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**nicoprod** · 03-23-2014, 09:57 AM

Originally posted by tibbitts

Since I never had an issue with any of them wide open at infinity (well, other than the general not-quite-as-good wide open vs. stopped down a little characteristic that I think everyone would expect),

How can you expect a lens to be at its best or close to its best at large aperture and infinity? That's what I mean by "meant for". It's true for all fast primes. See example below. first at 1.7 , second at 5.6.

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ironlionzion · 03-23-2014, 10:03 AM

Originally posted by nicoprod

I kind of see what you mean. Can you take a picture of a brick wall at 5.6 or a cereal box at 2.8? I think it would be easier to determine if there's something wrong with your fa31.

I just took these. They aren't the most scientific, but I did try to align the camera and plane of focus parallel with the brick wall. Again, this is center point focused. Here's it at f2.8. The blur is apparent at f4 and f5.6, but less noticeable than at f2.8. Here's the full frame, left crop, and right crop.

**nicoprod** · 03-23-2014, 10:08 AM

Originally posted by ironlionzion

I just took these. They aren't the most scientific, but I did try to align the camera and plane of focus parallel with the brick wall. Again, this is center point focused. Here's it at f2.8. The blur is apparent at f4 and f5.6, but less noticeable than at f2.8. Here's the full frame, left crop, and right crop.

ok it's pretty clear now. However, the left side of the wall is closer than the right side in this picture.

ironlionzion · 03-23-2014, 10:14 AM

Originally posted by nicoprod

ok it's pretty clear now. However, the left side of the wall is closer than the right side in this picture.

Yup I just noticed that. I retook the pictures to align it better. Actually, it seems like there is some distortion going on as well that makes it seem like the left side is closer than the right. I actually had to aim a little left (making the right side closer) for it to look even. Here are results at f4. Still blurry on the right.

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**nicoprod** · 03-23-2014, 10:17 AM

Originally posted by ironlionzion

Still blurry on the right

It sure is. And only your FA31 is doing this? is the middle in focus like the left is?

ironlionzion · 03-23-2014, 10:19 AM

Originally posted by nicoprod

It sure is. And only your FA31 is doing this?

Yup, only the 31mm. I have a K-x (the snow sunset picture was shot with it) and a K-3 which I've used for the other shots. It's definitely not the camera.

**nicoprod** · 03-23-2014, 10:20 AM

Originally posted by ironlionzion

Yup, only the 31mm. I have a K-x (the snow sunset picture was shot with it) and a K-3 which I've used for the other shots. It's definitely not the camera.

That sucks. I hope it's under warranty...

ironlionzion · 03-23-2014, 10:29 AM

Originally posted by nicoprod

That sucks. I hope it's under warranty...

Unfortunately it's not. I bought it used from a reputable user on these forums. I have other lenses I used for landscapes, so the 31mm has been used mainly for portraits and at wide apertures, where this blur at the edge of the frame is not noticeable. I can't be sure whether this decentering was there when I bought it or occurred under my possession (though I've never dropped or mishandled it).

**nicoprod** · 03-23-2014, 10:31 AM

Pentax offers free estimate for repair. It's $100 per hour after estimate I think. You should call....

ironlionzion · 03-23-2014, 10:39 AM

Originally posted by nicoprod

Pentax offers free estimate for repair. It's $100 per hour after estimate I think. You should call....

Ouch, I'll give them a call tomorrow during their open hours. I'd imagine it would take them at least a couple hours to diagnose, disassemble, fix, calibrate, and reassemble the lens. In the meantime, I'll see if anyone wants a decentered 31mm on the marketplace.

tibbitts · 03-23-2014, 12:00 PM

Originally posted by nicoprod

How can you expect a lens to be at its best or close to its best at large aperture and infinity? That's what I mean by "meant for". It's true for all fast primes. See example below. first at 1.7 , second at 5.6.

As I said in the statement you quoted,

Quote:

other than the general not-quite-as-good wide open vs. stopped down a little characteristic that I think everyone would expect

We all accept that almost any lens will probably perform at its best two or three stops down. I'm only questioning the infinity part. I'm guessing the lens was designed/intended to be used wide open at infinity to the same extent it was designed to be used wide open at, say, 5ft.

However, you've raised an important point in lens testing. A lot of us use a test target. With a wide-angle or standard lens, maybe we print 5 8.5x11s and put them in the center and corners of the frame, but then you're looking at maybe 7 or 10 ft or so. You can't be far away because the targets just won't be large enough, and at those distances, you have to deal with distance differences between the centers and edges. Plus focus field curvature in some lenses. And with a telephoto, you might not easily have access to a flat surface large enough to get a sufficient distance away without something coming between the camera and target. So, most of us are limited in how we can test lenses at different distances, but I do believe some defects can show up more or less at different focusing distances, not just different apertures.

---------- Post added 03-23-2014 at 12:16 PM ----------

In looking at the 2nd set of example brick wall images, I can see the one side is a little worse than the other, but I can tell you that my Sigma showed more variation and still was "within spec." So you might be fighting an uphill battle there.

