[Lev]

Guys, looks like I have front/back focus issue on my 50-135 lens

Front focus appears on 50mm and back focus on 135

any thoughts? return it right?

* correction. It looks like front/back focus issue is present when not zooming. If I zoom all the way in to 135 then there focus becomes good.

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Last edited by Lev; 01-24-2018 at 05:35 AM.

[JohnX]

Return it? Not necessarily.

You are always likely to have differences at either end of the range with any zoom.

You could consider (a) an average AF adjustment between the ends, and/or (b) fine tune for the focal length you are likely to use most often.

[clickclick]

Have you checked it out for decentering? I think I would do that as part of the decision process.

[Lev]

Have you checked it out for decentering? I think I would do that as part of the decision process.

yes I have checked it, with zeiss circle and another ISO.... no problem with decentering



well, when I focus on 135 it's great, more I zooming out more back focusing appears and on 50mm it's completely useless. If I correct it in AF fine adjustment - then it backfocuses on 135mm no matter what setting I'll make it is always big difference between 50mm and 135mm...

[clickclick]

Doesn't seem right to be that severe. I can't check my copy until later today, but I don't feel like this has been an issue with it.

[Lev]

I've set it to +3 in Fine AF and for the first look it seems it's correcting but why it's happening this way. I know that if you need fine AF you do it but it should affect all focal ranges, or I'm wrong?

[clickclick]

The AF adjustment should apply to the lens as a whole, not a specific focal length, so the question would be why is the focus offset not consistent. I don't know enough about the optics involved to give a real answer. The lens does focus internally, so I believe it is a more "complicated" system than just moving the elements fore/aft as a group. This might explain the deviation, but I would not expect it from this lens based on my experiences with it.

[Lev]

Does this means that it probably decentered?

[JohnX]

Take a shot of a brick wall, or something with a regular pattern, where you are standing at right-angles to it, both vertically and horizontally. The bricks/pattern should be equally focussed all around the edge of the frame.

Don't know if this helps? https://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-ol...ts-a-makeover/

Or this? https://www.pentaxforums.com/articles/photo-articles/how-to-check-your-lens-...centering.html

[Lev]

Take a shot of a brick wall, or something with a regular pattern, where you are standing at right-angles to it, both vertically and horizontally. The bricks/pattern should be equally focussed all around the edge of the frame.

Don't know if this helps? Lens Rentals | Blog

Or this? How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com

John, thanks, ok I'll do that but do you find something strange in pictures above? That strange lines in out of focus areas appear both in test chart and in picture

[BarryE]

Isn't a variation between front and back focus across a zoom lens' range usual ? It has been on all the zooms I've ever owned - some more than others. Obviously, if it's extreme then there may be a problem, but with a complex range of optical parameters to handle, there's always going to be compromises. I've found that I've set my zooms up to be better on the end that I tend to use most, then ease off a little towards the required adjustment at the other end. So never is it perfect, but it's better where I tend to need it most. They I try to remember this. Isn't this normal ? I've discussed this with users of other brands and folk in the trade, and it seems it's an accepted compromise, but most users just don't notice the problem.

On my K-3s most of my lenses (zoom and primes) are adjusted between +5 and +7 (interestingly both K-3s are identical). My 50-150 varied by about +3 as I recall, but my 16-50 was more like +5. Some field curvature may be involved in the latter. My K-1 is completely different as I've left all my FF lenses at 0, a zoom too - been lucky.

Also, I set my AFs up by measuring a target at about 2 to 3 m, as I found measuring at shorter distances gave inconsistent and less helpful results for my usage. Any close-up work I tend to double check the focus anyway and for long distances the focus becomes less relevant.

[Adam]

Are you are re-focusing after you zoom, right?

[Lev]

Are you are re-focusing after you zoom, right?

Adam, of course I do. I was wondering why allmost all my photos are out of focus. Sadly but fact that even when I focus it somehow, pictures from my 16-85 look better in tele range. Do you think that the AF fine adjustment may differ from wide to tele range and that this is normal?

Does anybody have idea what these vertical lines mean in focus chart I have posted? or that strange out of focus area in corner of the first picture?

