Nikon Z 35/1.8 S - hard to find a well centered copy?

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1llusive
1llusive Veteran Member • Posts: 3,422
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

kenw wrote:

1llusive wrote:

kenw wrote:

Probably different standards and different ways of using the lens would be my best guess!

I think a good example of that would be something like the Samyang/Rokinon 135/2 which is a hit both with the portrait photography crowd and the astro crowd. The astro crowd finds a lot more "bad" copies because every photo they take of the sky near wide open is essentially a very sensitive test for "decentering". Meanwhile the portrait crowd is going to say all their lenses are just fine because of course the corners and edges of all their photos are out of focus anyway.

As you mention in your other post, beyond any reasonable distance you start to get blurring from the air. So while a shot of a star field seems to be an ideal test, I think shutter speeds need to be very short and I think several images need to be taken and analyzed to be sure you aren't knocking a lens corner when it's just the atmosphere.

Pointing up into the sky and pointing along the surface of the earth have dramatically different seeing profiles (along the surface of the earth is far, far, far worse). So while 100mm is often problematic pointing at a distant terrestrial target it is not often an issue pointing up at the zenith. But still, seeing varies *a lot* from night to night and the location you shoot an astro photo also impacts your local seeing a lot as well.

On a Z7 with a 100mm lens a pixel has about a 9 arcsecond angle of view (unless I screwed up the math). Seeing limits of 9 arcseconds at zenith are incredibly bad and are actually beyond the lowest seeing rating of typical scales used to grade astronomical seeing. So it sure seems like down around 135mm a star field near zenith is a perfectly reasonable test target? Or did I make a mistake somewhere?

What about wider focal lengths?

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OP quintana Regular Member • Posts: 447
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

I think I can safely rule out technological or atmospheric issues because I used shutter speeds of 1/500th of a second for f/2 and 1/125th of a second for f/4 plus IBIS handheld which usually results in a 100% success rate. When in doubt I also repeat the test.

The distance to the target was as always about 100 meters which should be close enough to rule out atmospheric causes for unsharp corners. 
i also have experience with a couple of dozens of lenses that I have tested with this method and from my regular photography I know how atmospheric issues look like in the photos.

i don’t want to say that my tests are always 100% flawless but I think that I can see from experience when the results look like something with the test went wrong.

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michaeladawson Forum Pro • Posts: 15,000
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

I think there's actually two separate topics here.  One is about seeing conditions and the other is about being picky about the potential IQ of the lens.

An astrophotographer is going to be much more picky about flat-fieldness, decentering, tilt, etc.  If they don't have a good lens, seeing conditions aren't going to matter.  And I don't think anyone was actually suggesting using a star field to test the lens.

Seeing conditions is a separate topic.  I haven't done any wide field astro photography so I can't comment there.  But I used to be an avid telescope user for looking at nebula, planets, etc.  And I can tell you that there can easily be bad viewing conditions pointing straight up.  Bad enough that it looks as if you're viewing Jupiter behind a shimmering Coke bottle.  Now, granted, that is highly magnified viewing that is going to significantly amplify the effects of any atmospheric turbulence.

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Mike Dawson

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kenw
kenw Veteran Member • Posts: 6,325
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

1llusive wrote:

kenw wrote:

1llusive wrote:

kenw wrote:

Probably different standards and different ways of using the lens would be my best guess!

I think a good example of that would be something like the Samyang/Rokinon 135/2 which is a hit both with the portrait photography crowd and the astro crowd. The astro crowd finds a lot more "bad" copies because every photo they take of the sky near wide open is essentially a very sensitive test for "decentering". Meanwhile the portrait crowd is going to say all their lenses are just fine because of course the corners and edges of all their photos are out of focus anyway.

As you mention in your other post, beyond any reasonable distance you start to get blurring from the air. So while a shot of a star field seems to be an ideal test, I think shutter speeds need to be very short and I think several images need to be taken and analyzed to be sure you aren't knocking a lens corner when it's just the atmosphere.

Pointing up into the sky and pointing along the surface of the earth have dramatically different seeing profiles (along the surface of the earth is far, far, far worse). So while 100mm is often problematic pointing at a distant terrestrial target it is not often an issue pointing up at the zenith. But still, seeing varies *a lot* from night to night and the location you shoot an astro photo also impacts your local seeing a lot as well.

On a Z7 with a 100mm lens a pixel has about a 9 arcsecond angle of view (unless I screwed up the math). Seeing limits of 9 arcseconds at zenith are incredibly bad and are actually beyond the lowest seeing rating of typical scales used to grade astronomical seeing. So it sure seems like down around 135mm a star field near zenith is a perfectly reasonable test target? Or did I make a mistake somewhere?

