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Author Topic: What's wrong with this lens?  (Read 1028 times)

shadowblade

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What's wrong with this lens?
« on: December 14, 2019, 11:00:10 am »

My 12-24 f/4 G seems to be playing up. Specifically, I can't get all four corners sharp at the same time, even stopped down to f/8 or f/11, at any focal length wider than 17mm or so (it seems usable at longer focal lengths). I had it serviced, but it came back the same way.

Some examples at 14mm f/4. All images taken at ISO 100, manually focused on a tripod, activated by timer, with an A7r3. Not the best test subject, but it's all I have right this minute.

Focus on centre

Focus on top left chart

Focus on top right chart

Focus on bottom left chart

Focus on bottom right chart

What's wrong with my lens? Is there a decentered or tilted element, or something else?
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Paul_Roark

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Re: What's wrong with this lens?
« Reply #1 on: December 14, 2019, 08:36:53 pm »

... I can't get all four corners sharp at the same time, ...

I usually see that as evidence of a de-centered element.

FWIW

Paul
www.PaulRoark.com
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shadowblade

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Re: What's wrong with this lens?
« Reply #2 on: December 14, 2019, 11:24:55 pm »

I usually see that as evidence of a de-centered element.

FWIW

Paul
www.PaulRoark.com

Thing is, every one of the four corners is sharp when focused. Just that, when one corner is in focus, one or two of the other corners are completely out of focus, and no amount of stopping down can bring it back into focus. Happens both close-up and at infinity.

I didn't see any obvious asymmetry doing the defocus test on a Siemens star chart.
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Shiftworker

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Re: What's wrong with this lens?
« Reply #3 on: December 15, 2019, 05:50:12 am »

Unless you have bought this lens to do flat copy work at close distances I'd test it in a real world situation at long distances.
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shadowblade

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Re: What's wrong with this lens?
« Reply #4 on: December 15, 2019, 08:06:10 am »

Unless you have bought this lens to do flat copy work at close distances I'd test it in a real world situation at long distances.

That's the main reason I tested it in the first place - it was giving me unusable real-world shots.

Previously, it was giving me sharp corners for all sorts of photos. Then, after a long, bumpy car ride, the problem appeared - at that time, I thought it was just the bottom right corner that was blurry. Sent it in for repair. Came back 'within spec'. The problem was still there. So I tested it against test charts on a wall and found this. Every corner is sharp if I focus on it, but only at the expense of making one or two other corners so out-of-focus that even stopping down at f/11 or shooting at any subject distance doesn't fix it.
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Paul_Roark

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Re: What's wrong with this lens?
« Reply #5 on: December 15, 2019, 11:28:32 am »


>... Previously, it was giving me sharp corners for all sorts of photos. Then, after a long, bumpy car ride, the problem appeared ...

So, it was presumably damaged.

>Sent it in for repair. Came back 'within spec'. The problem was still there.

I think that says more about the company than the optical formula.

>So I tested it against test charts on a wall ...

In the past I also used use wall charts, but I found a better & easier way.  I'm fortunately that I have a mountain ridge at (effectively) "infinity" and clear air.  So I do diagonal shots of the ridge and look at the sharpness across the field, in an "X" pattern.  That is when I gave up on Canon and switched to prime Leica M glass on a Sony body (with the KoloriVision thin cover-glass conversion).  The optical issues disappeared.  It's hard being a perfectionist, but I found a solution that works for me.

> ... one or two other corners so out-of-focus that even stopping down at f/11 or shooting at any subject distance doesn't fix it.

I sold a camera from a major manufacturer that had that problem.  I have no idea if it was a terrible optical formula or bad assembly.  But it was useless for me.

(BTW, the founder of the small company, which became part of Perken Elmer, and build the Hubble told me that the original flaw was an element that had been installed incorrectly -- flipped over.  I have no idea if that was true.  This guy had a house on Emerald Bay, overlooking a popular beach just north of Laguna Beach in California.  In his living room was a 10" reflector telescope [not much longer than it was wide] -- aimed down at the beach.  Hmm ...)

Paul
www.PaulRoark.com



[/quote]
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shadowblade

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Re: What's wrong with this lens?
« Reply #6 on: December 15, 2019, 03:23:47 pm »

I've got no doubt the optical formula is great. In the 16-20mm range, it's as sharp as any other UWA. In the 12-15mm range, it's sharper than most of the primes available, including the popular Voigtlander Heliars.

I suspect something lost alignment that day and caused the problem. But I'm trying to describe the problem so that they can look into it properly, rather than simply sending it back again because, when they focused into each corner one at a time, that particular corner was acceptably sharp.

Unfortunately, I don't have a nearby wide-open landscape ti test it on, where I can get all four corners at infinity.  So, test charts it is...
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Shiftworker

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Re: What's wrong with this lens?
« Reply #7 on: December 16, 2019, 02:49:25 am »

Unfortunately, I don't have a nearby wide-open landscape ti test it on, where I can get all four corners at infinity.  So, test charts it is...
You don't need an open landscape - just focus on a distant object in the center of the frame and then without altering the focus recompose so that object is in all 4 corners.
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JaapD

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Re: What's wrong with this lens?
« Reply #8 on: December 16, 2019, 06:13:28 am »

Your image shows heavy tilting, meaning the camera’s sensor is not plan parallel to the window and its test charts.
As a result you may expect decentering related test ressults.

Regards,
Jaap.
« Last Edit: December 16, 2019, 06:22:37 am by JaapD »
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degrub

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Re: What's wrong with this lens?
« Reply #9 on: December 16, 2019, 09:43:58 pm »

Issue with the adapter or mount .
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shadowblade

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Re: What's wrong with this lens?
« Reply #10 on: December 17, 2019, 01:37:14 am »

Your image shows heavy tilting, meaning the camera’s sensor is not plan parallel to the window and its test charts.
As a result you may expect decentering related test ressults.

Regards,
Jaap.

It was only very slightly tilted (limitation due to the mini-tripod I had in my bag when shooting it during a break at work), but, at 14mm, that shows up as heavy keystoning. I can shoot some better test shots using my geared head when I get some time, but the problem should be obvious enough even with an imperfectly aligned shot. In any case, the keystoning is only in vertical tilt, not horizontal - both top corners are the same distance as each other, as are both bottom corners, so, when the bottom left is in focus, the bottom right should also be in focus. But that's not the case here. Also, at the testing distance and aperture, everything should have been more or less in focus anyway. It's certainly obvious enough in real-life shots - not just a bit of corner unsharpness, but all details in one or two corners smeared to oblivion, even at f/8.

Although I'm starting to think I might lose less money just selling it and buying another one rather than going through back-and-forth arguments with the local repair/service company.
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shadowblade

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Re: What's wrong with this lens?
« Reply #11 on: December 17, 2019, 01:41:04 am »

Issue with the adapter or mount .

It's a native lens, so no adapter. It's also the only lens that's affected, and it's affected the same way on three different bodies, so it's not the mount either.
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