Lens issue?

**Astropaps** · 06-07-2017, 09:24 AM

Hi, all

I recently purchased a 30-135mm Pentax lens from another member here. I was checking for back or front focus and noticed that one side (left) of the image seems softer than the other. I'm trying my best to get the everything line up perfectly. So is it operator error or is there an issue with the lens? Thanks for your input.

Amy

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**jatrax** · 06-07-2017, 09:38 AM

Hi Amy, not sure what lens you have. I'm not aware of any 30-135 lens for Pentax so could you give us the make and model please?

As to your testing, yes it is possible there is a problem. However, in testing like this precision is of the utmost importance. And setting up on a tripod and getting the camera precisely, squarely aligned is necessary for a good test. Not close, but exactly square. Your image might be showing that the test chart is not quite flat for example.

**c.a.m** · 06-07-2017, 09:43 AM

Originally posted by jatrax

Hi Amy, not sure what lens you have.

The EXIF data indicates DA* 50-135.

I agree with @jatrax - the test setup needs to be precise. I see in the EXIF data that the aperture was set at f/2.8. Depending on the focal length and distance to the focus target, the depth of field could be very narrow. A small amount of variation in the left-right position of the target could produce the out-of-focus appearance.

- Craig

**Astropaps** · 06-07-2017, 09:58 AM

Thanks for responding. Sorry -- it's the 50-135mm Pentax set at 135mm. I suspect that the camera is not quite square but every adjustment I tried to make produced the same result, with the left side of the image being softer than the right. I would have though that operator error would have varied the result.

Amy

photoptimist · 06-07-2017, 10:31 AM

Even if this kind of angled-target setup isn't perfectly straight, square, or level, there can't help but be some place on both sides is: 1) in the plane of focus; 2) should be in good focus; and 3) shows the same amount of sharpness. The worst that an imprecise set-up like this can do is make it seem like one side is front-focusing and the other side is back-focusing.

Thus, I'd say there is some issue with the lens but whether it is "bad" is hard to say. The mechanisms in some zooms can can show optical oddities when pushed to the hard-stop of the range of motion. Backing-off from the extreme just a tiny a mount can lead to better sharpness. Does it show this at other settings of the focal length?

**jatrax** · 06-07-2017, 10:32 AM

Originally posted by c.a.m

The EXIF data indicates DA* 50-135.

Missed that, thanks.

Originally posted by Astropaps

I suspect that the camera is not quite square but every adjustment I tried to make produced the same result

For your test setup:
Are you using a tripod?
Using a bubble level on the camera or other leveling means
Using a tape measure or other means to make sure the camera is precisely square to the test chart
Is the test chart itself absolutely flat? Just a tiny bit is enough to make a difference. From your image it looks to me that the test chart might have a bit of a bow in it. Check that first.

**Astropaps** · 06-07-2017, 10:32 AM

I'll check. I put on the 200mm and did not see the same issue. Will report back.

**jatrax** · 06-07-2017, 10:35 AM

Also try printing out a regular AF1951 flat chart and taping it to a wall. Try that (or even a detailed flat brick wall) to make sure the lens is not decentered before you keep trying the AF testing.

**Astropaps** · 06-07-2017, 10:37 AM

Well, I backed it off from 135 and see the same issue. Soft all along the left side compared to the right, which again, I don't see with the 200mm, so I suspect it's not an issue with the setup. I'll have to take it out and see if I notice anything in actual photos.

DeadJohn · 06-07-2017, 12:50 PM

Originally posted by jatrax

Also try printing out a regular AF1951 flat chart and taping it to a wall. Try that (or even a detailed flat brick wall) to make sure the lens is not decentered before you keep trying the AF testing.

In the absence of a flat chart or nearby brick wall, you can use graph paper or a taut window screen.

**Astropaps** · 06-07-2017, 02:16 PM

Just for comparison, here's a shot I took with the 200mm, same setup. As you can see, there seems to be even sharpness near or at the center of the target on both sides, although I do see a bit of front focus, I think. If there is an issue with the 50-135mm, what is it? Decentered? And if so, where can I get it fixed?

Na Horuk · 06-07-2017, 03:12 PM

Originally posted by Astropaps

I think. If there is an issue with the 50-135mm, what is it? Decentered?

