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How to Fix Front Focus and Back Focus
Posted By: Adam, 12-04-2011, 08:00 PM

We've written an article detailing what front- and back-focus is, and now to fix it!


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12-05-2011, 10:47 AM   #2
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Thanks for the article. I know I've been needing to check my lenses for FF/BF for a while, but I've been busy buying every other new photographic gizmo under the sun, and a calibration target just hasn't been a priority, but they're actually less expensive than I thought they'd be.

I still have a couple questions, maybe someone here will know the answer:

1) If focus with a particular lens just seems a bit inconsistent (sometimes dead on, sometimes frustratingly just off), that's probably not indicative of FF/BF, but rather the AF module, right? The reason I ask is I can put my camera on a tripod, have the camera AF on a static subject, take 5 different shots, and get 5 different interpretations of critical focus. This happen even in good light with obvious straight lines. I never allow the camera to choose the focus point, but I don't always use the center point (because it's on a tripod).

2) Can a zoom lens exhibit BF at one end, and FF at the other? If so, can the built in fine tuning correct for that? Would this be enough to consider a lens defective?

3) I realize that wide apertures don't leave much room for error. Is the AF module theoretically more likely to detect that critical focus is off when using a large aperture, or does that not matter? What about if I'm using a K or M series lens with no A setting on the aperture ring.

4) The articles mentions that the K7 can store corrections for up to 20 lenses, does it remember the lens by serial number or something? If I were to mount an FA 50, an M 50, and a Sigma 50, would it know the difference and apply the right correction for each?

Thanks in advance.
12-05-2011, 11:48 AM   #3
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to your #2, yes. i have even experienced a tele lens that exhibited BOTH FF and BF at the same focal length, depending on whether it was near or far focus. In other words, at 135mm near subjects were FF and far subjects were BF.

#4 - interesting question and i'd like to know this too.
I also would like to know if the AF fine adjustments work when using a TC (auto contacts type) or if that data is not retained.
12-05-2011, 05:42 PM   #4
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Adam,

I have the Lensalign and think it is a good way to get some consistency to the AF adjust process. I would add a couple of things to the process:


1. Buy a small bubble level to put inside the Lensalign fixture on the flat bottom part. If you have a bubble on your tripod, you can get both the camera and the fixture equally level.

2. I am not sure if this can be done in other software packages or not, but a great way to more clearly see the focus zone is to crop the image so you have just the Lensalign target. Then apply the emboss filter, starting with a setting of 230/100 (IIRC). Adjust contrast if needed and sharpen a bit. The results are like shown below:



Ray

12-05-2011, 06:01 PM   #5
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@maxfield_photo:
#1: Right. AF systems are not 100% accurate. They will claim 'correct focus' within a certain range of 'spot on'. A smallish aperture will mask the error due to the depth of field, whereas the errors may be blatantly visible at, say, f/1.4.
#2: mikeSF answered that above.
#3: I don't really know for sure but I think that a large aperture lens is harder to get to auto focus accurately due to the lower contrast of the image being analyzed. But this is speculation. Contradicting my belief, for the test reported in the article, the author used a FA*85mm/1.4 lens and it just focused spot on every time. Of course, that lens is of a mechanical quality with tight tolerances and that isn't manufactured anymore. Now we're in the plastic fantastic age.
#4: The M lens you mention isn't an auto focus lens so FF/BF is irrelevant. The lens is remembered by lens type, not serial number. If you have more than one FA 50/1.4, the camera cannot detect which one you use, just that it is a FA 50mm/1.4.
12-05-2011, 06:04 PM   #6
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@Ray Pulley:
Thanks! That's a great tip!
12-05-2011, 06:51 PM   #7
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QuoteOriginally posted by Ole Quote
#4: The M lens you mention isn't an auto focus lens so FF/BF is irrelevant. The lens is remembered by lens type, not serial number. If you have more than one FA 50/1.4, the camera cannot detect which one you use, just that it is a FA 50mm/1.4.
Heh, guess I didn't think that one out. But still I've noticed when I manually focus my M lenses, the in-focus indicator comes on and stays on for maybe 8 to 10 degrees of rotation. I think it's like you say, the camera claims the image is in focus, but critical focus may still be off. I don't know if the K5 is any better about this than the K20D, but I'm hoping if they do release a pro level full frame camera that accuracy can improved considerably. That is a feature for which I would pay a lot.
12-05-2011, 07:39 PM   #8
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QuoteQuote:
The lens is remembered by lens type, not serial number. If you have more than one FA 50/1.4, the camera cannot detect which one you use, just that it is a FA 50mm/1.4.
I was just about to ask a related question. It seems you've verified my fears.
I have two 16-50 lenses, each a little bit off in opposite directions. It is a shame the memory is not by serial. I am stuck either with one lens way out of focus while the other is perfect, or with two lenses slightly out of focus. Pentax didn't really think that through very well.

