine adjustment*, but it is not the lens that is out of failing per se and it is not the lens that is being adjusted. The same lens may be fully capable of "bang-on" manual focus or CDAF focus in live view. The full answer is fairly long and complex, but the short answer is that the PDAF system accepts an ambiguous signal as being not-out-of-focus when it still is. The AF fine adjust applies a numeric bias such that that the PDAF system tells the focus motor to stop a little bit either side of that ambiguous point.


Steve

* I hesitate you use the word "most" because that has not been my personal experience with the AF lenses on my shelf. Of the seven I own, only one has required a fine-adjust setting. If one is seeing consistently soft results across the board with all lenses in both live view and with the optical viewfinder, the problem may lie somewhere other than focus.


Last edited by stevebrot; 03-12-2018 at 06:42 PM.
03-12-2018, 06:58 PM   #9
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QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
I would say that unless you see a glaring problem with a lens, AF fine tuning is not required.

The first thing is that you have to be quite certain that any perceived focus problem are not caused by something else (eg bad camera technique, wrong AF settings, motion blur, bad UV filter etc).

And if you do decide to go down the path of AF fine adjustment, the procedure has to be done carefully, otherwise it could make any problem worse.
03-12-2018, 07:57 PM   #10
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I focused-adjusted a Sigma 30 using a crack in the sidewalk that has some grass in it. Took a rough sequence of settings, then a finer one. Took only a few minutes. This, of course, was after shooting a whole family get-together that was consistently back-focused...
03-12-2018, 08:05 PM   #11
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No, the lenses are fine.

QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
There is a variability between camera mounts and lenses. A lens that looks spot on perfect on my camera my be slightly off on yours, and vice-versa. The differences are measured in hundredths of an inch. I bought FocusTune and my keeper percentage went way up. The differences are subtle, and not too noticeable if you don’t pixel peep, but what fun is spending thousands of dollars on camera gear if you don’t pixel peep and say “WOW” once in a while!
03-12-2018, 10:09 PM   #12
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I've had to fiddle with a couple of my lenses, only the faster ones as I couldn't really see any problems with lenses of 2.8 or slower (depth of field hid the problems).

I just used an advertising brochure which had tidy small text, a proper chart might work better, but that was good enough for me - When I used to rebuild film cameras i used to just use a page from a newspaper
03-12-2018, 11:57 PM   #13
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QuoteOriginally posted by TXPentaxK50 Quote
So are most lenses slightly out of focus and they do in fact need adjusting?
Through 4 different K-mount digital bodies and ~12-15 AF lenses, I have yet to access the AF fine-tune feature.

Just my experience so far. Then again, seems like a good way to kill a rainy day!
03-13-2018, 06:03 AM   #14
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Just to clarify, fine focus adjustment only applies to the phase detect focus module which is separate from the image sensor. It doesn't affect contrast detect focus, so it doesn't affect live view focusing. Live view focusing uses the image sensor, so if there is enough detail on the subject it should be accurate.

A quick test to see if you even need to adjust af for a lens is to take a few shots on a tripod with the viewfinder and then a few in Live View. (Use two second delay to avoid camera shake, and ISO 100 with the appropriate shutter speed so you get the clearest detail.) If they're basically the same you don't need to adjust anything. If the live view ones are obviously better you probably need fine AF adjustment.
03-13-2018, 10:28 AM   #15
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QuoteOriginally posted by TheOneAndOnlyJH Quote
A quick test to see if you even need to adjust af for a lens is to take a few shots on a tripod with the viewfinder and then a few in Live View. (Use two second delay to avoid camera shake, and ISO 100 with the appropriate shutter speed so you get the clearest detail.) If they're basically the same you don't need to adjust anything. If the live view ones are obviously better you probably need fine AF adjustment.
^ ^ ^ ^ This...

If one is strongly compelled to take a scientific approach to fine tuning (I sort of am), my intended route would be to get the LensAlign product FocusTune software. The protocol and tools used ensures proper camera alignment and accounts for statistical spread of results.* If one is particularly compelled and the lens accepts the Sigma Dock, those lenses support fine adjust of the lens itself (have no idea how that works) for multiple focus distances and (for zooms) focal lengths.** This is equivalent to sending lens and camera in for adjustment as a system.


Steve

* Much perceived front/back focusing is simply imprecision of the PDAF system. Even at best, it ain't that great. Proper assessment should be based on at least 10 focus attempts before concluding that there is a front/back bias.

** Yes, front/back bias may vary by distance and/or focal length due to the way that modern internal focus/zoom lenses do their magic.
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