Yeah, uneven sharpness across the frame is usually a sign of decentering. The 50-135mm has a good reputation and the sample photo you posted doesn't seem sharp.
BUT please take another photo. Tripod, 2 sec timer (or remote trigger), natural light, ISO 100, near minimum focus distance (not closer than the MFD, though!), focused using Live view with Focus peaking. Av mode at lowest f-number. Test shots at 50mm and 135mm (you might need to move the focus chart for the shots)

Originally posted by Astropaps

And if so, where can I get it fixed?

Well, where do you live? Check for local Pentax dealers and shoot them an email. If the lens is out of warranty you can contact other lens repair centres as well. They usually let you send the lens to them and give you a quote on how much it would cost to repair before attempting the repair. If you decide not to repair with them, the shipping and a flat fee are on you.

**c.a.m** · 06-07-2017, 03:47 PM

Originally posted by Astropaps

here's a shot I took with the 200mm, same setup.

Does anyone else notice that the target appears slightly non-symmetrical to the camera's line of sight? Compare the left and right hand rows of triangles: the left is aligned with the left edge of the image while the right side is at an angle. Another indicator that the target is not precisely aligned to the camera lens axis is the central blue symbol - it's not a square (length is slightly longer than the height).

Would this cause unbalanced focus on the target?

**Astropaps** · 06-07-2017, 05:09 PM

Yes, I think that first photo indicates that the camera wasn't square to the target. I'll work on the setup and post more shots at different focal lengths.

Amy

abruzzi · 06-07-2017, 08:54 PM

to my eyes, both look a little skewed. I was reading some articles on decentering (new thing for me) and (correct me if I'm wrong here) it seems it would be more reliable to test with defocusing on a star chart. From what I've read, it seems that with that approach it is less critical to be at a perfect right angle.

The idea is you print the document below, focus on the page, then defocus a little. If it deforms shape when it blurs (i.e. becomes oval instead of circular) the lens is decentered. Is there a drawback to that method?

Lens Rentals | Blog

https://upload.wikimedia.org/wikipedia/commons/f/f5/ImageTestStarJMW.pdf

(see image at lower right)

06-07-2017, 09:43 AM	#3
c.a.m Site Supporter Join Date: Jan 2013 Location: Ottawa, Canada Photos: Gallery \| Albums Posts: 4,193	Originally posted by jatrax Hi Amy, not sure what lens you have. The EXIF data indicates DA* 50-135. I agree with @jatrax - the test setup needs to be precise. I see in the EXIF data that the aperture was set at f/2.8. Depending on the focal length and distance to the focus target, the depth of field could be very narrow. A small amount of variation in the left-right position of the target could produce the out-of-focus appearance. - Craig Last edited by c.a.m; 06-07-2017 at 09:50 AM.

06-07-2017, 03:12 PM	#12
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	Originally posted by Astropaps I think. If there is an issue with the 50-135mm, what is it? Decentered? Yeah, uneven sharpness across the frame is usually a sign of decentering. The 50-135mm has a good reputation and the sample photo you posted doesn't seem sharp. BUT please take another photo. Tripod, 2 sec timer (or remote trigger), natural light, ISO 100, near minimum focus distance (not closer than the MFD, though!), focused using Live view with Focus peaking. Av mode at lowest f-number. Test shots at 50mm and 135mm (you might need to move the focus chart for the shots) Originally posted by Astropaps And if so, where can I get it fixed? Well, where do you live? Check for local Pentax dealers and shoot them an email. If the lens is out of warranty you can contact other lens repair centres as well. They usually let you send the lens to them and give you a quote on how much it would cost to repair before attempting the repair. If you decide not to repair with them, the shipping and a flat fee are on you. Last edited by Na Horuk; 06-07-2017 at 05:26 PM.

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06-07-2017, 09:38 AM	#2
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Hi Amy, not sure what lens you have. I'm not aware of any 30-135 lens for Pentax so could you give us the make and model please? As to your testing, yes it is possible there is a problem. However, in testing like this precision is of the utmost importance. And setting up on a tripod and getting the camera precisely, squarely aligned is necessary for a good test. Not close, but exactly square. Your image might be showing that the test chart is not quite flat for example.