What about wider focal lengths?

Then it is even *less* of an issue of course...

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Ken W
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kenw
kenw Veteran Member • Posts: 6,325
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?
1

michaeladawson wrote:

I think there's actually two separate topics here. One is about seeing conditions and the other is about being picky about the potential IQ of the lens.

An astrophotographer is going to be much more picky about flat-fieldness, decentering, tilt, etc. If they don't have a good lens, seeing conditions aren't going to matter. And I don't think anyone was actually suggesting using a star field to test the lens.

Yes, that was my point.  An astrophotographer is going to notice IQ issues in their day to day (well, night to night) photography without even doing any testing that a portrait photographer isn't.

Using a star field as an actual lens screening tool is probably not a good idea except for an experience astrophotographer who knows what to expect and how to evaluate conditions and the appropriateness of doing so for any given setup.

Seeing conditions is a separate topic. I haven't done any wide field astro photography so I can't comment there. But I used to be an avid telescope user for looking at nebula, planets, etc. And I can tell you that there can easily be bad viewing conditions pointing straight up. Bad enough that it looks as if you're viewing Jupiter behind a shimmering Coke bottle. Now, granted, that is highly magnified viewing that is going to significantly amplify the effects of any atmospheric turbulence.

Oh my goodness yes it can be really bad at high magnification.  Having done amateur astronomy from a desert I can attest to that!  But for reference Jupiter is about 30 arc seconds in size at the moment, or just around 3 pixels for a 100mm lens on a Z7.  So even if Jupiter were to jump half the width of its disk in your telescope (which would be unimaginably bad seeing) it would be a tiny amount of blur for a 100mm on a Z7.

But, anyway - we are drifting well off topic!  The important take away is actually "testing" a lens on a distant terrestrial target is not a particularly critical test and especially for longer focal lengths can even lead someone to think there might be something wrong with their lens when there isn't.  For shorter focal lengths as Jim pointed out it does have the benefit of being difficult to screw up and readily available to most people - with all the caveats Jim outlined.  I use it and variations on it as part of my lens screen testing as it is one of the fastest and easiest tests to do quickly.

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Ken W
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kenw
kenw Veteran Member • Posts: 6,325
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?
4

quintana wrote:

I think I can safely rule out technological or atmospheric issues because I used shutter speeds of 1/500th of a second for f/2 and 1/125th of a second for f/4 plus IBIS handheld which usually results in a 100% success rate. When in doubt I also repeat the test.

Red flag! Red flag!  DO NOT USE IBIS!!!

This may not be the issue with your current test, but a big problem with using IBIS is that of course you *shouldn't* expect the corner performance to be symmetric since when IBIS is running the sensor is not centered on the optical axis of the lens.

Since you say you did the test a bunch of times and you were able to make the softness go away with refocusing on the corner it sounds like there is something actually tilted with the lens so maybe IBIS isn't an issue here.  But in general don't use IBIS (or OIS) if trying to compare corners or edges of a lens.

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1llusive
1llusive Veteran Member • Posts: 3,422
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

kenw wrote:

1llusive wrote:

kenw wrote:

1llusive wrote:

kenw wrote:

Probably different standards and different ways of using the lens would be my best guess!

I think a good example of that would be something like the Samyang/Rokinon 135/2 which is a hit both with the portrait photography crowd and the astro crowd. The astro crowd finds a lot more "bad" copies because every photo they take of the sky near wide open is essentially a very sensitive test for "decentering". Meanwhile the portrait crowd is going to say all their lenses are just fine because of course the corners and edges of all their photos are out of focus anyway.

As you mention in your other post, beyond any reasonable distance you start to get blurring from the air. So while a shot of a star field seems to be an ideal test, I think shutter speeds need to be very short and I think several images need to be taken and analyzed to be sure you aren't knocking a lens corner when it's just the atmosphere.

Pointing up into the sky and pointing along the surface of the earth have dramatically different seeing profiles (along the surface of the earth is far, far, far worse). So while 100mm is often problematic pointing at a distant terrestrial target it is not often an issue pointing up at the zenith. But still, seeing varies *a lot* from night to night and the location you shoot an astro photo also impacts your local seeing a lot as well.

On a Z7 with a 100mm lens a pixel has about a 9 arcsecond angle of view (unless I screwed up the math). Seeing limits of 9 arcseconds at zenith are incredibly bad and are actually beyond the lowest seeing rating of typical scales used to grade astronomical seeing. So it sure seems like down around 135mm a star field near zenith is a perfectly reasonable test target? Or did I make a mistake somewhere?

