Back focus issues and zoom lenses

[anirbanbanerjee]

<p>I bought a Tokina 80-200 f/2.8 AF on eBay from a good (99.3%) seller. I am seeing it backfocusing 3-4 inches at 80mm but almost correct at 200mm (with tungsten light as well as in daylight). I am using the ancient K100D, which is the only Pentax DSLR I have access to.</p>

<p>Is a variation of backfocus with zoom setting indicative of a bad lens?</p>

<p>I am hoping at this point that the lens is not defective as I spent USD 200 on my country's protectionist customs duties.</p>

[aron_burday]

<p>I can't answer your question definitely. I hope the following will help.</p>

<p><a href="http://www.lensrentals.com/blog">This guy</a>, who runs a lens rental company here in the USA, analyzes his company's quality control test results and blogs about what he has found. It's pretty interesting. (Right now you need to scroll down a little to get to the good posts.) He's the only person I'm aware of who is publishing lens tests based on reasonably large samples. Unfortunately he hasn't looked at your specific question. He has found that zoom lenses may be sharper at one end of the zoom range than the other. He has also found that there will be observable variation in sharpness among good samples of a given lens model. Based on those results, I doubt that your lens is defective. If there can be variations in sharpness from one end of the zoom to the other, it seems likely that there could be variations in how the lens works with the camera's AF system. You probably just need to figure out whether you can live with the focus variation you have observed. But again, I can't be sure. Hope that's somewhat helpful. It is worth taking a look at the lensrentals.com blog in any case.</p>

[Robin Smith]

<p>This strikes me as a not unusual issue with third party lenses, particularly noticeable in tele-zooms as their DOF is not large enough to cover up focus errors. This may be due to differences between the Tokina and Pentax AF systems. Is it a new lens? If so then it should be better. If it is a secondhand lens, then who knows what has happened to it? The only people who can really help are Tokina or a good lens repair shop, or you could send it back for a refund. Certainly that degree of lack of focus makes it pretty useless.</p>

[michael_kuhne]

<p>It may actually be the same at 80mm and 200mm in terms of percentage of the field. "Almost correct" at 200mm may translate to 3-4 inches at 80mm, unless you are moving in closer at 80mm. Since this lens is not a varifocal zoom, if it were RIGHT ON focus at 200mm, then zoomed back to 80mm without changing location, it should then be still in focus at 80mm or at least close.</p>

<p>The only way I know to find out if this is a lens issue or a lens/body AF mis-match, is to try the lens on a different model body! </p>

[mggm59]

<p>Unfortunately I have the same issue on a K5 with a Pentax 12-24 I paid 700€, and before this one I had another (replaced by Pentax) that had different focus left and right. <br>

This means that one can have right AF on one side of the focal excursion with the right autofocus correction (in my case is around -5) but not on the other (where it would need to be +3 or 4), or have teh extremes slightly out of focus and chose an average value.<br>

Also, on this and other wide angle zooms, I see that it does not reach perfect focus at infinity, just because the available excusion AFTER the infinity mark is not enough in some conditions (it bangs the focus ring upon reaching the end stop and beeps that it cannot focus, I lost a lot of shots because of this).<br>

If I remember correctly I had the same problem when I tested them on a K20, so it could seem to be the lens, but I also had a K10 (actually 2, both with the same problem, Pentax refused a refund because according to them it was within their tolerances) which could not focus properly with a manual focus 50 f1.4 A. My conclusion is that the focus plane, due to the stabilization mechanism, has some tolerances that can vary greatly from one camera to the other, and the lens cannot cope. The problem of different focus at the two ends is probably a different issue which might be lens related, or AF related, or both.<br>

I would call all this annoying if it were cheap stuff: on a total of 2000€ of equipment (and the same happens on a 28-80 f2.8 FA* worth even more when it was new) it's something I cannot define in polite company.</p>

[anirbanbanerjee]

<p>Thanks for the responses. Unfortunately, because of my travel I could not test this lens earlier.</p>

<p>I'll try the lens on my film MZ-S, and until I get my K-5, which won't be for another 6 months at least, I am hoping some smart Russian hacker will release a hacked firmware :-) (I have the latest firmware where the known debug mode trick doesn't work).</p>

[mggm59]

<p>Unfortunately I have the same issue on a K5 with a Pentax 12-24 I paid 700€, and before this one I had another (replaced by Pentax) that had different focus left and right. <br>

This means that one can have right AF on one side of the focal excursion with the right autofocus correction (in my case is around -5) but not on the other (where it would need to be +3 or 4), or have teh extremes slightly out of focus and chose an average value.<br>

Also, on this and other wide angle zooms, I see that it does not reach perfect focus at infinity, just because the available excusion AFTER the infinity mark is not enough in some conditions (it bangs the focus ring upon reaching the end stop and beeps that it cannot focus, I lost a lot of shots because of this).<br>

If I remember correctly I had the same problem when I tested them on a K20, so it could seem to be the lens, but I also had a K10 (actually 2, both with the same problem, Pentax refused a refund because according to them it was within their tolerances) which could not focus properly with a manual focus 50 f1.4 A. My conclusion is that the focus plane, due to the stabilization mechanism, has some tolerances that can vary greatly from one camera to the other, and the lens cannot cope. The problem of different focus at the two ends is probably a different issue which might be lens related, or AF related, or both.<br>

I would call all this annoying if it were cheap stuff: on a total of 2000€ of equipment (and the same happens on a 28-80 f2.8 FA* worth even more when it was new) it's something I cannot define in polite company.</p>

[jeremy_corbin]

<p>Anirban, That's a bummer about the russian hack. I used the debug mode on my *istDS and nailed in the focus. The K10 I have actually backfocused horribly when I got it, required a 160um adjustment when it was said and done. The K10 is bad enough that I need to pull out the focusing screen and shim it to be correct for my MF lenses when I quit being lazy and just get it done.</p>

<p>To me, it sounds like that AF adjustment needs to be made. Try searching for "K100 debug mode" instead of "K100 russian hack" to see if you get a different answer. My DS did <strong>not</strong> require a firmware upgrade in order to access the debug mode... I just needed to have the right file on a SD card and push the right buttons on power-up.... Oh... and be able to read in Cryptonian.</p>

<p>Good Luck!</p>