Testing the distortion of my lenses with a star test using my M11M

[JOE58BC]

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Besides doing landscape photography I also enjoy a reasonable amount of astrophotography and have several telescopes. I like milky way photography. I was at my very dark sky site in central Oregon and thought I would do a quick fun distortion test of my lenses at their widest aperature and at F4. 

The night skies this trip were very clear and are near pristine dark skies so the viewing was as ideal as I could expect so it would be a fun exercise. My thinking was this, rather than using a grid or a checkerboard to photograph and test distortion I would photograph a star-field in the milky way shooting at zenuth (straight up into the constillations Lyra and Cygnus).  Each of the stars being a fine point source is a test of the optical train onto the sensor at that point in the image. As a test perfectly round and tight stars  will have a low HFR. ( half flux radius Radius where half the energy, Photons, of the star is contained in your image) Stars out of focus or dostorted for whatever reason will have a larger gaussian blur and larger HFR. 

I used my M11M body and the following lenses.

Leica 90mm apo summicron asph F2,

Leica 50mm summicron f2,

Leica 35mm Summilux f1.4 asph and a

Zeiss 25mm Boigoon F2.8.

I have since aquired  21mm super elmar f3.4 asph but will have to wait to test that. 

All images were taken untracked on a fixed tripod at sufficient ISO to give similar exposures (it was a quick test) I took an image at wide open and at f4 except for the Zeiss Lens which I took at f 5.6 for some reason. The power of the low noise full 60 MP sensor is impressive.

You can compare each lens wide open and stopped down at F4 in the provided link of jpg images. It is easiest to detect distortion by looking at the star shapes in the corner of the images.  General radial types of distortion can be easily seen in all lenses when wide open but  they vary significantly. The lenses are  really remarkable stopped down to F4. I think I understand the distortion better looking at the shape of the star point sources. It would be good to get some feedback from others. Obviously there is so much more beauty and com[pplexity going on in peoples terrestrial photos and I find the lenses spectacular in their performance. 

The jpg images are in this link with some annotation https://adobe.ly/4mM3p37. I could not add 8 images to this post  and thus exceeding the size limits but I added one image here as a teaser. 

For interest I used an Image grading app to grade the images and count the stars and quality of the stars in the image the output was informative for sure. The HFR, (half flux radius Radius) dropped for all the lenses when stopped down meaning the average quality of the star images improved significantly( rounder and tighter). Not surprising but nice to see it in numbers. I'm not sure of the accuracy of the star counts but again as the field of view narrowed the number of stars in the image dropped significantly. Still the numbars are impressive. I did not have time to verify the count but if you have time, have a go with the raw files and get back to us. These are very short exposures but the detection of so many satellites and a few planes was impressive. 

Image Name	HFR	Stars
25ZEIS_BIO_2_8.fit	1.74	60664
25ZEIS_BIO_5.6.fit	1.57	78643
35LUX_ASPH_f1_4.fit	2.36	62031
35LUX_ASPH_f4.fit	2.02	58431
50CRON_M_f2.fit	2.34	41155
50CRON_M_f4.fit	1.67	52420
90CRON_APO_f2.fit	1.73	36529
90CRON_APO_f4.fit	1.5	31980

 

 

This is a link to the raw files if you wish to look through in better detail on your own.  Shot is identified in the filename

https://adobe.ly/3VapXPk

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[JOE58BC]

I just updated the JPG link to the correct images.😁

[pippy]

Interesting comparison; thanks for posting.

Might I ask which version of the 50mm Summicron were you using?

Philip.

[spydrxx]

Very innovative. Thanks for posting.

[JOE58BC]

@Pippy The 50mm is a Version 5

[Michael Geschlecht]

Hello  Joe,

Welcome to the Forum.

Nice photo.

Thank you for doing this test.

I may be missing something here, but, it appears to me that the wider the angle of coverage of a lens, the more stars that would be in the image.

Also, in a theoretically perfectly corrected lens: The wider the aperture: The higher the resolution.

And, the smaller the aperture: The higher the contrast.

So, wouldn't that mean:

The better corrected the actual lens is across the entire field: The closer the proportional relationship of the star count at a larger aperture is to the star count of the same lens stopped down? And, in  a very well corrected lens: The star count of the wider aperture image might exceed the star count of the stopped down image under certain circumstances?

Best Regards,

Michael

Edited August 21 by Michael Geschlecht

[JOE58BC]

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Hey Michael

 

Quote

I may be missing something here, but, it appears to me that the wider the angle of coverage of a lens, the more stars that would be in the image.

Well that is what is generally observed in the data provided above. 25mm average= 69653, 35mm average = 60231 , 50mm average 46787 and 90mm average=34254. I'd call that a trend though I can't speak to much onthe verasity of the absolute numbers. I did not write the counting program and it is a bit complex to analyze an image, identify seperate local peaks that vary significantly in their size in pixels and count them. I thought I would include it in the post for fun. As a scientist I may have a skewed idea of fun.

