Does the 5N 'exaggerate' field curvature?

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Archive 2011 · Does the 5N 'exaggerate' field curvature?

Sosua
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p.1 #1 · Does the 5N 'exaggerate' field curvature?

Hello all,

Been through a few lenses trying to find a rangefinder normal on my NEX-5N at the lower end of the cost spectrum (tried a 35mm Skopar and 28mm Ultron in store for 30 minutes but they had poor edge performance when I examined the images at home later).

I found a few used deals locally and have had a Leica 35mm F2.8 Summaron and a Voigtlander 40mm F1.4 for the past few days.

Now, in a perfect world I guess I would have a ZM 35 2.8 and a 35mm ASPH lux, but that ain't happening anytime soon!

My use is mainly travel and landscape shooting. I don't do a lot of low light, but the Nokton obviously adds a few more possibilities there.

My below impressions:

The Summaron is sharp wide open over, but a little lower in contrast and is spectacular across the frame at F5.6 onward. Field curvature seems to be a minor issue that I can work around, but need to be aware of (focus as deep as possible while retaining acceptable focus on the middle).

The Nokton is 'usable' wide open centrally although a bit hazy and fringy and quickly falls off. Fine I guess for low light portraits. More sharpness and contrast at F2 but it seems to suffer from even more noticeable field curvature which means I need to be at F8 to get great results across the field for 3D scenes. Its great there - as good as the Summaron and I'd suspect around as good as it gets. For my main uses, its academic I guess.

I do slightly prefer the wider FOV of the Summaron and it is easier to get great results at middle apertures where i'm likely to use it. However the Nokton gives me a low light option too so I guess i'll just need to keep them both for a while and see how it goes!

I am wondering however now if field curvature was the reason I noticed poor edges with the Ultron and Skopar too (it took me a while to realize what was happening with the Nokton and Summaron as I was getting 'baffling' results at first)

So, do you think the supposed angled microlenses on the 5N (which appear to work a treat with my CV 15) could be aggravating lenses with existing field curvature? Or am I speculating on things I don't really understand

Dec 06, 2011 at 03:52 AM

Smiert Spionam
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p.1 #2 · Does the 5N 'exaggerate' field curvature?

It's an interesting idea. If it's purely a matter of field curvature, then images focused on a point at the corner should be sharp -- but that doesn't always do the trick.

I think a more likely explanation is that the NEX is the first interchangeable lens camera with wide lens compatibility, a very shallow registration distance, and an APS-sized sensor. As much as we love it, that combination simply leads to problems -- and the fact that Sony addressed them as well as they have in the second generation of this system is pretty amazing. I would of course love even better performance, but Sony has managed to get this system dialed in rather well, and rather quickly.

I think another piece of the puzzle is that because of the frenzy of interest in figuring out which lenses work well on this system, there's just a lot more information circulating about performance quirks. Very few of my Nikkor lenses are really all that great deep in the corners on a DSLR, either.

Dec 06, 2011 at 04:01 AM

Sosua
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p.1 #3 · Does the 5N 'exaggerate' field curvature?

Excellent point - especially with the number of older lenses, designed for film being subject to scrutiny on a high pixel dense sensor...

It would be interesting how these lenses perform on the M8/9 and GXR.

Dec 06, 2011 at 04:19 AM

Sosua
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p.1 #4 · Does the 5N 'exaggerate' field curvature?

And yes - even at 2.8, if I focus at the edge, the images are sharp on the edge (albeit significantly softer in the centre)

Dec 06, 2011 at 04:22 AM

Smiert Spionam
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p.1 #5 · Does the 5N 'exaggerate' field curvature?

It would be interesting to see some very careful comparisons with the same lens on an NEX-5n and an M8/9, shot off the same tripod of the same subject (with the Leica cropped to the APS sensor area). I'm sure there would be some interesting differences -- and wouldn't it be interesting if one of them was a shift in field curvature?

Hmm. Stranger things have happened.