06-07-2017, 09:58 AM	#4
Astropaps Loyal Site Supporter Join Date: Mar 2017 Posts: 121 Original Poster	Thanks for responding. Sorry -- it's the 50-135mm Pentax set at 135mm. I suspect that the camera is not quite square but every adjustment I tried to make produced the same result, with the left side of the image being softer than the right. I would have though that operator error would have varied the result. Amy

06-07-2017, 10:31 AM	#5
photoptimist Pentaxian Join Date: Jul 2016 Photos: Albums Posts: 5,128	Even if this kind of angled-target setup isn't perfectly straight, square, or level, there can't help but be some place on both sides is: 1) in the plane of focus; 2) should be in good focus; and 3) shows the same amount of sharpness. The worst that an imprecise set-up like this can do is make it seem like one side is front-focusing and the other side is back-focusing. Thus, I'd say there is some issue with the lens but whether it is "bad" is hard to say. The mechanisms in some zooms can can show optical oddities when pushed to the hard-stop of the range of motion. Backing-off from the extreme just a tiny a mount can lead to better sharpness. Does it show this at other settings of the focal length?

06-07-2017, 10:32 AM	#6
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Originally posted by c.a.m The EXIF data indicates DA* 50-135. Missed that, thanks. Originally posted by Astropaps I suspect that the camera is not quite square but every adjustment I tried to make produced the same result For your test setup: Are you using a tripod? Using a bubble level on the camera or other leveling means Using a tape measure or other means to make sure the camera is precisely square to the test chart Is the test chart itself absolutely flat? Just a tiny bit is enough to make a difference. From your image it looks to me that the test chart might have a bit of a bow in it. Check that first.

06-07-2017, 10:35 AM	#8
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Also try printing out a regular AF1951 flat chart and taping it to a wall. Try that (or even a detailed flat brick wall) to make sure the lens is not decentered before you keep trying the AF testing.

06-07-2017, 10:37 AM	#9
Astropaps Loyal Site Supporter Join Date: Mar 2017 Posts: 121 Original Poster	Well, I backed it off from 135 and see the same issue. Soft all along the left side compared to the right, which again, I don't see with the 200mm, so I suspect it's not an issue with the setup. I'll have to take it out and see if I notice anything in actual photos.

06-07-2017, 12:50 PM	#10
DeadJohn Pentaxian Join Date: Jan 2011 Location: New York Posts: 4,834	Originally posted by jatrax Also try printing out a regular AF1951 flat chart and taping it to a wall. Try that (or even a detailed flat brick wall) to make sure the lens is not decentered before you keep trying the AF testing. In the absence of a flat chart or nearby brick wall, you can use graph paper or a taut window screen.

06-07-2017, 03:47 PM	#13
c.a.m Site Supporter Join Date: Jan 2013 Location: Ottawa, Canada Photos: Gallery \| Albums Posts: 4,193	Originally posted by Astropaps here's a shot I took with the 200mm, same setup. Does anyone else notice that the target appears slightly non-symmetrical to the camera's line of sight? Compare the left and right hand rows of triangles: the left is aligned with the left edge of the image while the right side is at an angle. Another indicator that the target is not precisely aligned to the camera lens axis is the central blue symbol - it's not a square (length is slightly longer than the height). Would this cause unbalanced focus on the target?

06-07-2017, 05:09 PM	#14
Astropaps Loyal Site Supporter Join Date: Mar 2017 Posts: 121 Original Poster	Yes, I think that first photo indicates that the camera wasn't square to the target. I'll work on the setup and post more shots at different focal lengths. Amy

06-07-2017, 08:54 PM	#15
abruzzi Veteran Member Join Date: Apr 2017 Location: New Mexico Posts: 788	to my eyes, both look a little skewed. I was reading some articles on decentering (new thing for me) and (correct me if I'm wrong here) it seems it would be more reliable to test with defocusing on a star chart. From what I've read, it seems that with that approach it is less critical to be at a perfect right angle. The idea is you print the document below, focus on the page, then defocus a little. If it deforms shape when it blurs (i.e. becomes oval instead of circular) the lens is decentered. Is there a drawback to that method? Lens Rentals \| Blog https://upload.wikimedia.org/wikipedia/commons/f/f5/ImageTestStarJMW.pdf (see image at lower right)