Fine Tuning a Lens

<p>What is the best way to fine tune focus a lens? I've tried it with a tripod, single point focus, using a yard stick on an angle... but still not happy with my "judgement" on where the exact focus point is.</p>

<p>Help!</p>

<p>This site has a pdf of Tim Jackson's chart along with explanation, photos, & instructions. I've used Tim's chart for years quite satisfactorily. You do need to make sure the camera, chart and lens are in proper alignment relative to each other to achieve accurate results. I taped my chart to a 45 degree carpenter's square and level everything before proceeding. <br>

http://www.kscameraclub.org/docs/pdfs/focus_test_chart_edited.pdf</p>

<p>I find fine tuning quite time consuming and not at all trivial what the best method is. I have a Spyder LensCal target but I think it is too small to be sure that the focus sensor only sees the target when a realistic distance to target is used. By doing focus fine tuning at close distance, inaccurate results at long distances can be obtained. I recommend at least 30 x focal length as the distance to the target, preferably more. Repeat the focus operation several times and plot the errors measured on a slanted ruler as a scatter graph in some software (e.g. Excel), then fit a line using linear regression and see where the errors goes to zero. Unfortunately the graph may be nonlinear sometimes, but the line fitting gives a straightforward answer. I typically use 6 focus operations for each fine tune setting and get a series of shots, then evaluate the photos and find the sharpest part of the ruler and type in the position on the ruler into the spreadsheet.</p>

<p>Another method is to first focus to the target using contrast detect (Live view) AF, then switch to optical viewfinder operation, and adjust fine tune setting and see when the focus confirmation dot stops being visible (or starts blinking). Do this in both directions so you get the + and - limits of focus fine tune which judge the LVAF result to be in focus. You can then e.g. select the mean of those two values as fine tune setting to use. This isn't necessarily accurate as the near depth of field is typically shorter than the far depth of field. But it gets you close (without taking a single exposure!) and then you can refine the focus fine tune setting based on real world usage results. </p>

<p>The two most recent bodies I have purchased (D7100 and especially the D810) require less fine tuning to use than my D800 did.</p>

<p>I honed mine by making formal tests, then honing them in over time with real-world results. Apparently the whole thing isn't written in stone--different lens openings matter, distances matter, etc. Real results are better than theoretical ones, and if you don't see a problem in the real world results, is there one?</p>

The best way?

 

The home brew club won't like my suggestion but what works best (and I say this after trying multiple techniques) is the

LensAlign Mk II target combined with FocusTune software to analyses the results.

 

Note that Nikon (and Canon) recommend a 50x focal length distance between camera and target while Michael Tapes

(creator of the LensAlign and FocusTune) recommends 25x and my findings agree with his.

<p>"....but still not happy with my "judgement" on where the exact focus point is." - Surely then Wade, that means any residual focus error doesn't matter?</p>

<p>My reasoning is; that if you can't tell exactly where the point of best focus is on a fine-detailed high contrast subject designed for the purpose, then you also aren't going to be able to see the difference in everyday use of the lens.</p>

<p>"Note that Nikon (and Canon) recommend a 50x focal length distance between camera and target..." What drugs were they on when they came up with that nonsense? A 1/49th scale image has far too much D-o-F at any sensible aperture to accurately determine a focus error. For example, a 50mm lens at f/1.4 and 2.55 metres focus distance has nearly 200mm of "accepted" D-o-F on full-frame. Increase the magnification to 1/24th and that drops to 48mm.</p>

<p>I'm in search of why fine tuning a lens does anything. <br>

As long as the three light paths inside the camera body are consistent with each other (lens to primary mirror to ground glass, lens to secondary mirror to AF sensors, and lens to image sensor) why wouldn't the AF system automatically adjust the lens for best focus on the selected AF point?<br /><br />If the focus is too close, turn the lens one way, if the focus is too far, turn the lens the other way. I don't see what is in the lens that might need fine tuning.</p>

<p>The lens fine tuning does make a difference. My 80-400 on a D800 shows a consistent focus error, so I have to fine tune that lens. At least we're in the digital era where you can check this instantly rather than waiting 2 hours for your slides to be developed.</p>

<p>Aaron, that doesn't quite answer the question. </p>

<p>If you 80-400 has a consistent focus error, you could correct it by manually tweaking the focus, right? And if you can manually tweak the focus, why can't the AF system recognize the same tweak.</p>

 

<p>Tom, assuming that the first half of your question isn't purely a rhetorical one to set up the second half: Who is going to--or can--manually tweak the focus for each exposure when photographing the active and often relatively distant subjects that an 80-400 tends to be used for (e.g., wildlife)? Sure, those of who have used entirely manual cameras used to do this all the time--I learned to shoot with a Minolta SRT-200--but expectations are entirely different in this era of autofocus.</p>

<p>(I can't address the underlying causes of imperfection in an AF system.)</p>

<p>I guess what I'm really trying to ask is what underlying cause of imperfection in an AF system allows or requires a fine tuning adjustment to be made for a lens. </p>

<p>It would make sense to me if the AF motor in the lens had some random error plus or minus some small amount. But I don't know anything about the construction of the AF motor in a lens. So what makes sense to me might not have any bearing on the electromechanical facts.<br /><br />If someone could definitely state that the design of ring motors is such that there is always a consistent error in the same direction, it might make a little more sense to me. But I'd still wonder why the camera wouldn't recognize that the image is out of focus that slight bit and move the lens to compensate. I'd also want to understand that underlying design of ring motors.<br /><br />I might be obtuse about this because I just don't get it. But I'm not trying to be rhetorical at all. <br>

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