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10-21-2014, 06:41 AM   #1
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Focus issues on Pentax K30

Greetings!

I've been a forum member for quite some time, but actually have lately forgotten to use its resources!

I've been a Pentax owner since 2009, first having a K200, then a Kr and now a K30. Each time I upgraded because I was frustrated with focus issues. FIVE times I have had to send in my camera for repair and each and every time it was an Auto Focus System Repaired issue. It was once for the K200, three times [in two years] for the Kr and my K30 was in last summer and I am beginning to notice issues again.
I am not a beginner photographer, having been taking photos for over 40 years, but I have never really been technically knowledgeable about settings and how they work. However, I don't buy professional level cameras and I didn't think it was rocket science to get a clear, sharp image from an entry level dSLR.

I mainly use my 35mm 2.4 and 50-135 2.8 lenses. I usually shoot in Av mode.
At this time, I can not afford a Spyder-type calibration tool, so I set up a shot on my dining room table, camera on tripod. For each shot I focused on the 6" mark on the ruler. I did not focus and re-compose, just shot as is from the tripod.

Here are the results. Any advice, opinion or help would be appreciated.

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10-21-2014, 07:49 AM   #2
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If you're focus testing (a) you need to be about 10x the focal length of the lens away from your target, (b) wide open aperture (i.e.f2.4 for your 35mm), and (c) in day-light (assuming it's the most common light source for your photography.

All of that said, it appears from your first shot that you have back focussing with the 35mm lens.
10-21-2014, 07:59 AM   #3
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Thanks, JohnX. So, 10x the focal length..... 35mm x 10?
10-21-2014, 08:34 AM   #4
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You have the best looking focus calibration setup I have seen so far

10-21-2014, 08:55 AM   #5
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QuoteOriginally posted by Panache Quote
Here are the results. Any advice, opinion or help would be appreciated.
  • The AF system on your K-30 is not able to consistently discern the intended point of focus in your ramp set up. Neither will the AF system on any camera on the market.
  • Do your testing against a flat, high contrast target of sufficient size to provide an unambiguous surface to focus on. If you want a ramp to evaluate front/back focus, place it along side the real target.


Steve

Last edited by photolady95; 10-21-2014 at 10:17 AM.
10-21-2014, 08:59 AM   #6
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I have a K-30 and the 35mm 2.4 also. I would say your results look about the same as mine when I do test shots for focus. Maybe it's my copy or technique, but I've never been thrilled with the sharpness of this lens for detail, even in manual focus. I don't seem to have problems with sharpness on my other autofocus lenses, FA 50 1.4, 18-135wr.
10-21-2014, 09:48 AM   #7
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Dou you have manually selected the center focusing point ? If not, it means you let the camera choose for you and there's no way the camera will know where you want to focus. In auto mode, all the pictures above could be fine depending on which of the K-30 11-AF points the camera locked on...

As Stevebrot mentioned above, it would be much better to have a flat focusing target. Then, you'll exactly know where the camera has focused. In your setup, even if the center focusing point is selected, the camera could still have chosen to focus on the 5, the 7, or the apples on either side of the 6...

For a quick check, I like tto do this simple "battery test". It's easy to do and work well enough for checking a camera. If the center battery is sharp, either your camera has no focusing issue or it's not important enough that you should care about. Since this means that under normal shooting condition your focus point will be well inside your focusing plane.
10-21-2014, 10:23 AM   #8
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Thanks for the advice so far.
I do have my focus set to Centre.
I did a quick print-up of this adjusting chart and started over. I'm still not sure what distance to place the camera. I adjusted for back-focus issues at +2, +4, +6, +8 and +10 on the AF Fine Adjustment setting.
Am I getting closer to figuring this out?

Please, remember that I need layman's terms to understand some of the technical replies. Let's just say I'm a complete newbie where camera settings and lenses are concerned.

Thanks so much for helping me try to figure this out. My warranty is up in less than a month. If I can do troubleshooting on my own to save some money, I'm all for it.

As before, taken with self-timer on tripod.

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10-21-2014, 10:30 AM   #9
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QuoteOriginally posted by CarlJF Quote
the camera could still have chosen to focus on the 5, the 7, or the apples on either side of the 6...
Exactly, though the situation is even dumber than that. The AF system "chooses" nothing. It merely signals that focus is attained when the phase peak for the AF sensor maximizes. The distance at which that happens is fully dependent on the subject and independent of the photographer's intent.

FWIW and despite my advice regarding a flat surface, I have found that shooting my aerosol "canned air" can (lots of black-on-white fine print on the back) at about 10x the focal length to be useful and entertaining. The parallel sides make it easy to frame to the center point and the curved surface make it easy to see back-focus. Front focus is detected by nothing being sharp.


