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06-30-2011, 07:43 PM   #1
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Sigma 135mm 1.8 backfocusing problem

I picked up a Sigma 135mm 1.8 from Ebay a couple days ago.

I was doing some testing wide open tonight and have come to the conclusion that it has a serious backfocucing problem.

I was standing about 15 feet from what I was trying to focus on. It looked like it was focused just right and my K10D was telling me that it was in focus.

So I took the pic and it came out very blurry but I noticed that the stuff behind my subjest was perfectly in focus.

I then focused on something a little over a foot in front of my subject and took a picture, My subject was now perfectly in focus in the resulting picture.

I also noticed this a little last night when I was testing close focus (about 3 feet from my subject). The part that was in focus in the resulting picture was about 1-1.5 inches behind what I was focusing on.

Is this a normal kind of thing to happen?
It could be that my camera has been backfocusing ever since I got it and this lense just shows it really well as I have noticed that even on some autofocus lenses that things seem to be backfocused sometimes.

From what I have seen online of this lense, it tends to get a lot of negative comments about it being really soft wide open. I am wondering if it is because it just brings out back and/or front focusing problems to a higher degree than other lenses.

06-30-2011, 09:18 PM   #2
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Bah... adjusting the AF focus setting is not going to help unless I start ignoring the view finder except for the red square and beep when it detect that it is in focus.

What could the actual problem be?
Could it be that the view finder focusing is screwed up? Could it be that the sensor is not in the correct place?

Which one of those is more likely? I tend to think it is the sensor as when it looks in focus in the view screen, the AF focus thinks it is in focus as well.

I do have the diopter all the way to the right. Anything towards the left and it is completely blurry.

Last edited by cyclone3d; 06-30-2011 at 09:28 PM.
06-30-2011, 09:50 PM - 1 Like   #3
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Part of the problem is the focusing screen. The stock screen exaggerates the depth of field available with fast lenses and you probably won't see a difference in depth of field between wide open and about f/3.5 or so. There are a number of old discussions here on the forum about different aftermarket screens and their advantages/disadvantages with regards to depth of field, metering and focus accuracy.

It is also possible that your current focus screen would benefit from a different shim to adjust the point where objects appear to come into focus. Have you tried any other fast lenses on the camera?

Sample discussion thread: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/118633-split-p...libration.html

The diopter setting should have more to do with your eyesight than it does with where the camera focuses. You adjust the diopter until the display in the viewfinder appears sharp and then you should be able to leave it.
07-01-2011, 04:14 AM   #4
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Hrmmm. I just tested with my converted minolta 58mm 1.4 and it looks like it has the same exact problem. No wonder the few times I have actually used it I had a horrible time getting pics to come out focused.

I do have some old SLRs that I could steal the focusing screen out of to make one for my K10D, including a non-working Pentax Program Plus that the film advance lever won't work. Looks like before I got it somebody banged on the bottom of the camera to try to get it to work.


Last edited by cyclone3d; 07-01-2011 at 04:41 AM. Reason: fix error
07-01-2011, 04:46 AM   #5
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The K10D does have a focus adjustment, but you need to get into the service program to adjust it. Search this site and you will find a way in through a "Hacked" program and testing / adjustment procedures. I did this years ago. I found slightly different adjustments for the several lenses I have, so I just averaged them and set it up that way. It works fine for me now.

Of course it's that ever so shallow depth of field of the 135mm lens at f1.8 that brought this to the forefront for you.
07-01-2011, 06:57 AM   #6
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QuoteOriginally posted by calicojack Quote
The K10D does have a focus adjustment, but you need to get into the service program to adjust it. Search this site and you will find a way in through a "Hacked" program and testing / adjustment procedures. I did this years ago. I found slightly different adjustments for the several lenses I have, so I just averaged them and set it up that way. It works fine for me now.

Of course it's that ever so shallow depth of field of the 135mm lens at f1.8 that brought this to the forefront for you.
From what I have read, that will only affect the AF, and does not have anything to do with manual focus. I also tried it last night and it didn't make any difference.

I am wanting the manual focus lenses to be in focus when they look in focus in the view finder AND when the little red in-focus indicator pops up in the view finder.
07-01-2011, 06:59 AM   #7
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I am going to try making a focusing screen out the focusing screen from my broken Pentax Program Plus.