---------- Post added 01-24-18 at 11:45 AM ----------

Isn't a variation between front and back focus across a zoom lens' range usual ? It has been on all the zooms I've ever owned - some more than others. Obviously, if it's extreme then there may be a problem, but with a complex range of optical parameters to handle, there's always going to be compromises. I've found that I've set my zooms up to be better on the end that I tend to use most, then ease off a little towards the required adjustment at the other end. So never is it perfect, but it's better where I tend to need it most. They I try to remember this. Isn't this normal ? I've discussed this with users of other brands and folk in the trade, and it seems it's an accepted compromise, but most users just don't notice the problem.

On my K-3s most of my lenses (zoom and primes) are adjusted between +5 and +7 (interestingly both K-3s are 1identical). My 50-150 varied by about +3 as I recall, but my 16-50 was more like +5. Some field curvature may be involved in the latter. My K-1 is completely different as I've left all my FF lenses at 0, a zoom too - been lucky.

Also, I set my AFs up by measuring a target at about 2 to 3 m, as I found measuring at shorter distances gave inconsistent and less helpful results for my usage. Any close-up work I tend to double check the focus anyway and for long distances the focus becomes less relevant.

Well, at 50mm I have to set it to +3 or +4 to make it just acceptable, not great, this way the focus point jus enters area in focus but if I want it to be great I have to set it to +10! You might think ok, let it be like that, but now at 135 you're completely out of focus

[Adam]

Adam, of course I do. I was wondering why allmost all my photos are out of focus. Sadly but fact that even when I focus it somehow, pictures from my 16-85 look better in tele range. Do you think that the AF fine adjustment may differ from wide to tele range and that this is normal?

Does anybody have idea what these vertical lines mean in focus chart I have posted? or that strange out of focus area in corner of the first picture?

---------- Post added 01-24-18 at 11:45 AM ----------



Well, at 50mm I have to set it to +3 or +4 to make it just acceptable, not great, this way the focus point jus enters area in focus but if I want it to be great I have to set it to +10! You might think ok, let it be like that, but now at 135 you're completely out of focus

Regarding the front and back focus test, you need to re-do it using a proper setup in order to get conclusive results. However, my personal recommendation would be to get out there and take some real pictures, and revisit the focusing adjustment only if you get photos that aren't "keepers" when they should be.

The lines in the 2nd and 3rd shots are likely from your light source. Try using a shutter speed < 1/50s.

[Lev]

Isn't a variation between front and back focus across a zoom lens' range usual ? It has been on all the zooms I've ever owned - some more than others. Obviously, if it's extreme then there may be a problem, but with a complex range of optical parameters to handle, there's always going to be compromises. I've found that I've set my zooms up to be better on the end that I tend to use most, then ease off a little towards the required adjustment at the other end. So never is it perfect, but it's better where I tend to need it most. They I try to remember this. Isn't this normal ? I've discussed this with users of other brands and folk in the trade, and it seems it's an accepted compromise, but most users just don't notice the problem.

On my K-3s most of my lenses (zoom and primes) are adjusted between +5 and +7 (interestingly both K-3s are identical). My 50-150 varied by about +3 as I recall, but my 16-50 was more like +5. Some field curvature may be involved in the latter. My K-1 is completely different as I've left all my FF lenses at 0, a zoom too - been lucky.

Also, I set my AFs up by measuring a target at about 2 to 3 m, as I found measuring at shorter distances gave inconsistent and less helpful results for my usage. Any close-up work I tend to double check the focus anyway and for long distances the focus becomes less relevant.

Barry, if it's set to 0 and I'm focusing to subject from about 3-4 meter and lens is set to somewere 70mm, then it backfocuses for about 10cm. e.g. sharp focus is behind subject for about 10cm. I have also checked my 16-85 and 55-300. Last one had just very tiny bit of backfocus on wide angle. Easily will be corrected with I think +1 setting in AF fine tune. with 50-135 it's a whole different thing, with setting to 0, on front/back focusing chart, on wide angle and from 1,2m distance, it backfocuses so that the focus goes all the way up for several centimeters, 2 or 3. So to fully compensate it I have to set AF fine tune to +10! Now the question, would you buy the lens with this kind of behavior?