Best practice for autofocus fine tuning?

<p>I have a D300 and have tried fairly unsuccessfully to use the autofocus fine-tuning for my lenses. I was using a focus chart that I got from Thom Hogan's D300 guide (guide highly recommended!). It is a high-contrast target which you are supposed to shoot at perpendicular to the chart. Thom says that when the image is best in focus, you should see a moire pattern with the grey-scale dots, as well as having the sharp, black lines in focus. I use a tripod with cable release and a reasonable shutter speed (ca. 1/500 s). I have a very hard time figuring out what is the best-focused image is. I know my eyes are 47, but 100% magnification on the images are fairly ambiguous over a range of steps. <br>

Are there other steps I should be taking or a better focus target that people have been using successfully? Do they shot a chart that is meant to be shot at a 45 degree angle? Do you need to print the charts on a high-quality laser printer vs. a good inkjet printer? <br>

Any help appreciated. <br>

Cheers, David.</p>

<p>Autofocus has always worked precisely on every Nikon camera that I've owned (N80, F100, D70, D200, D700). I suspect that the AF on most cameras works fine. In the rare cases where it doesn't work, I would leave the adjustments to a Nikon-approved service technician. Every time I hear of someone making AF adjustments on their own, the end result is worse than it was in the beginning. It's probably better to understand how AF works and develop reliable AF technique - yes, there is a technique - than to fool around with critical internal adjustments.</p>

<p>Dan: Thanks for the response, but there is a user-accessible function in all the recent DSLR models for doing a per-lens autofocus adjustment. Even the pros have said that they use the facility. I suspect that Nikon would not have included the option if it were not useful. After all, even for the best Nikon lenses (e.g., the constant aperture lenses) there are manufacturing variability and on the variable aperture lenses the problem is more acute. There have been some threads on it, but I'm polling for best practices. </p>

<p>You may not be looking to spend this kind of money, (and I've never used these, or any focus chart for that matter), but maybe something like this would provide at least more consistent results. The video tutorials (halfway down the page) make it look fairly straightforward. I've never tried to fine tune my AF because I haven't had the money to spend on a good chart or getting it done in a shop, and I never felt just using a print out would help all that much.</p>

<p>http://www.rawworkflow.com/lensalign</p>

<p>No harm in trying it yourself since you can always set your camera back to default AF.</p>

<p>Here is a copy of a previous post I made, I hope it is beneficial.<br /> I AF tuned my 80-400VR and my 24-120VR and was very pleased with the results. I focused at a target of cars a city block away. The adjustments were in 2 units per picture. I started at +18 and went to -18. I used live view to focus. This was done at 80mm, 200mm and 400mm on the 80-400VR and 24mm, 70mm,and 120mm on the 24-120VR. I picked cars so that I had sharp lines in all directions and sharp contrasts including various colors.<br /> <br /> If you use NX2, the AF tuning number is displayed. I then looked at 300% at each shot and selected the best ones and tabulated the AF numbers for those. I averaged the numbers for the best looking shots. Some shots were too close to tell a difference so I took the next worst shots and wrote those down. For example +10 slight blur +2 slight blur. Split the difference or halfway between those and you get +6. Do this for min focal lenth then again for medium and last for max focal length. When you get all three average them again. I found no difference in the numbers between the various focal lengths on each particular lens.<br /> The 24-120 wound up at +8 and the 80-400 at -4 AF tune numbers. There is a good improvement doing this. All shots on a tripod with MLU and shutter delay to stop vibrations.<br /> <br /> To get used to AF tuning play with the AF tune function in the menu and see what happens. You can't screw anything up and it is a feature you should know how to use. Nikon added this feature because they knew there is a problem.</p>

<p>James,<br>

<br />Thanks for sharing your experience. I'm glad that your results were positive.</p>

<p>I have a question. You tuned the lens for focus on optical infinity. Did this have any effect on close-up focus? I.e. was close-up focus similarly improved when adjustments were made to sharpen the lens' focus on infinity?</p>

<p>If at all, I use the chart from <a target="_blank" href="http://focustestchart.com/chart.html">http://focustestchart.com/chart.html</a> and it works well. You photograph it from an angle of 45 degrees. There is a bar that you focus on, and it's very easy to see the result, i.e. if the focus is in front of the bar or behind, and how much so.</p>

 

<p>My experiences with AF fine tuning can be summed up as follows:</p>

<ul>

<li>I have not found any lens that could not be corrected.</li>

<li>I have never found a zoom lens that needs different correction values for different focal lengths</li>