What about wider focal lengths?

Then it is even *less* of an issue of course...

Okay, that's what I thought but the way you worded your post made it sound like the opposite. So you're saying a star test is less accurate at long focal lengths? That would make sense to me.

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JimKasson
JimKasson Forum Pro • Posts: 32,542
Short exposures and atmospheric disturbances

Short exposures don't mitigate atmospheric disturbances. They do make the effects in the image more erratic from exposure to exposure, though. Not what you want for lens testing.

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OP quintana Regular Member • Posts: 447
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

OK, I will remember that.

I already was sceptical about using IBIS because of the issues that you wrote about but I have never read anything about this in a forum (until now) so I didn’t really investigate further.

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EricTheAstroJunkie Contributing Member • Posts: 815
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?
2

kenw wrote:

1llusive wrote:

kenw wrote:

Probably different standards and different ways of using the lens would be my best guess!

I think a good example of that would be something like the Samyang/Rokinon 135/2 which is a hit both with the portrait photography crowd and the astro crowd. The astro crowd finds a lot more "bad" copies because every photo they take of the sky near wide open is essentially a very sensitive test for "decentering". Meanwhile the portrait crowd is going to say all their lenses are just fine because of course the corners and edges of all their photos are out of focus anyway.

As you mention in your other post, beyond any reasonable distance you start to get blurring from the air. So while a shot of a star field seems to be an ideal test, I think shutter speeds need to be very short and I think several images need to be taken and analyzed to be sure you aren't knocking a lens corner when it's just the atmosphere.

Pointing up into the sky and pointing along the surface of the earth have dramatically different seeing profiles (along the surface of the earth is far, far, far worse). So while 100mm is often problematic pointing at a distant terrestrial target it is not often an issue pointing up at the zenith. But still, seeing varies *a lot* from night to night and the location you shoot an astro photo also impacts your local seeing a lot as well.

On a Z7 with a 100mm lens a pixel has about a 9 arcsecond angle of view (unless I screwed up the math). Seeing limits of 9 arcseconds at zenith are incredibly bad and are actually beyond the lowest seeing rating of typical scales used to grade astronomical seeing. So it sure seems like down around 135mm a star field near zenith is a perfectly reasonable test target? Or did I make a mistake somewhere?

You are correct that with the Z7 and a 100mm lens you have a 9 arc-sec pixel scale, the reality is that you are sooooo far undersampled at 9 arc-sec that seeing is something that shouldn't even be remotely worried about. Generally seeing is something that comes into play when you image at sub-1 arc-sec resolutions, I have several telescope setups ranging from 0.86 arc-sec up to 3 arc-sec scales (with dedicated astro cams, if I use my Z7 or D5300 it goes down to 3.6 arc-sec), where we live (mountains outside SLC, Utah) the seeing is generally below average most of the time and when it is I don't use the 0.86" setup, I'll stick to my 2" setup unless I'm doing narrowband imaging. In any case, when testing a lens for aberrations or decentering the seeing conditions if imaging zenith shouldn't impact the test at all unless you are shooting much longer focal lengths. With the Z7 if you wanted a sub-1" pixel scale you'd need 900mm+ focal length.

I'd be much more worried about atmospheric pollution (during the winters here we get really bad inversions, for instance) impacting infinity distance tests that atmospheric seeing. Ironically we had 2 clear-ish nights the last few nights with some of the best seeing I've ever experienced at our home, got a lot of good narrowband h-alpha imaging done of a tough target (IC63).

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Dan DeLion Contributing Member • Posts: 900
Pompous
1

kenw wrote:

quintana wrote:

JimKasson wrote:

To the OP: please post images like the ones that Ken posted. Are the lenses decentered or tilted? You can tell be focusing on the target in the bad corner.

Currently I am not at home so I can not post the pictures at the moment because they are on my hard drive.

I don’t test my lenses indoors but like this: http://www.gletscherbruch.de/foto/test/dezentrierung/dezentrierung.html

It might not be as sophisticated as the test Ken did but still it should result in 4 corners with the same level of sharpness when the copy of the lens is perfect.

I use a test like that myself as a quick screening. For normal to wide angle lenses like the 35/1.8 it is a pretty good quick check. Often if this kind of test looks good then I don't bother doing the more careful Siemens star test. If this kind of test looks poor then I start doing more testing to be sure.