If that doesn't take care of it, it looks like I am going to have to add a shim or two to the focusing screen to get it to be correct.

See this thread for info on the shims:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/45309-shims-focus-screen-k10d.html

Last edited by cyclone3d; 07-01-2011 at 07:14 AM. Reason: spelling
07-01-2011, 06:59 AM   #8
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Sigma makes a 135/1.8?

07-01-2011, 07:04 AM   #9
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Chances are you need to shim the focusing screen, not replace it. There are a lot of threads around here about calibrating the screen, found this one with google.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/45309-shims-focus-screen-k10d.html

See the similar threads at the bottom when you click the link.

(looks like we both found the same article)

07-01-2011, 07:11 AM   #10
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QuoteOriginally posted by Clinton Quote
Sigma makes a 135/1.8?
They used to make one in the ys mount (T-mount with aperture control).

The ys-Pentax-k mount was specially installed at the factory and was not available to buy seperately. The ys-Pentax-k mount was also specific to what aperture range a lens had.

Another thing of note is that the ys-whatever mounts were not the same in between lenses. The one for my 135/1.8 has a single arm coming out of it that attaches to the lens, while my 300/4 has two arms that come out and attach to the lens.

Mine were both for Canon, so I am just using a t-mount adapter currently.

I want to make an actual modified t-mount adapter that will allow me to use stop down metering instead of having to use it as a fully manual lens, especially for the higher f-stops.
07-01-2011, 07:13 AM   #11
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QuoteOriginally posted by JeffJS Quote
Chances are you need to shim the focusing screen, not replace it. There are a lot of threads around here about calibrating the screen, found this one with google.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/45309-shims-focus-screen-k10d.html

See the similar threads at the bottom when you click the link.

(looks like we both found the same article)

Yeah, but I kinda want the split focusing circle plus the micro prism that is in the Program Plus. The split focusing circle makes it a whole lot easier to focus manual lenses.
07-01-2011, 08:25 AM   #12
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QuoteOriginally posted by cyclone3d Quote
Yeah, but I kinda want the split focusing circle plus the micro prism that is in the Program Plus. The split focusing circle makes it a whole lot easier to focus manual lenses.
You may Still have to shim it. I had a Katzeye on the K10/20 cameras and they Are nice to have.

07-02-2011, 08:59 AM   #13
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It's not just a shimming issue. The basic problem is that the stock screen shows more DOF than your lens is delivering - it is incapable of rendering a DOF as shallow as you get at 135/1.8. No amount of shimming will change that. It will *always* be the case that some things appear in focus but turn out not to be when shooting very shallow DOF with the stock screen. Only if the things that that *are* actually in focus in the shot *don't* look in focus in the viewfinder would there be a problem that shimming would help with. But that's rare.
07-02-2011, 10:46 AM   #14
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I just finished trimming down a focusing screen from an old broken/missing parts konica FT-1.

I was going to make one from the screen out of the broke Pentax Super Program but I slipped and messed it up.

The one I trimmed down I made the split prism angled. It is less than 45 degrees but that is how it fit.

The depth of field ring is awesome for the faster lenses AND exposure is not near as far off as it was before.

I already shimmed it a little bit with seme clear scotch type tape, but I think I need to add another layer as it is still backfocusing some.

Marc - it is doing exactly that. I have to focus on something closer to me in order to get the thing I want in focus to be in focus in the resulting picture.
07-02-2011, 10:54 AM   #15
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QuoteOriginally posted by Marc Sabatella Quote
It's not just a shimming issue. The basic problem is that the stock screen shows more DOF than your lens is delivering - it is incapable of rendering a DOF as shallow as you get at 135/1.8. No amount of shimming will change that. It will *always* be the case that some things appear in focus but turn out not to be when shooting very shallow DOF with the stock screen. Only if the things that that *are* actually in focus in the shot *don't* look in focus in the viewfinder would there be a problem that shimming would help with. But that's rare.
According to Rachel Katz, many of the K-5s have required shim changes in order to manually focus properly - mine did. The factory shipped with a .4, I needed .25. With the split prism, I can focus my 50mm f1.4 very precisely (even though, as you say, I can't actually SEE how thin the DOF is).
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