AF Fine Tuning - does you camera have it?

Caisiel

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13 Sep 2012 - 7:21 PM

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I have messed about with the af micro adjust on a number of my lenses only to finally revert back to
'0' af micro adjust on my Sony a850. I was worried my small lens collection was failing to accurately focus. This paranoid adjusting was applied to my zeiss 24-7" and 85mm zeiss, I soon realised that it was more inefficient focussing by me, as opposed to an issue with the optics. Hence deemed this a useless feature, until I set up and tested my older Minolta 100mm macro, and 135mm af lenses. They received adjustments of -6 and +9 respectively. Increasing my hit rate considerably. My question to those more educated ones out there, do lenses lose accuracy with AF with age?? If so this feature could prove to be essential for some of us who find it hard to leave good optics in the drawer!

Steppenwolf

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13 Sep 2012 - 8:24 PM

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Quote: Yes I do (well on some AF systems) as I mentioned the lens passes information on where it is in its focus range to the camera. If it measure that incorrectly then the calculation of how much to move the lens will be wrong as so a focus error will occur. This is why you have lens by lens adjustment. It is what is relied on to make classic phase AF systems.


Sorry but this doesn't make sense to me. I see no reason why the camera should be interested in where the lens is in its focus range. The way the Minolta/Sony AF works (on DSLR/SLTs) is that the camera makes an initial calculation as to how much to move the lens focus. It then moves it and checks (when in single focus mode). If it's not in focus it makes another calculation and moves the lens and checks. It basically iterates until the picture is in focus. With a compatible lens you only hear one movement - there may be small corrections but they're too fast to detect. However, I had a Kenko TC with a duff chip once and you could see and hear the lens oscillating back and to until it finally found focus. It was plainly telling the camera the wrong information (probably on gearing, although it may have been focal length) but the camera still found focus - it just took so long it was pointless.

If a camera's in continuous focus or tracking focus mode then it seems to just make one calculation, but in single focus mode it iterates.

Sooty_1 (Critique Team)

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13 Sep 2012 - 8:38 PM

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The other thing in lenses is backlash in the focus drive it might depend which direction the lens is focussing how much error you get.

Steppenwolf

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14 Sep 2012 - 9:50 AM

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Quote: It's actually not called "manufacturing tolerance" in that manufacturers don't tolerate it

You're either being facetious or you don't know much about 'tolerances'..... I don't see any smileys..?

Everything is manufactured to a tolerance...the degree to which an item can deviate from the design norm and still be acceptable. Some items have larger tolerances than others, but even on an optical product, there is still a variation in manufacture. The fact that a 'correction factor' is added to the autofocus computation, to compensate for a measured error, does not mean the the camera/lens leaves the factory absolutely spot-on for focus. That correction factor will also have a tolerance.

The question is really "how much is too much" to fail quality control. The reasons that old Soviet cameras had such varied quality was the agricultural production methods used gave rise to large tolerances, as opposed to the West Germans with their highly skilled and accurate processes. Thus with the Eastern European stuff, if you got two items at the same end of tolerance you got a good product, etc etc.

As Mike alluded to previously, if you have items at the opposite ends of the tolerance scale, there could well be noticeable errors.

Nick

I questioned "Mike's" allusion to "the tolerance scale" in the case of lenses because I wanted him to explain (which he hasn't done, I notice) how a lens can be set up wrongly to cause a camera to deliver wrong focus. Is he saying that the lens elements can be misaligned/mismanufactured to cause wrong focus? This can certainly happen in the case of a lens that has "differential focus" or where the lens elements are misaligned. Or is he alluding to inaccuracies in the lens ROM data? If the lens ROM data is wrong (e.g. it tells the camera the wrong gearing or wrong direction of gearing, etc) then this can cause reproducible FF or BF because, when the camera makes the final correction it will adjust the focus by the wrong amount. (BTW, backlash in focus drive won't produce reproducible FF or BF, just random errors).

"Mike" made an exceedingly glib statement saying "lenses are made within a tolerance so if your lens is at one end of the tolerance range and the body is at the other end blah blah". I'd be interested if he can expand on what that exactly means in the case of a lens.