As others already pointed out this kind of test can definitely be problematic with longer focal length lenses because you start to see atmospheric effects instead of lens effects! Even then though if your shutter speed is high enough (say about 1/250) the atmospheric effects result in small scale distortions rather than the blurriness and smearing of lens aberrations. So if you are familiar with how things should look from experience you can still use this kind of test on long focal length lenses to detect obvious asymmetry but it is far from ideal.

My typical procedure is to use something like the Gletscherbruch test first and if it doesn't look good then I do more careful and controlled testing. Whenever a lens test looks bad I usually assume the problem is with me or the test first. It is too easy to make mistakes and so demonstrated repeatability of the results using more than one method is a good practice before declaring anything "bad".

If I focus both not-so-perfect-copies of the Z 35/1.8 on the bad corner it gets as sharp as I would expect. This probably means that it’s tilted and not decentered, right? Until I read your answer I have to admit that I always thought that decentered and tilted mean the same.

That does suggest that it is likely "tilt" but isn't conclusive. As Jim pointed out though the target might not be appropriate for really showing what is going on. For example if the target has detail in only one dimension you might not detect astigmatism. If look at my tests you'll see that one axis of the star looks great while in the other everything is falling apart.

But yes, in the forums "decentered" has become the way many refer to any sort of asymmetry in aberrations even though strictly the lens might not have any "decentered" elements in it.

Ken - Seriously, your really starting to sound pompous.  Your "standards" are so high and your testing so primitive, I'm surprised any lens is good enough for you!

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kenw
kenw Veteran Member • Posts: 6,325
Re: Nikon Z 35/1.8 S - hard to find a well centered copy?

1llusive wrote:

kenw wrote:

1llusive wrote:

kenw wrote:

1llusive wrote:

kenw wrote:

Probably different standards and different ways of using the lens would be my best guess!

I think a good example of that would be something like the Samyang/Rokinon 135/2 which is a hit both with the portrait photography crowd and the astro crowd. The astro crowd finds a lot more "bad" copies because every photo they take of the sky near wide open is essentially a very sensitive test for "decentering". Meanwhile the portrait crowd is going to say all their lenses are just fine because of course the corners and edges of all their photos are out of focus anyway.

As you mention in your other post, beyond any reasonable distance you start to get blurring from the air. So while a shot of a star field seems to be an ideal test, I think shutter speeds need to be very short and I think several images need to be taken and analyzed to be sure you aren't knocking a lens corner when it's just the atmosphere.

Pointing up into the sky and pointing along the surface of the earth have dramatically different seeing profiles (along the surface of the earth is far, far, far worse). So while 100mm is often problematic pointing at a distant terrestrial target it is not often an issue pointing up at the zenith. But still, seeing varies *a lot* from night to night and the location you shoot an astro photo also impacts your local seeing a lot as well.

On a Z7 with a 100mm lens a pixel has about a 9 arcsecond angle of view (unless I screwed up the math). Seeing limits of 9 arcseconds at zenith are incredibly bad and are actually beyond the lowest seeing rating of typical scales used to grade astronomical seeing. So it sure seems like down around 135mm a star field near zenith is a perfectly reasonable test target? Or did I make a mistake somewhere?

What about wider focal lengths?

Then it is even *less* of an issue of course...

Okay, that's what I thought but the way you worded your post made it sound like the opposite. So you're saying a star test is less accurate at long focal lengths? That would make sense to me.

Apologies if my post wasn't very clear!  Yes, I'm saying at longer focal lengths seeing becomes more of an issue and at shorter focal lengths less of an issue.

I was just pointing for the 135mm Samyang I was tossing out that it is unlikely to ever be an issue for a star field near zenith.  At some longer focal lengths you'll need to start to worry.

And again to be clear, shooting a distant terrestrial target like a building or clock tower is subject to far worse effects than pointing up into the sky.  And thus impacted at shorter focal lengths.

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Ken W
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1llusive
1llusive Veteran Member • Posts: 3,422
Re: Short exposures and atmospheric disturbances

JimKasson wrote:

Short exposures don't mitigate atmospheric disturbances. They do make the effects in the image more erratic from exposure to exposure, though. Not what you want for lens testing.

But longer exposures will blur the entire image, so pick your poison. The idea behind short exposures is a sanity check to not fool yourself into thinking you have a "bad copy" just because one image had a bad corner.

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JimKasson
JimKasson Forum Pro • Posts: 32,542
Re: Short exposures and atmospheric disturbances

1llusive wrote:

JimKasson wrote:

Short exposures don't mitigate atmospheric disturbances. They do make the effects in the image more erratic from exposure to exposure, though. Not what you want for lens testing.