I'd suggest printing the 5 test targets and positioning them in the extreme corners and center, and then testing at different apertures. It's just easier to evaluate the relative performance from test targets than from the brick and mortar, although I've used both. On the other hand, test target images didn't help in my case, so what do I know...

People here have sometimes scoffed at my reports of problems with 5 out of 5 55-300s, but I've gotten worse brick and mortar pictures (worse as in more differences from corner to corner, not considering absolute resolution) from every one of them than what I'm seeing in that 2nd set of pictures.

---------- Post added 03-23-2014 at 12:19 PM ----------

Originally posted by ironlionzion

Ouch, I'll give them a call tomorrow during their open hours. I'd imagine it would take them at least a couple hours to diagnose, disassemble, fix, calibrate, and reassemble the lens. In the meantime, I'll see if anyone wants a decentered 31mm on the marketplace.

I don't think they'll tell you, especially on the phone, whether they're even capable of "calibrating" the lens. Calibrating autofocus, maybe. Centering - I'm not sure they can even test for it (as in measure it with scientific equipment, not by shooting brick walls), much less correct it, unless maybe the lens was just not assembled properly in the first place (has a loose element, for example.) I'd love for somebody who actually knows to tell me I'm wrong.

**nicoprod** · 03-23-2014, 02:20 PM

Originally posted by tibbitts

I'm only questioning the infinity part. I'm guessing the lens was designed/intended to be used wide open at infinity to the same extent it was designed to be used wide open at, say, 5ft.

This is where I disagree. Wide open at infinity only if it is your only way to get the image (not enough light). Why over feeding your sensor with light?

tibbitts · 03-23-2014, 02:42 PM

Originally posted by nicoprod

This is where I disagree. Wide open at infinity only if it is your only way to get the image (not enough light). Why over feeding your sensor with light?

But that has to do with the amount of light on the subject, not your distance to the subject. You can say the same thing - why overfeed the sensor - at 5 feet or infinity. You may be trying for selective focus at infinity, where you blur the foreground (foliage, etc.), and only the infinity parts are in focus. Same with a portrait at 5ft, you might be trying to blur the background. I'm only disagreeing that the lens was specifically designed to give better wide-open performance at 5ft vs. infinity - or even that if it was, that there would be a way for the consumer to determine this.

**nicoprod** · 03-23-2014, 03:04 PM

Yea I get what you're saying... For me shooting wide open at infinity is a little crazy, unless that's the only way to get the picture. Large aperture photography is a precision thing. The DOF is too small to shoot a landscape for instance. I actually never go wider than f2.2 with the 31.

**Digitalis** · 03-23-2014, 06:03 PM

Originally posted by nicoprod

For me shooting wide open at infinity is a little crazy, unless that's the only way to get the picture.

Sometimes it IS the only way to get the image:

Pentax K5IIs SMCP-FA 31mm f/1.8 ASPH limited- ISO 800 @ f/1.8 20s

03-23-2014, 06:03 PM	#150
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,695	Originally posted by nicoprod For me shooting wide open at infinity is a little crazy, unless that's the only way to get the picture. Sometimes it IS the only way to get the image: Pentax K5IIs SMCP-FA 31mm f/1.8 ASPH limited- ISO 800 @ f/1.8 20s Last edited by Digitalis; 03-23-2014 at 06:38 PM.

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03-23-2014, 10:08 AM	#138
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Originally posted by ironlionzion I just took these. They aren't the most scientific, but I did try to align the camera and plane of focus parallel with the brick wall. Again, this is center point focused. Here's it at f2.8. The blur is apparent at f4 and f5.6, but less noticeable than at f2.8. Here's the full frame, left crop, and right crop. ok it's pretty clear now. However, the left side of the wall is closer than the right side in this picture.

03-23-2014, 10:17 AM	#140
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Originally posted by ironlionzion Still blurry on the right It sure is. And only your FA31 is doing this? is the middle in focus like the left is?

03-23-2014, 10:19 AM	#141
ironlionzion Veteran Member Join Date: Dec 2010 Location: Dallas, TX Posts: 409	Originally posted by nicoprod It sure is. And only your FA31 is doing this? Yup, only the 31mm. I have a K-x (the snow sunset picture was shot with it) and a K-3 which I've used for the other shots. It's definitely not the camera.

03-23-2014, 10:20 AM	#142
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Originally posted by ironlionzion Yup, only the 31mm. I have a K-x (the snow sunset picture was shot with it) and a K-3 which I've used for the other shots. It's definitely not the camera. That sucks. I hope it's under warranty...

03-23-2014, 10:29 AM	#143
ironlionzion Veteran Member Join Date: Dec 2010 Location: Dallas, TX Posts: 409	Originally posted by nicoprod That sucks. I hope it's under warranty... Unfortunately it's not. I bought it used from a reputable user on these forums. I have other lenses I used for landscapes, so the 31mm has been used mainly for portraits and at wide apertures, where this blur at the edge of the frame is not noticeable. I can't be sure whether this decentering was there when I bought it or occurred under my possession (though I've never dropped or mishandled it).