12-16-2011, 07:47 AM   #9
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QuoteOriginally posted by amoringello Quote
I was just about to ask a related question. It seems you've verified my fears.
I have two 16-50 lenses, each a little bit off in opposite directions. It is a shame the memory is not by serial. I am stuck either with one lens way out of focus while the other is perfect, or with two lenses slightly out of focus. Pentax didn't really think that through very well.
Seems unusual to have two of the same model to me and I'm sure that Pentax saw that as a very unlikely event.
12-18-2011, 08:01 AM   #10
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I have focus problems with all my manual lenses on my K5. I have tried to adjust the focus as described in the article. I have a FF probelm. When I adjust I can make it worse but not better even if I go to 10 in adjustment the FF problem dont change in that direction but gets worse when I go in th other direction. Is ther a problem with my camera?
PA
12-18-2011, 04:44 PM   #11
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Chances are that there's nothing wrong with your camera at all, you will have to live with the fact that the AF confirmation system is approximate for manual lenses. If you use them a lot, your best bet would be a split-screen focusing screen.

Adam
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07-17-2012, 10:01 AM   #12
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Here's another auto-focus adjust trick I sort of stumbled across but that I've found is nearly as accurate as visual analysis...

* Set your camera to shoot full resolution JPEGs, maximum quality.

* Run a series of AF adjustments (I go the full +/-10 in increments of 2).

* WITHOUT opening the images in LR or Photoshop, look at the file size of each image. On a Mac, simply highlight all the images and press <Command>+<I> for "Get Info". All the info windows will open for each image. Look at the exact file size in bytes and pick the 2 or 3 LARGEST file sizes and note the file/image names. On Windows, you would simply right-click and select Properties to get the exact file size.

* Now open all the images in Photoshop and visually inspect them. What I do is tile all the images on screen, pick one of them and set to 100% pixel view and then move the image in the window so you can see your focus target...then use "Match Zoom and Location" to get all the tiled images to snap to the same view/location and start inspecting. You can usually throw out 1/2 to 2/3 of the images right off the bat because of poor focus...then you narrow down to the final 2-3 images. Running either the Emboss or Find Edges filter is another technique you can use per the LensAlign instructions.

* After doing the visual inspection, see if this is the SAME 2-3 images you chose from looking at the file sizes.....my guess is they will be the same. (The reason has to do with the fact that the sharply focused images have more detail and thus are not as easily compressed in file size as the fuzzy images.)

The ONLY time this method has not worked for me is when I included too much background behind the focusing target and the back-focusing was so bad that the background was more sharply focused than the focusing target! This happened to me recently where I was about 50x the focal length away from the LensAlign target which included too much background, otherwise this technique of finding the largest file size has proven to be pretty consistent. A way to avoid this is to simply shoot a flat textured surface such as a brick wall.

If any of you try this technique, let me know how it worked for you.

Terry
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