Dec 06, 2011 at 04:30 AM

Jacob D
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p.1 #6 · Does the 5N 'exaggerate' field curvature?

I'd pretty much agree with your impressions of the Nokton 40. I find it fairly vintage looking from wide open to about f2.5. It has some soft glow there. Starting around f2.8 it sharpens up very nicely, but not really suitable for landscapes until 5.6, and better at f8 (possibly better yet at f11, I don't know exactly where diffraction sets in). I definitely find the 35 Skopar a better lens for landscape, so I'm not sure what gives, unless I have a bad Nokton and a great Skopar.

I've talked to users who owned both the Summaron and Skopar who felt the Skopar pretty easily surpassed the Summaron and was closer in performance to the Summicron. It's not to hard to find comments backing that up with some time spent Googling around.

Regarding field curvature, I have no idea if the NEX exaggerates it.

Dec 06, 2011 at 04:30 AM

Sosua
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p.1 #7 · Does the 5N 'exaggerate' field curvature?

Hmmm... what sold me on giving the Summaron a whirl was people had said it was as good as it gets stopped down (and a guy had one locally for $450 NZD)... I think they are all likely much of a muchness at F5.6-8.

I kind of want to try another Skopar now - as I may have not given it a fair shake.

Dec 06, 2011 at 04:46 AM

douglasf13
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p.1 #8 · Does the 5N 'exaggerate' field curvature?

The ZM 35/2 that I've tested on my 5N is sharper in the corners than on my 5.

Dec 06, 2011 at 06:06 AM

Sosua
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p.1 #9 · Does the 5N 'exaggerate' field curvature?

Those ZM 35's are known for having a very flat field of focus, no?

Dec 06, 2011 at 08:19 AM

uhoh7
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p.1 #10 · Does the 5N 'exaggerate' field curvature?

maybe only those kodak/leica sensors can really cope. It's a digital thing no?

They got the colorshift way better-- and I think somebody proved the zm 25/2.8 corners were better too. And the 16 is better now too--a bit no?

But edges can be weird. I was taking some first looks at the leica 28/2. It should be best at f/2, but for infinity landscape anyway---not. F/8 pretty good though

Dec 06, 2011 at 09:30 AM

denoir
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p.1 #11 · Does the 5N 'exaggerate' field curvature?

Sosua wrote:
So, do you think the supposed angled microlenses on the 5N (which appear to work a treat with my CV 15) could be aggravating lenses with existing field curvature?

No. The micro lenses are used to offset the chief ray angle. When the exit pupil of the lens is very close to the sensor the angle at which the light hits the pixel sites at the edges/corner can be extreme. This is what the micro lenses are for.

Field curvature on the other hand is when you plane of focus isn't a plane but a curved surface. This is a global property of the lens and can't in any way be changed by the surface you project it on.

So the answer is no, the micro lenses can't affect field curvature one way or another.

Dec 06, 2011 at 09:36 AM

Peire
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p.1 #12 · Does the 5N 'exaggerate' field curvature?

I did not notice anything like this with NEX 5N and WA lenses that I tried (Zeiss C/Y Distagons:25/2.8,28/2.8,35/2.8; Zeiss Z Distagons: 18/3.5,21/2.8,25/2.8,28/2,35/2;OM Zuiko: 21/3.5 and 24/2.8).More - it gives sharper images in the corners wide opened,compared to NEX 5 and NEX 3 of previous generation.

Dec 06, 2011 at 01:44 PM

Specularist
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p.1 #13 · Does the 5N 'exaggerate' field curvature?

The microlenses don't affect field curvature, but the glass filter in front of the sensor certainly does.

Luckily for you, the latest paper by H. H. Nasse at Zeiss discusses this very phenomenon, on page 12. In that paper Nasse calls it astigmatism, which is a difference in focus between the tangential and sagittal surfaces (i.e. field curvature of at least one of them). Specifically, the filter causes the focus for tangential structures to shift to greater distances, to a greater degree with increasing image height.