03-23-2014, 10:31 AM	#144
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Pentax offers free estimate for repair. It's $100 per hour after estimate I think. You should call....

03-23-2014, 10:39 AM	#145
ironlionzion Veteran Member Join Date: Dec 2010 Location: Dallas, TX Posts: 409	Originally posted by nicoprod Pentax offers free estimate for repair. It's $100 per hour after estimate I think. You should call.... Ouch, I'll give them a call tomorrow during their open hours. I'd imagine it would take them at least a couple hours to diagnose, disassemble, fix, calibrate, and reassemble the lens. In the meantime, I'll see if anyone wants a decentered 31mm on the marketplace.

03-23-2014, 12:00 PM	#146
tibbitts Pentaxian Join Date: Apr 2007 Location: Oklahoma USA Posts: 2,196	Originally posted by nicoprod How can you expect a lens to be at its best or close to its best at large aperture and infinity? That's what I mean by "meant for". It's true for all fast primes. See example below. first at 1.7 , second at 5.6. As I said in the statement you quoted, Quote: other than the general not-quite-as-good wide open vs. stopped down a little characteristic that I think everyone would expect We all accept that almost any lens will probably perform at its best two or three stops down. I'm only questioning the infinity part. I'm guessing the lens was designed/intended to be used wide open at infinity to the same extent it was designed to be used wide open at, say, 5ft. However, you've raised an important point in lens testing. A lot of us use a test target. With a wide-angle or standard lens, maybe we print 5 8.5x11s and put them in the center and corners of the frame, but then you're looking at maybe 7 or 10 ft or so. You can't be far away because the targets just won't be large enough, and at those distances, you have to deal with distance differences between the centers and edges. Plus focus field curvature in some lenses. And with a telephoto, you might not easily have access to a flat surface large enough to get a sufficient distance away without something coming between the camera and target. So, most of us are limited in how we can test lenses at different distances, but I do believe some defects can show up more or less at different focusing distances, not just different apertures. ---------- Post added 03-23-2014 at 12:16 PM ---------- In looking at the 2nd set of example brick wall images, I can see the one side is a little worse than the other, but I can tell you that my Sigma showed more variation and still was "within spec." So you might be fighting an uphill battle there. I'd suggest printing the 5 test targets and positioning them in the extreme corners and center, and then testing at different apertures. It's just easier to evaluate the relative performance from test targets than from the brick and mortar, although I've used both. On the other hand, test target images didn't help in my case, so what do I know... People here have sometimes scoffed at my reports of problems with 5 out of 5 55-300s, but I've gotten worse brick and mortar pictures (worse as in more differences from corner to corner, not considering absolute resolution) from every one of them than what I'm seeing in that 2nd set of pictures. ---------- Post added 03-23-2014 at 12:19 PM ---------- Originally posted by ironlionzion Ouch, I'll give them a call tomorrow during their open hours. I'd imagine it would take them at least a couple hours to diagnose, disassemble, fix, calibrate, and reassemble the lens. In the meantime, I'll see if anyone wants a decentered 31mm on the marketplace. I don't think they'll tell you, especially on the phone, whether they're even capable of "calibrating" the lens. Calibrating autofocus, maybe. Centering - I'm not sure they can even test for it (as in measure it with scientific equipment, not by shooting brick walls), much less correct it, unless maybe the lens was just not assembled properly in the first place (has a loose element, for example.) I'd love for somebody who actually knows to tell me I'm wrong.

03-23-2014, 02:20 PM	#147
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Originally posted by tibbitts I'm only questioning the infinity part. I'm guessing the lens was designed/intended to be used wide open at infinity to the same extent it was designed to be used wide open at, say, 5ft. This is where I disagree. Wide open at infinity only if it is your only way to get the image (not enough light). Why over feeding your sensor with light?

03-23-2014, 02:42 PM	#148
tibbitts Pentaxian Join Date: Apr 2007 Location: Oklahoma USA Posts: 2,196	Originally posted by nicoprod This is where I disagree. Wide open at infinity only if it is your only way to get the image (not enough light). Why over feeding your sensor with light? But that has to do with the amount of light on the subject, not your distance to the subject. You can say the same thing - why overfeed the sensor - at 5 feet or infinity. You may be trying for selective focus at infinity, where you blur the foreground (foliage, etc.), and only the infinity parts are in focus. Same with a portrait at 5ft, you might be trying to blur the background. I'm only disagreeing that the lens was specifically designed to give better wide-open performance at 5ft vs. infinity - or even that if it was, that there would be a way for the consumer to determine this.

03-23-2014, 03:04 PM	#149
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Yea I get what you're saying... For me shooting wide open at infinity is a little crazy, unless that's the only way to get the picture. Large aperture photography is a precision thing. The DOF is too small to shoot a landscape for instance. I actually never go wider than f2.2 with the 31.

I'm terrified... 31mm comparisons

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