How to test Pentax 15-30 for decentering?

awscreo · 12-06-2017, 03:00 PM

Now i'm totally confused. I've shot a ton of sunset pics just now, on a tripod with ES, and everything comes out reasonably sharp in the corners and edges. Not sure what to make of it, hand-held i'm struggling to get a sharp corner to corner image, even thought the center is always sharp, but on tripod I have no issues. I shot Rokinon 14 hand-held and never had issues. There might be a user error somewhere in there, but I can't nail what exactly I'm doing wrong that prevents me from having good results hand held. I always thought UWA lenses are easier to deal with in terms of getting sharp results since the dof is so deep..

**clackers** · 12-06-2017, 05:08 PM

Originally posted by awscreo

Now i'm totally confused. I've shot a ton of sunset pics just now, on a tripod with ES, and everything comes out reasonably sharp in the corners and edges.

If the lens was decentred, it would be soft in all conditions, so let's put this down to some of all the other factors ... technique, setup, subject matter, shutter shake, etc ... rather than the lens itself.

A quick and dirty way of testing decentering is to take four pictures of the same scene rotating the body 90 degrees each time. If one side is soft, that will 'follow' the rotation in the images.

awscreo · 12-06-2017, 05:34 PM

Originally posted by clackers

If the lens was decentred, it would be soft in all conditions, so let's put this down to some of all the other factors ... technique, setup, subject matter, shutter shake, etc ... rather than the lens itself.

A quick and dirty way of testing decentering is to take four pictures of the same scene rotating the body 90 degrees each time. If one side is soft, that will 'follow' the rotation in the images.

Yup, I'm guessing it's something else. I mean the corners are not completely even, but I think it's within reasonable expectations. I'm just curious how come my hand held shots are sharp in center but soft on edges.

**clackers** · 12-06-2017, 05:40 PM

Originally posted by awscreo

Yup, I'm guessing it's something else. I mean the corners are not completely even, but I think it's within reasonable expectations. I'm just curious how come my hand held shots are sharp in center but soft on edges.

Well, what you do is you do a bunch of tests, changing just one variable at a time, examining the results ...

**sbh** · 12-06-2017, 06:02 PM

I tested by shooting level in a right angle to a wall. It's important to be in a right angle and level. Your brick wall picture shows that you shot slightly to the right because the lines are not straight. They should be perfectly horizontal and vetical.
Set the aperture to 2.8 where it is most noticeable and compare the results with another lens. Tape pieces of newspaper to the wall in the edges of the frame. 30mm should be fine.

calsan · 12-07-2017, 12:50 AM - **1 Like**

At this point, I would suggest you stop doing tests and just send it back. You won't ever be happy with it and if you keep it, it will only annoy you!

awscreo · 12-07-2017, 12:53 PM

Originally posted by calsan

At this point, I would suggest you stop doing tests and just send it back. You won't ever be happy with it and if you keep it, it will only annoy you!

I did send it back. I did more tests and I think it wasn't a great copy anyway. Corners were inconsistent, upper left and bottom left were always softer than right side, like half of photos taken on tripod show some softness on the left side as well. Hope next copy would be better, and hopefully better packaged.

awscreo · 12-16-2017, 07:30 PM

Ok so I got the replacement, this one definitely sits better on the camera (other one felt slightly loose despite the rubber gasket), but after running just quick test shots I still found the issue to persist while I'm hand-holding the camera vs on tripod (0 issues there). So I went around the web, digging. I was convinced that issue was my limited understanding of UWA optics, it felt like I'm missing something or doing something wrong rather than the second lens also having the same issue.

Well, I think this article is the answer: Field Curvature, A Tricky Problem in Photography

It does feel that I'm getting this issue only when I'm tilting the camera while taking the shot. I guess the bulbous frontal element of the lens has something to do with this, the focal plane is probably curved so what I'm seeing is the effects described above. I've never ran into this with any of my other lenses, so I simply didn't know what it was. I'll still take as many test shots as I can to make sure hardware is not at fault.

Lesson learned

Sandy Hancock · 12-16-2017, 09:00 PM - **1 Like**

You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made.

My DFA15-30 had an obvious problem in the right upper quadrant.

I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed.

I reckon your copy was/is fine.

awscreo · 12-16-2017, 09:05 PM

Originally posted by Sandy Hancock

You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made.

