[akptc]

Hi, I just need to confirm what I've gathered through reading older posts and my own experience with the Pentax lens. These two points appear to be true - please kindly comment.

1. Pentax 16-50 is a varifocal lens. As such, it may be impossible to perfectly fine-tune the AF on both the wide and the long ends at the same time. If one end is well-tuned, the other will most likely back/front-focus.

2. Tamron 17-50 is a par-focal lens and can achieve perfect AF at all focal lengths with a single in-camera AF correction.

I have the Pentax 16-50 and have been fighting with the AF tuning for a while now. Tried averaging the error but can never achieve AF quality on both ends to my satisfaction. I am hoping the Tamron lens will free me from this stress.

Looking forward to your advice.

Andy

[Unregistered User 8]

Hi Andy, and seasons greetings. I don't have the Tamron but I do own the DA* 16-50. Initially I was having similar issues with my copy but found the compromise that suited me and my style of shooting. Waaaayyyy back in 2015 another member was having trouble and I discussed my thoughts with them in the linked thread below. The 16-50 can be a little tricky, but a wonderful lens IMO. Good luck.

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/308662-my-...2-8-sharp.html

[Trickortreat]

Tamron is a better lens if you dont mind losing weather sealing. It is probably the best normal zoom for crop (yes even better than the Sigma one) on all DSLR systems. Even better on Pentax because of ibis

[nobody]

As far as I know Tamron is varifocal lens too. At least my copy is. If you have zoom lens I think, the best way to adjust AF issues is to adjust it on a longer "side" of the lens, in this particular case 50mm. It is due to DOF which is shallower on 50mm than on 17mm (comparing the same Fstop). That means the DOF on 17mm is deeper than on 50mm so possibility of taking sharp images on 17mm is higher even the focus issues are not properly adjusted.

[Trickortreat]

Tamron isnt varifocal
Quick question on Tamron 17-50mm vs Sigma 17-50mm - PentaxForums.com
That said, no zoom lens is 100% parfocal. When I micro af adjusted mine, I used ~40mm focal length.

[tibbitts]

I only have the Tamron. I had a problem with it losing contact with any body that Tamron fixed on the 2nd try. At least in the U.S. I'm guessing the body (probably just one) that Tamron uses for calibration is off by 10, which is a fairly huge factor given that my Pentax bodies have only an adjustment range of +/-10, since they calibrated my lens twice now with the same result.

[shardulm]

As far as I know Tamron is varifocal lens too. At least my copy is. If you have zoom lens I think, the best way to adjust AF issues is to adjust it on a longer "side" of the lens, in this particular case 50mm. It is due to DOF which is shallower on 50mm than on 17mm (comparing the same Fstop). That means the DOF on 17mm is deeper than on 50mm so possibility of taking sharp images on 17mm is higher even the focus issues are not properly adjusted.

Great recommendation. I would do this too. Mine is actually already tuned at the long end. So I never bothered resetting it.


I have a great copy of the Tamron but just don't like the rendering albeit sharper from DA* in some situations.

[akptc]

[...] my thoughts [...] in the linked thread below. The 16-50 can be a little tricky, but a wonderful lens IMO. Good luck. Is my Pentax 16-50mm f2.8 sharp? - PentaxForums.com

Thank you PJV, I found a decent ruler and plan to take my time adjusting AF at 50mm on the DA tomorrow. Will report on the results. Also thank you everyone who contributed. Btw, I will probably end up also getting the Tamron just to "taste the sharpness".

[Unregistered User 8]

Good luck, and looking forward to your results.

[torashi]

Try to make sure your focusing screen is in perfect focus by comparing the view through the viewfinder and AF to live view. If the focusing screen is off, you need to shim it somehow to get it to the equivalent distance to the sensor. Once you made sure it's ok, you can proceed with calibration with a ruler.

[HippyHippo]

I own both.
I could never get the Pentax 16-50 to focus properly across the full range of focal lengths either, despite extensive efforts to do so with a calibration tool. It was also unusable at f/2.8. Then the AF died.
The Tamron is noisy and not weather-sealed...but is adequately sharp throughout the range, much better at f2.8 and lighter (plus the AF actually works!).

[sungibr]

Hi, I just need to confirm what I've gathered through reading older posts and my own experience with the Pentax lens. These two points appear to be true - please kindly comment.

1. Pentax 16-50 is a varifocal lens. As such, it may be impossible to perfectly fine-tune the AF on both the wide and the long ends at the same time. If one end is well-tuned, the other will most likely back/front-focus.

2. Tamron 17-50 is a par-focal lens and can achieve perfect AF at all focal lengths with a single in-camera AF correction.

I have the Pentax 16-50 and have been fighting with the AF tuning for a while now. Tried averaging the error but can never achieve AF quality on both ends to my satisfaction. I am hoping the Tamron lens will free me from this stress.

Looking forward to your advice.

Andy

I have the 16-50 and use it extensively.

From my experience, the the major problem of get a reasonable af calibration is decentering. And the 16-50 is famous for that.
If you could get yours serviced, try to look for decentering. If you copy are defective, it'll be almost impossible to get a decent calibration.
I get mine somehow adjusted and now are pretty much fine. Reasonable sharpness on 50mm f2,8 and very good upwards.

Also, if that's not the case, doing the calibration on the longer focal lenght is the best way, as it was pointed out already.

Both pictures are f2,8. The first is 50mm and the second 16mm

Hope it helps

[akptc]

After about two hours of fine tuning the AF on the 16-50, I have concluded that this specimen is the spawn of the devil himself and as such needs an exorcism by CRIS.

I tried everything, assuring myself of the angles and distances, used a variety of rulers and other objects and still, just as it looked that I have finally converged on the right fine-tune setting, if I then focused on something a little closer or farther, the lens would again be severely off-focus. I am physically exhausted by this process and have used up my entire curse word vocabulary several times over. This lens is going in for a major surgery. Or the trash can. I do own a large hammer...

Thanks everyone for your advice, I greatly appreciate each and every post.

Edit: Apparently CRIS is not the place for Pentax repairs, I will be sending the lens to Precision Camera

Andy

[Unregistered User 8]

I have concluded that this specimen is the spawn of the devil himself and as such needs an exorcism by CRSI.

Very sorry to hear this Andy, and also sorry for giving you " false " hope. Please let us know how the EXORCISM goes.

[akptc]

PJV, not at all, I am kind of glad to have suffered through it, I think I can now fine tune any healthy zoom quickly, with my eyes (almost) closed