Steve

---------- Post added 10-21-14 at 10:41 AM ----------

QuoteOriginally posted by Panache Quote
I'm still not sure what distance to place the camera.
You want to be close enough so that there is a reasonable amount of focus throw. If you are too far away, you are challenging the mechanical precision of the system. There is no sense in doing that since it makes it almost impossible to calibrate for accuracy. A good rule of thumb would be 10x the focal length if the lens is capable of focusing that close. Do your calibration in natural light.

For your zooms, you might want to check the calibration at more than one focal length.


Steve

Last edited by stevebrot; 10-21-2014 at 10:48 AM.
10-21-2014, 10:50 AM   #10
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@stevebrot, what exactly is the 20x focal length? I've got a can of air. Gonna try that.
Even though I'm adjusting at wide open, do I have to go through any of the calibration tests at other apertures?
10-21-2014, 12:22 PM   #11
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QuoteOriginally posted by Panache Quote
I did a quick print-up of this adjusting chart and started over. I'm still not sure what distance to place the camera. I adjusted for back-focus issues at +2, +4, +6, +8 and +10 on the AF Fine Adjustment setting.
Am I getting closer to figuring this out?
Have you made a test without adjustment ? It may be just fine...

QuoteQuote:
For your zooms, you might want to check the calibration at more than one focal length.
And I will add that it may be tricky with a zoom lens, if not impossible, to find a perfect fine adjustment that will work with all focal lengths and all focusing distances. Unless the lens/body is grossly front/back focusing consistently under all conditions, it may be a pain to try to pinpoint a fine focus adjustment. For example, I have a zoom that shows a slight front focus when used at his maximal focal length and shortest focusing distance. If I'm doing a fine adjustment for this, it will then shows back focus at all other focal lengths or distances... So, instead of trying to find an optimal fine adjustment, I'm just stepping down the lens to increase the DOF or switch to MF when I'm shooting in this specific condition.

So, if you're finding out that your zoom is working acceptably well for all focal length -the focus point being somewhere in the focal plane but not necessarily always in the middle of it- it's probably better not tying to adjust it. It may end up doing more wrong than good. Once you know the conditions giving a tricky AF with a specific lens, it's relatively easy to circumvent the issue by either stopping down the lens, correcting the focus with Quickshift, switching to MF or simply using LV instead of the VF.

Last edited by CarlJF; 10-21-2014 at 12:34 PM.
10-21-2014, 12:50 PM   #12
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QuoteOriginally posted by CarlJF Quote
Have you made a test without adjustment ? It may be just fine..........Once you know the conditions giving a tricky AF with a specific lens, it's relatively easy to circumvent the issue by either stopping down the lens, correcting the focus with Quickshift, switching to MF or simply using LV instead of the VF.
Yes, I have made tests without adjustment. Last week, I took a few photos of a couple/friends I know. That's when I noticed that something was off.
Normally my subjects are landscapes [where focus point isn't often an issue] or my grandchildren, where I use Auto a lot!

But...not sure I understand your "relatively easy" solutions. I'm not sure my eyes are trustworthy enough for using MF. I rarely use LV, so will give that a try too. I have no idea what Quickshift is so will look it up in my manual.

Thanks
10-21-2014, 02:03 PM   #13
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QuoteOriginally posted by Panache Quote
what exactly is the 20x focal length?
For 35mm focal length: 35mm X 20 = 700mm (70 cm)

I suggest testing focus at 10X the focal length if the lens will focus that close.


Steve
10-21-2014, 02:14 PM   #14
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I think the second last one looks best in that set, though it's hard to tell. Also the test should be made under neutral lighting if you really want to be picky to avaid any risk of color temperature throwing it off.
10-21-2014, 05:24 PM   #15
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QuoteOriginally posted by Panache Quote
Yes, I have made tests without adjustment. Last week, I took a few photos of a couple/friends I know. That's when I noticed that something was off.
Yes, but I mean with the same setup that you have above with the +2, +4, +6 +8 adjustements. It would be useful to have a reference shot with 0.

QuoteOriginally posted by Panache Quote
But...not sure I understand your "relatively easy" solutions. I'm not sure my eyes are trustworthy enough for using MF. I rarely use LV, so will give that a try too. I have no idea what Quickshift is so will look it up in my manual.
LV is also quite useful to check for front/back focus issue. You take a picture of your setup, and you get the reference image of what it should look like. If the AF from the VF looks the same, you don't have a focusing issue.

Quickshift is only a feature of many Pentax lenses. When AF is done, Quickshift allows you to manually fine tune the focus from the focusing ring without having to switch to MF.

For children, although I'm also not a big fan of LV, I must admit that LV set on face detection usually works quite well.
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