<li>I have never found a lens that needs different correction values for near and far focus</li>

<li>Always open up the lens to the maximum aperture, e.g. f2.8.</li>

<li>Use the test chart for near focus testing</li>

<li>Make a second test outside by focusing on a far away building. 100 meters is certainly far enough.</li>

<li>Set the correction value according to the focus chart. Use a big value first, see what you get, if you have overshot, use something in the middle (binary search). Repeat until you reach an optimum.</li>

</ul>

<p>Hi Dan,<br>

The problem seems to be in the variation of components in the electronics to my way of thinking. The performance was improved equally at all distances.</p>

<p>I found that two of my more favorite Nikon lenses needed a bit of fine tuning. All the others were spot on with the D300, but two of them were focusing slightly long - one much worse than the other. </p>

<p>I am so glad that I went to the trouble. I did not print the pics, I just looked at them on the screen and then adjusted a little more until I had gone too far, then backed off. It is the most productive 1 or 2 hours I have spent in a long time. I did all the shooting with the camera mounted on a sturdy tripod, shooting at roughly 45 degrees to the chart.</p>

<p>One lens was my new 35mm f/1.8 prime where in aperture priority mode I shot with it wide open to minimize the depth of field. The other was a new and relatively expensive Nikon zoom lens (some people who rate it call it overpriced). To minimize depth of field I shot with the aperture wide open, and with the lens fully zoomed out to telephoto.</p>

<p>My homemade method seems to have worked fine for me. It is so nice that the D300 lets one fine tune the autofocus by lens. This is a great feature!</p>

 

<p>Just a quick question to clarify the fine-tuning procedure. If there is back focus, is it corrected by positive or negative correction?</p>

<p>By negative. Think that you need to move the focus point forward at the screen`s graphic.</p>

<p>That is why you to do not want to shoot perpendicular to the chart. By shooting at a roughly 45 degree angle to the chart it is obvious whether the lens is focusing in front of the desired location (what I call short), or behind it (which I call long). </p>

<p>Most of my Nikon lenses were right on the nose, but two were off. One slightly, and the other a whole lot. Both were focusing long, i.e. behind the intended location. </p>

<p>Negative on the D300 moves the focus point towards you and positive moves it further away.</p>

<p>David,</p>

<p>Good luck if you are trying to correct a back focus/front focus problem w/ a zoom.</p>

<p>Contrary to what some might believe, zooms are almost impossible to compensate for at various FL's...I've tried. If this is the case, you'll have to find a happy medium.</p>

<p>Now primes are another story obviously. They can be corrected for FF/BF if a problem exists.</p>

<p>Some people swear by the D-300's ability to AF fine tune; others swear at it.<br>

Another possibility...Poor collimation of the lens itself.<br>

What lens are you uhappy with?</p>

<p> </p>

<p>did someone say they checked their AF by focusing in live view? wouldn't this completely defeat the purpose of what you are checking? to be fair i have not read the manual on this aspect of my 700 so it seems quite strange to me.</p>

<p>This thread inspired me to try AF tuning on my D700 with a 24-70 f/2.8 mounted. I set the camera on a tripod at a 45-degree and put a page with distinct lines and fine-print text on a table top. The initial tests indicated that the actual plane of focus was closer to the camera then the distinct, straight black line that my sensor was pointing to. An adjustment of +6 or +7 compensated for the difference.</p>

<p>I was thinking, "Boy, I'm really glad that I tested this lens." But life is not always as simple as we'd like it to be.</p>

<p>I ran the test again with another page of text and distinct lines. Everything was in the same position; I didn't move the camera at all. I set the focus adjustment back to the OFF position and took a shot. Perfect! Right on the money. No adjustment necessary.</p>

<p>So, why did one test tell me that my AF was off by +7, and the other test was right on the money at zero? I don't know, and I'll have to run some additional tests, but I would be extremely cautions about making AF fine-tuning adjustments based upon the results of a single test. I suspect that my lens has no defect whatsoever, but that the D700's AF mechanism has inherent quirks with regard to what it selects to focus upon.</p>

<p>The D700 manual states that AF fine tuning should NOT be used under normal circumstances. I'm not sure when they intend us to use it, but until I can replicate the appearance of a defect in my lenses, I'm leaving the setting in the OFF position.</p>

<p>Caveat photographer!</p>