[Tim A]

Can you give me a link to a proper test chart?

Adam, I hope I was not being unreasonable with this request. I should have been more clear, because I can find focus charts by doing a Google search, but they come in lots of flavors. I was more wanting to know which chart you thought I could use to properly evaluate my lens.

CRIS has told me the lens meets Pentax specifications and from what I hear specs for lenses can have substanial tolerances, especially wide angle lenses like mine. It is obvious I am going to have to return the lens at my expense. I have already paid for one round trip. While I still have the lens, though, to work with I would like to have some result that would be convincing to show them. I gather you might have such a test in mind, but I don't know what it is. I have taken many photos, but none of them show anything that is not in the two I have posted here.

The decentering test showed the lens is not decentered and the focus always seems to be good except for the one area that is always less focused than the rest.

[sTi]

The decentering test showed the lens is not decentered and the focus always seems to be good except for the one area that is always less focused than the rest.

There are different kinds of decentering which need different tools to detect.
One kind is that the lens elements are parallel but not lined up exactly at their center.
The other kind is that one or more elements are at an angle (i.e. tilted), which leads to a tilted focal plane and the typical one-sided unsharpness. Your lens clearly(!) suffers from the tilt problem. If I were you I wouldn't hestitate to send the lens back with some pictures that demonstrate the one-sided unsharpness and point out the possible tilt problem in your accompanying letter. No lenses are perfect, but if the asymmetric sharpness distribution can even be detected on such scaled-down images this is simply unacceptable, especially for a premium-priced prime lens.

[asp1880]

You can also try to use a tripod and liveview focusing, moving the focusing square as far into the upper right corner as it will go.
That should be able to tell you if that corner can be sharp at all and remove any doubts about field curvature.

Also, remember to do all tests wide open, ie. f/4.

Regards,
--Anders.

[Tim A]

There are different kinds of decentering which need different tools to detect.
One kind is that the lens elements are parallel but not lined up exactly at their center.
The other kind is that one or more elements are at an angle (i.e. tilted), which leads to a tilted focal plane and the typical one-sided unsharpness. Your lens clearly(!) suffers from the tilt problem. If I were you I wouldn't hestitate to send the lens back with some pictures that demonstrate the one-sided unsharpness and point out the possible tilt problem in your accompanying letter. No lenses are perfect, but if the asymmetric sharpness distribution can even be detected on such scaled-down images this is simply unacceptable, especially for a premium-priced prime lens.

Thank you for the tilted lens element diagnosis. I found this to be an informative article about tilted lens elements.

LensRentals.com - A Tilted Element Demonstration

I sent some more photos to CRIS that really don't do anything more than show the problem is consistent. They say they are evaluating them, but seem to be taking their time. I would expect better service since they obviously sent me a bad lens saying it met Pentax specs.

[Hemppo]

Im suffering exactly same problem with my 15mm Ltd.

In my case I think that at least one of the elements has become loose with time as I'm able to capture some decent images if I shake the front barrel a bit with my fingers. The front barrel has noticeable play so I'm afraid that the front barrel grooves have weared a bit on use also.

I put a sample photo here. One can see that the right side of the image is all blurry when the center area is fine.

[tibbitts]

I don't endorse the use-a-chart philosophy entirely, unless you have the ability to print a several-thousand-square-foot test chart. I can tell you that some of my 55-300s have behaved differently at 5ft to 15 ft. - what you're looking at with a chart - than at infinity. It depends on what's wrong with the lens. You can use test chart results, but you also need examples from distances that are greater than you can achieve with a test chart.

With wide angles, I do sometimes print 5 test charts and attach them to a wall in the center and corners. But focus field curvature is sometimes an issue, so I only do this at more moderate apertures, and mostly to compare the results of focusing on different corners. Even if field curvature isn't a design issue with the lens, I'm guessing that the center being at such a different distance from the lens as the edges would compromise the results.

Regarding where to focus, I'd focus with the center of screen on the object you're going to be evaluating, and then reposition the camera so the object is in a corner or edge, rather than try to focus (lv or otherwise) in the corner or edge of the screen.

[Bernie_Rain]

I have the SMC 15mm Limited and I like it, but the corner sharpness is quite disappointing below f/8 - has nothing to do with your problem, just a side note.

[Digitalis]

The sigma 8-16mm f/4.5-5.6 @ 15mm is consistently sharper than the DA15mm f/4 in the corners - with the DA15 you have to focus slightly in front of your target to get the corners sharp - stopping down to f/11 also improves the consistency of image quality across the frame by diffraction equalizing resolution across the frame. Field curvature is just another of the quirks of the DA15mm f/4.

[Tim A]

There are different kinds of decentering which need different tools to detect.
One kind is that the lens elements are parallel but not lined up exactly at their center.
The other kind is that one or more elements are at an angle (i.e. tilted), which leads to a tilted focal plane and the typical one-sided unsharpness. Your lens clearly(!) suffers from the tilt problem. If I were you I wouldn't hestitate to send the lens back with some pictures that demonstrate the one-sided unsharpness and point out the possible tilt problem in your accompanying letter. No lenses are perfect, but if the asymmetric sharpness distribution can even be detected on such scaled-down images this is simply unacceptable, especially for a premium-priced prime lens.

UPDATE

The following is what CRIS says is the current repair status. I thought for sure the problem was a tilted lens element as you suggested, but from this status I gather the problem was in the focus mechanism.

REDO
REPAIRED FOCUS MECHANISM
ADJUSTED FOR OPTIMUM RESOLUTION
CLEANED AND CHECKED TO MANUFACTURER'S SPECIFICATIONS

[sterretje]

ADJUSTED FOR OPTIMUM RESOLUTION

Resolution was not optimal at the right hand side. Might still have been an alignment (tilt) issue. Hope you get a decent working lens back.

[Tim A]

Resolution was not optimal at the right hand side. Might still have been an alignment (tilt) issue. Hope you get a decent working lens back.

Thank you.

I was probably to quick to consider the adjustment they did as something routinely done on all lenses rather than a fix for my problem. If that was the fix, I hope they didn't try to compensate for a tilted lens element by adjustment rather than realigning the element.

In any case, the lens has been shipped. They are having it take the slow scenic route.

[Tim A]

I just got an email from CRIS that said:

Our technician found the lens was not focusing correctly. When the focus was adjusted it became tight which required shifting the focus mechanism. He also adjusted the optics for proper centering. The lens should focus properly once you get it back.

[Hemppo]

Im suffering exactly same problem with my 15mm Ltd.

In my case I think that at least one of the elements has become loose with time as I'm able to capture some decent images if I shake the front barrel a bit with my fingers. The front barrel has noticeable play so I'm afraid that the front barrel grooves have weared a bit on use also.

I put a sample photo here. One can see that the right side of the image is all blurry when the center area is fine.

Here is a little update to my situation.

I sent the lens for a repair to a non official pentax service. They tightened the rear lensgroup and adjusted focus. When I got the lens back it revealed that now the lens was sharp only at the very center of the image all corners were extremely unsharp and blurry. I sent the lens back and they changed the rear group holder and tried to adjust focus but now I caught the bad news from them saying that they were not able to get corners any better.

Now I'am waiting the lens to be posted back and was considering if I should send the lens to an official pentax service. The problem is that nowadays there ain't pentax service in my country anymore and I should send it to Sweden...

The lens has already been on its' service trip for 7 months and I find it very frustrating to send it abroad for who knows how long...

Reading this topic I have understood that this little bugger can be pretty hard to adjust properly? So I was thinking if official service could have more knowledge with this lens.