But longer exposures will blur the entire image, so pick your poison. The idea behind short exposures is a sanity check to not fool yourself into thinking you have a "bad copy" just because one image had a bad corner.

Short exposures can do just that, even worse than long ones. With short exposures, the amount of sharpness and distortion varies more from shot to shot.

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Rhaegar Regular Member • Posts: 218
Re: Short exposures and atmospheric disturbances

might have missed it, but is there any shorter, quicker version of these numerous tests one can do indoors to just be assured you didn't get a lemon?

some of these tests described are so far out of the reach of what your standard hobbyist is going to attempt

kenw
kenw Veteran Member • Posts: 6,325
Re: Short exposures and atmospheric disturbances
1

Rhaegar wrote:

might have missed it, but is there any shorter, quicker version of these numerous tests one can do indoors to just be assured you didn't get a lemon?

some of these tests described are so far out of the reach of what your standard hobbyist is going to attempt

Jim Kasson's test is actually really easy for any hobbyist to do with the only real constraint being having enough distance between the camera and the target which usually forces you outdoors. But that's a pretty low burden all things considered for a test which is very difficult to get wrong.  If you live above the Arctic circle I'm sure the burden is quite a bit worse...

For Jim's test you don't actually need to use a big Siemens star target either though he recommends it for good reasons. Really anything with high contrast detail will work that is large enough will work. If you have a small printer you can just tape together a larger target on a piece of cardboard. The exact details of the target really aren't critical for seeing if you've got a "lemon" or not.

The theory of the test and the execution are really simple:

https://blog.kasson.com/lens-screening-testing/theory-of-the-test/

Basically just look up on Jim's plots the minimum distance your target needs to be for a particular focal length and aperture and then shoot away.  Again, the distance is just a rough minimum and so there is no need to be precise at all in the exact distance you set the target from the camera.  And by design Jim's test requires no careful alignment of the test target to the focal plane.

Indoor tests often end up shot with test charts or targets that are small enough and shot at distances close enough that aligning the focal plane and the target to be parallel is critical and this is what most people mess up.  If you really want to shoot indoors use as large a test target as possible shot from as far away as possible and use whatever tools and understanding of geometry you can to ensure the target and the focal plane are parallel.

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Ken W
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JimKasson
JimKasson Forum Pro • Posts: 32,542
Re: Short exposures and atmospheric disturbances

Rhaegar wrote:

might have missed it, but is there any shorter, quicker version of these numerous tests one can do indoors to just be assured you didn't get a lemon?

some of these tests described are so far out of the reach of what your standard hobbyist is going to attempt

Shooting an ISO 12233 target is quick and can be done indoors, but it takes highly precise setup, a large target, and doesn't work for short lenses. Analysis of the results is a bit tricky if you don't use a slanted edge program.

Some people have gotten good results by shooting a Siemens Star and defocusing. I find interpretation of those shots difficult.

What's wrong with testing outdoors?

Jim

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JimKasson
JimKasson Forum Pro • Posts: 32,542
Re: Short exposures and atmospheric disturbances

kenw wrote:

Jim Kasson's test is actually really easy for any hobbyist to do with the only real constraint being having enough distance between the camera and the target which usually forces you outdoors. But that's a pretty low burden all things considered for a test which is very difficult to get wrong. If you live above the Arctic circle I'm sure the burden is quite a bit worse...

For Jim's test you don't actually need to use a big Siemens star target either though he recommends it for good reasons. Really anything with high contrast detail will work that is large enough will work.

If you don't use a Siemens star, it will more difficult for others to interpret your tests. One of the advantages of the approach I've developed is that there is a kind of internal quality control. Someone who is familiar with the test can look at the captures and get an idea of how well the test was conducted.

If you have a small printer you can just tape together a larger target on a piece of cardboard. The exact details of the target really aren't critical for seeing if you've got a "lemon" or not.

Joe Holmes has gotten good results by using a particular utility pole near his house, but Joe is an expert photographer and tester.

Jim

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Rhaegar Regular Member • Posts: 218
Re: Short exposures and atmospheric disturbances

Some of us shoot in cities in which getting the space needed for the outdoors shots is just not an easy thing to do

JimKasson
JimKasson Forum Pro • Posts: 32,542
Re: Short exposures and atmospheric disturbances

Rhaegar wrote:

Some of us shoot in cities in which getting the space needed for the outdoors shots is just not an easy thing to do

Some people use city parks for that. You'll, need a folding, lightweight easel for your target. The kind used for flip charts is great, and cheap.

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