My DFA15-30 had an obvious problem in the right upper quadrant.

I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed.

I reckon your copy was/is fine.

No i had other shots I didn't post. I spent 2 days testing and sent it back because I thought there was an issue with the lens.

---------- Post added 12-16-17 at 09:14 PM ----------

Originally posted by Sandy Hancock

You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made.

My DFA15-30 had an obvious problem in the right upper quadrant.

I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed.

I reckon your copy was/is fine.

If you want - have a look at the sample that got me worried. Full res here: https://drive.google.com/open?id=1p5inCXUHwc9LjUNBfXyHsyla8OvOm2A_

If you've never seen focal plane this curved, wouldn't you be worried that your new $1400 isn't right?

Sandy Hancock · 12-16-2017, 09:31 PM

Well why didn't you start with that image? The left edge is *much* worse than the right.

awscreo · 12-16-2017, 10:00 PM

Originally posted by Sandy Hancock

Well why didn't you start with that image? The left edge is *much* worse than the right.

The church image shows the same thing actually. But weirdly enough when I used a tripod I couldn't replicate the issue, so I'm thinking it's my user error rather than anything being wrong with the lens.
I need to do some testing of the second copy, but the article about field curvature seems to indicate that it's probably what I'm running into, maybe I'm not focusing on a correct object/distance to have everything
in focus and there is a learning curve, I don't know.

awscreo · 12-17-2017, 02:47 PM

bleh, I'm so sad. I decided to compare my rokinon 14 2.8 vs this replacement lens, and I don't know what to think of it. Rokinon clearly is better across the frame, but especially that left side

If 15-30 is optically the way it should be, do I need to settle on an idea that a $300 prime designed a long time ago is that much better than a brand new pro zoom that costs over $1300??
Or it's the second decentered copy I got in the row? Has someone cursed me with bad copies?

This whole buying a new exciting pro zoom deal has me so disappointed.

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12-06-2017, 03:00 PM	#16
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Now i'm totally confused. I've shot a ton of sunset pics just now, on a tripod with ES, and everything comes out reasonably sharp in the corners and edges. Not sure what to make of it, hand-held i'm struggling to get a sharp corner to corner image, even thought the center is always sharp, but on tripod I have no issues. I shot Rokinon 14 hand-held and never had issues. There might be a user error somewhere in there, but I can't nail what exactly I'm doing wrong that prevents me from having good results hand held. I always thought UWA lenses are easier to deal with in terms of getting sharp results since the dof is so deep..

12-06-2017, 05:08 PM	#17
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by awscreo Now i'm totally confused. I've shot a ton of sunset pics just now, on a tripod with ES, and everything comes out reasonably sharp in the corners and edges. If the lens was decentred, it would be soft in all conditions, so let's put this down to some of all the other factors ... technique, setup, subject matter, shutter shake, etc ... rather than the lens itself. A quick and dirty way of testing decentering is to take four pictures of the same scene rotating the body 90 degrees each time. If one side is soft, that will 'follow' the rotation in the images.

12-06-2017, 05:34 PM	#18
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Originally posted by clackers If the lens was decentred, it would be soft in all conditions, so let's put this down to some of all the other factors ... technique, setup, subject matter, shutter shake, etc ... rather than the lens itself. A quick and dirty way of testing decentering is to take four pictures of the same scene rotating the body 90 degrees each time. If one side is soft, that will 'follow' the rotation in the images. Yup, I'm guessing it's something else. I mean the corners are not completely even, but I think it's within reasonable expectations. I'm just curious how come my hand held shots are sharp in center but soft on edges.

12-06-2017, 05:40 PM	#19
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by awscreo Yup, I'm guessing it's something else. I mean the corners are not completely even, but I think it's within reasonable expectations. I'm just curious how come my hand held shots are sharp in center but soft on edges. Well, what you do is you do a bunch of tests, changing just one variable at a time, examining the results ...

12-06-2017, 06:02 PM	#20
sbh Site Supporter Join Date: Oct 2012 Location: Black Forest, Germany Photos: Gallery Posts: 849	I tested by shooting level in a right angle to a wall. It's important to be in a right angle and level. Your brick wall picture shows that you shot slightly to the right because the lines are not straight. They should be perfectly horizontal and vetical. Set the aperture to 2.8 where it is most noticeable and compare the results with another lens. Tape pieces of newspaper to the wall in the edges of the frame. 30mm should be fine.

