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I own a nikon D7000 with 18-105 lens. I am not happy with sharpness I get with autofocus in portraits. I read about front/back focusing, but I am not sure whether my gear has this issue. Is this issue related with autofocus only? How can I figure if camera/lens need any tuning or calibration and what is involved in the process.

Some sample photos

1.http://www.flickr.com/photos/91114978@N05/8530128087/in/photostream

2.http://www.flickr.com/photos/91114978@N05/8530125481/in/photostream

EDIT

I did the test again this time not hand held but on timer mode.

Here are some more pics and details

Image 1. http://www.flickr.com/photos/91114978@N05/8685865958/ (Focused on Orange bottle)

Image 2 : http://www.flickr.com/photos/91114978@N05/8685870554/in/photostream/ (Focused on Center bottle)

Image 3 :http://www.flickr.com/photos/91114978@N05/8685868566/in/photostream/ (Focused on Green bottle)

EXIF data for three images : ISO 400 1/8 sec f5.0 Indoor, No Flash, AF-S Single point (Center focus point) Timer mode, JPEG Fine

No post processing.

Your comments are highly appreciated.

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Edited the title to keep this on topic as its own question, as the original question was really a duplicate of this: photo.stackexchange.com/questions/1/… –  dpollitt Mar 4 '13 at 19:54
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The easiest way to determine if front/back focusing is the problem is to try contrast detection AF using live view. Front/back focusing problems only affect phase detection AF. –  Michael Clark Mar 5 '13 at 5:47
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Have you tried contrast detection using Live view? This will show you the maximum sharpness your camera/lens is capable of. –  Michael Clark Apr 29 '13 at 13:04
    
Was the camera securely mounted on a tripod or sitting on the edge of the table? If the latter, a Tv of 1/6 second is vulnerable to vibration from the mirror. This, of course, could also be avoided by testing CDAF using Live View. –  Michael Clark Apr 29 '13 at 13:13
    
the camera was on table :( –  V.B Apr 29 '13 at 13:39

2 Answers 2

up vote 7 down vote accepted

There are several issues related to Phase Detection Auto Focus performance. You first must determine what the source of the problem is. It could be caused by one of several factors, or a combination of some or all of them. If you also have the problem when using the Contrast Detection AF in Live View, then the problem is somewhere else.

  • Front/Back focusing. If the body lens combo consistently misses in one direction, this can be corrected using AF Fine Tune. The most accurate methods use flat targets parallel to the sensor plane. Tilted targets are great at demonstrating the concepts involved, but determining exactly what the focus sensor array is aimed at is more problematic that commonly thought. If your viewfinder says your focus point is over the "zero" point, but your focus array is actually focused on the "2", your adjustment will not be correct. Is the sharpest point always a little closer than the spot you wanted? That is probably being caused by front-focusing. If the sharpest point is always a little further than the point you wanted it is back-focusing.

  • Focus point location. The squares for each focus point in your viewfinder are only an approximation of the actual spot the focus array is pointed for each specific focus point. The corresponding points on the array are not physically arranged in the same layout as what you see in the viefinder. See here for a detailed look at the 5DII focus system. All multi-point AF systems behave this way to one degree or another. If most of your photos focus on a point in the same direction from what you are aiming at regardless of whether it is nearer or further away than your aiming point, this could be your issue.

  • Focus consistency. Phase detection AF developed over the years with the emphasis on speed over accuracy. The camera measured focus once, decided how much and which direction the focus needed to move, moved the lens, and then took the picture without any feedback after moving the focus mechanism. It was an "open loop" system. The technology has now matured to the point that manufacturers are also designing systems that do include feed back from lenses that tell the camera exactly how far they moved in response to the instruction the camera sent. Roger Cicala discusses this issue in his blog entry at LensRentals.com. To gain the advantages of a semi-closed loop system, both the lens and the body must have the capability. If there is no discernible pattern to the AF errors, it may just be the limits of the D7000 with that lens.

I would begin by doing a proper AF Fine Tune. With a zoom lens such as your 18-105, do it at the focal length you use the most. If you use a wide range of focal lengths, do it at the longest one you use frequently.

You can also do a "pattern test" like Andre did with his 7D. Understanding your AF systems characteristics will help you learn how to use it more effectively.

You might try a prime lens. Any zoom lens that covers a focal range of 18-105mm has design compromises that affect image quality and sharpness. You may be expecting too much from that lens.