As for a camera's "tolerances", I don't think that the classic form of DSLR BF and FF is caused by manufacturing tolerances. David Kilpatrick wrote an article about this on photoclubalpha some years ago, but it seems to have gone. The gist of it was that it's too expensive to mass produce consumer DSLRs accurately enough to focus within reasonable tolerances, so they test each camera after production to determine the discrepancy in the light path lengths to the AF sensor and the image sensor. This data is then fed into the firmware and is used as a final correction to achieve accurate focus. Presumably if the camera is hopelessly out of line they reject it. However, provided this is done correctly the focus should be OK assuming that a fully compatible lens is used. The manufacturing "tolerance" here seems to be in the accuracy of the measurement of this data. I guess when people send their camera back for FF/BF they recheck this. The way DSLRs achieve focus is so complicated that there may be other ways for FF/BF to occur, though - I posted a link to another DK article about this on another thread.


Quote: I am certain you will agree that if you can know where in the lens range the focusing motor is and can tell how out of focus it is then you have a better chance of moving the lens to the correct point quickly. If you look at the latest Canon lens patents they have items for precisely knowing the rotations of the motor.

Maybe, but I don't buy your argument that the process of AF is always done by a single movement of the lens focus elements because I can hear the AF motors iterating to achieve correct focus. It's possible that Sony AF using a different approach to Canon, but I doubt it.

LenShepherd (e2 Member)

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14 Sep 2012 - 10:10 AM

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Quote: It's actually not called "manufacturing tolerance" in that manufacturers don't tolerate it


There is a lot of truth in this.
See my last reply for some detail - in my experience less than ideal AF targets result in far more poor AF accuracy issues than camera or lens tolerances

Raffeluccio

31 Jan 2013 - 2:46 PM

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I have a Pentax k-x. Officially it does not have AF fine tuning (i.e. there is not a menu item for it). However I found on the internet a procedure which allows the user to place the camera in "debug mode" and then access the AF fine tuning.

In short AF fine tuning makes a world of difference and has become a "must have" for me. I know users of the Nikon D5000, and the Nikon D5100 and before doing AF on my Pentax, their camera's were taking more sharply focused pictures then mine. After doing the adjustment on mine, my pictures blow away their pictures in terms of sharpness! I tested their cameras for focusing accuracy and sure enough their cameras are slightly back focused.
Unfortunately there is no AF fine tuning for their cameras so they are stuck with this problem.

There has been a lot of discussion as to the cause and remedies to this AF fine tuning problem. In my experience all the points made above are right and wrong at the same time! AF fine tuning can make a world of difference, but of course it has to be done properly.

I have tested four cameras; my k-x, a d5000 and 2 d5100, with various lenses. The focusing was at least slightly off on all of them. This would lead me to believe that most cameras probably settle for a compromised focus and that they could all benefit from AF fine tuning.

Here is some specific data for your consideration;

1 - Pentax K-x, with a pentax - F 35-70mm lens required an offset of +140
2 - Pentax K-x with a pentax - f 70-210mm lens required an offset of +140
3 - Pentax K-x with a pentax DAL 18-55mm lens required an offset of +60
4 - Pentax K-x with a pentax - A 50mm f1.7 required an offset of +60
All of the above were slightly back focusing trough the whole zoom range but the amount of back focus varied slightly as the zoom setting was changed. The settings above were a compromise which places the "sweet" spot near the middle of the DOF.
5 - The D5000 with the nikon 18-105mm lens was back focusing to the point that one could see it easily by naked eye on a laptop monitor. I tested it and found that it was back focusing more on the lower end of the zoom and much less on the higher end of the zoom. I sent a sample picture to nikon and they asked me to sent it in for adjustment i.e the confirmed the back focus problem.

6 - This one is the weirdest of all. I tested a demo model of the d5100 with the nikon 18-55mm kit lens. This combo showed back focus at 18mm, front focus at 35 mm, and back focus at 55mm!!

Before all you nay sayers jump on me as to my testing methods and accuracy etc. etc . I used the center focus spot only, a shutter speed of at least 180, turned off VR, and in the case of the nikons I used view nx2 to confirm the focal point.

As you can see, from the above, AF problems do exist. They are caused by the camera body as well as the lenses. AF fine tuning can offset the problem in most cases, but there are cases such as item 6 above that fine tuning would not be able to solve.

Hope this helps. I wish there was some way to get Nikon to issue new firmware for the D5000 and the D5100 to allow AF fine tuning.

Raffeluccio

31 Jan 2013 - 8:53 PM

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Pentax k-30 also has AF fine tuning.