The Leica M8 and M9 have especially thin filters to reduce this focus shift at the edges of the field.

Dec 06, 2011 at 06:45 PM

snapsy
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p.1 #14 · Does the 5N 'exaggerate' field curvature?

I'm not sure what your expectations/requirements are but there are a few lenses that perform well in the corners at reasonable apertures. At 35mm the best I've tested so far is the Canon 35mm FDn f/2. In the 40-50mm range the two best lenses I've used are the Contax G 45mm f/2 and the Minolta MC 50mm f/1.4, the latter of which is only barely behind the Contax but is much cheaper. Here's a recent comparison I did:

http://testcams.com/blog/2011/11/18/nex-manual-focus-lens-comparison-for-35mm-to-50mm/

Dec 06, 2011 at 07:00 PM

douglasf13
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p.1 #15 · Does the 5N 'exaggerate' field curvature?

There's a seemingly knowledgeable guy on another forum that I've seen mention that part of the problem is that Sony tends to actually leave a little bit of space between the AA filter and the sensor, in order to make sensor dust (or should I say sensor topping dust) less visible. I have no idea about the validity of this statement, but it could be a third element to this equation.

Dec 06, 2011 at 07:23 PM

sebboh
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p.1 #16 · Does the 5N 'exaggerate' field curvature?

uhoh7 wrote:
But edges can be weird. I was taking some first looks at the leica 28/2. It should be best at f/2, but for infinity landscape anyway---not. F/8 pretty good though

why should it be best at f/2?

Dec 06, 2011 at 07:50 PM

sebboh
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p.1 #17 · Does the 5N 'exaggerate' field curvature?

denoir wrote:
Field curvature on the other hand is when you plane of focus isn't a plane but a curved surface. This is a global property of the lens and can't in any way be changed by the surface you project it on.

technically not entirely true: if you project on to a curved surface that can certainly effect it. it would be very expensive, but i could imagine some type of curved filter being used to try to compensate for color shift (i don't think this would be a good idea without lenses designed for such a thing). that said i don't thing focus shift is being effected by the NEX sensor in any way other than making it more visible with its higher pixel density.

Dec 06, 2011 at 07:54 PM

Specularist
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p.1 #18 · Does the 5N 'exaggerate' field curvature?

sebboh wrote:
that said i don't thing focus shift is being effected by the NEX sensor in any way other than making it more visible with its higher pixel density.

But the glass filter above the sensor does introduce field curvature. Read page 12 of the Zeiss paper I linked to.

Dec 06, 2011 at 08:39 PM

sebboh
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p.1 #19 · Does the 5N 'exaggerate' field curvature?

Specularist wrote:
sebboh wrote:
that said i don't thing focus shift is being effected by the NEX sensor in any way other than making it more visible with its higher pixel density.

But the glass filter above the sensor does introduce field curvature. Read page 12 of the Zeiss paper I linked to.

i haven't had time to download and read it yet. i'm assuming it refers to refraction at the filter having a greater impact the more oblique the angle is, hence it increases tangentially? if this is the case it should be easy to test whether the NEX is producing much of such an effect with any lens known to have low astigmatism and a short distance from the rear element to the sensor.

Dec 06, 2011 at 09:28 PM

denoir
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p.1 #20 · Does the 5N 'exaggerate' field curvature?

Specularist wrote:
The microlenses don't affect field curvature, but the glass filter in front of the sensor certainly does.

Actually he speaks of astigmatism and not of field curvature. They're related but not the same. Field curvature affects the whole image space while astigmatism only one directional component.

technically not entirely true: if you project on to a curved surface that can certainly effect it. it would be very expensive, but i could imagine some type of curved filter being used to try to compensate for color shift (i don't think this would be a good idea without lenses designed for such a thing). that said i don't thing focus shift is being effected by the NEX sensor in any way other than making it more visible with its higher pixel density.

It depends if the curvature is in image space or in object space. The former can be corrected by a curved surface while the latter can't.

Dec 06, 2011 at 09:53 PM

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