Ken Rockwell says in his review of the Nikon 18-105mm VR:

The plastic-mount 18-105mm VR is a decent enough general-purpose lens for people who are in the price range of the D90 with which it is kitted, but for $400 ($300 in a kit with the D90), I'd rather buy something else. The photos are nice and sharp most of the time, but if you're looking closely, the 18-105mm is Nikon's fuzziest lens in the corners at 18mm. Even the $100 18-55mm is better.

The DxO Mark Scores for this lens are pretty low as well. DxO Mark scores compared to the Nikon 24-70mm f/2.8 and the Canon Ef 24-70mm f/2.8L II.

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Your additional examples bear out the point made in Roger Cicala's blog entry linked in the answer: All PDAF systems have a range of focus accuracy from one shot to the next. The more accurate (and expensive) systems have a lower standard deviation, but they still vary a little from one shot to the next. Mount the camera securely on a tripod and use Live View to find the best performance your lens/camera combination is capable of. If the results using CDAF via Live view is about the same as PDAF, then you have found the limits of your lens. –  Michael Clark Apr 29 '13 at 14:26
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If, on the other hand, the CDAF images are considerably sharper, then the problem lies with your AF performance. Take 3 test shots of each position of the cans on the table and reset the focus to infinity between each shot. Then compare the three shots and see if the sharpest point of focus moves around or stays the same distance each time. –  Michael Clark Apr 29 '13 at 14:29
    
Thanks for your suggestions. I am not sure whats the intent of setting the focus to infinity? and you mean to do this in CDAF? –  V.B Apr 30 '13 at 5:06
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Use CDAF to see what the best your camera/lens is capable of. It will be slower but more accurate than PDAF. Then compare the results using CDAF to the results using PDAF. The focus variation comes into play in PDAF. Moving the focus to infinity between each shot gives an "honest" result when taking several shots with the same settings to establish the range of deviation. –  Michael Clark Apr 30 '13 at 6:08

Yes. This is an autofocus only issue. Front or back focus simply means that the camera focuses the lens in front or in back, respectively, of where focus actually is. This is related to a particular camera and lens combination due to manufacturing tolerances.

The easiest way to see if your camera and lens together exhibit the problem is to perform a focus test. There are charts designed exactly for this. Some are commercial products made of plastic, others are PDF files you print yourself. You can do this equally well with a simply rule that has fine markings (millimeters will do).

Just aim the camera at the target, autofocus with the center-point and shoot. Make you do this with the camera on a tripod and stabilizer turned off to make sure something is not moving. Just inspect closely the result after. If something closer is sharper then were you focuses, you have a front-focus issue. If it is something further, you have back-focus. If where you focuses is sharpest you do not have a focus issue. It is best to do this with the lens wide-open to have the most shallow depth-of-field and avoid focus shift when stopping down.

The last part of your question - How to Fine-Tune - has been answered here before.

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FWIW. The ones at Pentax DSLR Blog work well and even with non-Pentax cameras. Click on the Autofocus Adjustment Chart image to get a PDF download. –  Itai Mar 4 '13 at 19:37
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Using angled targets to adjust AF Fine Tune/Micro Adjustment depends on the actual focus sensor array being pointed at the same exact spot as the viewfinder indicates. This is not always the case. For an example, see blogs.stonesteps.ca/showpost.aspx?pid=54 –  Michael Clark Mar 5 '13 at 4:01
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If you follow the instructions with the angled chart, the procedure greatly increases the changes of the focusing being in the right spot because the high-contrast boundary is isolated from everything else. So you get no possible focus lock until the actual sensor is aligned with the intended focus target. –  Itai Mar 5 '13 at 4:06
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Flat target methods, such as the moire one you recommended in your answer are the most accurate. Systems where the entire target is angled are the least accurate. The flat targets with angled rulers fall somewhere in between. At the distances recommended for longer focal length lenses, the target is so small that it is possible for the AF point to lock onto something nearer or further away than the flat part of the target, even when the square in the viewfinder is properly centered on the target. –  Michael Clark Mar 5 '13 at 5:41
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Here's an official chart from Canon for the 5DII. flickr.com/photos/33233371@N04/3098752372/in/photostream/… Notice that when only the center point is selected, the area of sensitivity extends all the way to the squares above and below the center point! The camera will focus on an area near the outer ends if it finds more contrast there than at the center of the cross. If the six AF Assist points are enabled, all of the red areas are also active when only the center point is chosen. –  Michael Clark Mar 5 '13 at 11:37

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