12-07-2017, 12:50 AM - 1 Like	#21
calsan Pentaxian Join Date: Nov 2008 Location: Perth, Australia Photos: Gallery Posts: 1,548	At this point, I would suggest you stop doing tests and just send it back. You won't ever be happy with it and if you keep it, it will only annoy you!

12-07-2017, 12:53 PM	#22
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Originally posted by calsan At this point, I would suggest you stop doing tests and just send it back. You won't ever be happy with it and if you keep it, it will only annoy you! I did send it back. I did more tests and I think it wasn't a great copy anyway. Corners were inconsistent, upper left and bottom left were always softer than right side, like half of photos taken on tripod show some softness on the left side as well. Hope next copy would be better, and hopefully better packaged.

12-16-2017, 07:30 PM	#23
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Ok so I got the replacement, this one definitely sits better on the camera (other one felt slightly loose despite the rubber gasket), but after running just quick test shots I still found the issue to persist while I'm hand-holding the camera vs on tripod (0 issues there). So I went around the web, digging. I was convinced that issue was my limited understanding of UWA optics, it felt like I'm missing something or doing something wrong rather than the second lens also having the same issue. Well, I think this article is the answer: Field Curvature, A Tricky Problem in Photography It does feel that I'm getting this issue only when I'm tilting the camera while taking the shot. I guess the bulbous frontal element of the lens has something to do with this, the focal plane is probably curved so what I'm seeing is the effects described above. I've never ran into this with any of my other lenses, so I simply didn't know what it was. I'll still take as many test shots as I can to make sure hardware is not at fault. Lesson learned

12-16-2017, 09:00 PM - 1 Like	#24
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,267	You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made. My DFA15-30 had an obvious problem in the right upper quadrant. I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed. I reckon your copy was/is fine.

12-16-2017, 09:05 PM	#25
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Originally posted by Sandy Hancock You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made. My DFA15-30 had an obvious problem in the right upper quadrant. I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed. I reckon your copy was/is fine. No i had other shots I didn't post. I spent 2 days testing and sent it back because I thought there was an issue with the lens. ---------- Post added 12-16-17 at 09:14 PM ---------- Originally posted by Sandy Hancock You sent a lens back because you had soft results hand held at 1/10 second in the fog? The other shots looked fine. The brick wall test shot was obviously not properly aligned, so no real conclusion could be made. My DFA15-30 had an obvious problem in the right upper quadrant. I sent it for assessment/repair under warranty. it went back to Japan and came back fully fixed. I reckon your copy was/is fine. If you want - have a look at the sample that got me worried. Full res here: https://drive.google.com/open?id=1p5inCXUHwc9LjUNBfXyHsyla8OvOm2A_ If you've never seen focal plane this curved, wouldn't you be worried that your new $1400 isn't right?

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12-16-2017, 09:31 PM	#26
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,267	Well why didn't you start with that image? The left edge is much worse than the right.

12-16-2017, 10:00 PM	#27
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	Originally posted by Sandy Hancock Well why didn't you start with that image? The left edge is much worse than the right. The church image shows the same thing actually. But weirdly enough when I used a tripod I couldn't replicate the issue, so I'm thinking it's my user error rather than anything being wrong with the lens. I need to do some testing of the second copy, but the article about field curvature seems to indicate that it's probably what I'm running into, maybe I'm not focusing on a correct object/distance to have everything in focus and there is a learning curve, I don't know.

12-17-2017, 02:47 PM	#28
awscreo Veteran Member Join Date: May 2016 Photos: Gallery \| Albums Posts: 2,722 Original Poster	bleh, I'm so sad. I decided to compare my rokinon 14 2.8 vs this replacement lens, and I don't know what to think of it. Rokinon clearly is better across the frame, but especially that left side If 15-30 is optically the way it should be, do I need to settle on an idea that a $300 prime designed a long time ago is that much better than a brand new pro zoom that costs over $1300?? Or it's the second decentered copy I got in the row? Has someone cursed me with bad copies? This whole buying a new exciting pro zoom deal has me so disappointed. Full res images available here - Update your browser to use Google Drive - Google Drive Help