Quote

Also, in a theoretically perfectly corrected lens: The wider the aperture: The higher the resolution.

And, the smaller the aperture: The higher the contrast.

So, wouldn't that mean:

The better corrected the actual lens is across the entire field: The closer the proportional relationship of the star count at a larger aperture is to the star count of the same lens stopped down? And, in  a very well corrected lens: The star count of the wider aperture image might exceed the star count of the stopped down image under certain circumstances?

Interesting but complex. The star counting was for fun as stated... however: looking at real world optical systems ( camera lenses) and resolution then wider aperatures generally reduced resolution. So when lenses are stopped down a few stops the lens reaches maximum resolution. As you keep going to smaller aperatures then diffraction will reduce resolution regardless of the quality of the glass as this is a property of the wave nature of light.

Resolution is shown using Modulation Transfer Function(MTF) charts. and is typically done measuring the number of lines per mm.  Here is an example I found of an MTF chart of various lenses but all camera lenses have this pattern I believe.

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[Michael Geschlecht]

Hello Joe,

Generally speaking, scientists have a good idea of fun: Having a good time while searching for the reality of truth. Keeping in mind that sometimes truth is a moving target.

If we leave diffraction limitation to the side for this part of the discussion & deal with a single given angle of coverage of the same piece of the sky. And if we keep in mind that nobody has built a perfect lens yet:

Lens design & construction is like black & white photography: Mostly shades of gray.

Meaning: Since nobody has built a perfect lens today, people build lenses that are compromises.

A larger aperture allows more separate pieces of information to enter the optical system & land on the same image capture surface with the same ISO (Sensor/Film) in the same amount of time. 

I'll be back.

Best Regards,

Michael

Edited August 22 by Michael Geschlecht

[lincoln_m]

"My God it's full of stars!"

Sorry I couldn't resist that quote from 2001: A Space Odyssey 

Re: 35mm Summilux at f4. Top right. Is that NCC-1701 going into warp drive? Looks like a the USS Enterprise, 3 section spaceship, from the future trying to get to their time after realising they shouldn't break the Prime Directive with our M-Class planet.

Oh no! its just a normal aircraft flying over with 3 flashing wing and tail lights during (I'm guessing) a 5 second exposure.

The resolving power of the 50mm summicron at f4 ( you can just see that double double stars) is impressive and confirms it is still an amazing optic even though it was designed in 1979, 46 years ago.

We should enjoy these dark skies while we can because there are currently over 8000 Space X Starlink Satellites orbiting. Astronomers need to know when they are overhead so as to not bother making measurements. One day we won't see the stars because of the light reflecting off the Starlinks in the way then we will really feel like we are in a simulation as Elon suggests we are.

Very informative images, thanks.

[farnz]

21 hours ago, JOE58BC said:

Resolution is shown using Modulation Transfer Function(MTF) charts. and is typically done measuring the number of lines per mm.

Apologies for picking up a seemingly minor point but MTF is measured in line pairs per mm, which means the resolution will be double what 'lines per mm' would be, which in empirical terms is substantial.

(You can mark that down as an Engineer's skewed idea of fun. )  Now ... where did I leave those neutrinos ... ?

Pete.

[JOE58BC]

One last edited image with the Lux35 @F4 8sec ISO 12500

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[adan]

9 hours ago, farnz said:

Apologies for picking up a seemingly minor point but MTF is measured in line pairs per mm, which means the resolution will be double what 'lines per mm' would be, which in empirical terms is substantial.

(You can mark that down as an Engineer's skewed idea of fun. )  Now ... where did I leave those neutrinos ... ?

Pete.

Just be wary of slipping into https://en.wiktionary.org/wiki/measurebation . 😜

[lincoln_m]

Maybe it's me but if you play with the black/white point and contrast then there are many suspicious dotted lines and curves of dots ........  all over this Milky Way at different angles and curves? In the top part there looks like writing or windy rivers with darker (fewer stars) parts and brighter (river banks). Is Elon aligning his Starlink satellites so as to put a watermark copyright in the night sky?

Or is it some sort of jpeg artefact ? If these 5M pixel jpegs are great then the original 60M pixel RAW images must be out of this world!

We don't get clear night skies in England unless you're in the right place at the right time, so perhaps these lines of stars have always been there but I'm seeing them for the first time with these fantastic images?

[UliWer]

On 8/20/2025 at 6:35 AM, JOE58BC said:

It is easiest to detect distortion by looking at the star shapes in the corner of the images.  General radial types of distortion can be easily seen in all lenses when wide open but  they vary significantly.

I think you are mixing up distortion and optical coma. The irregular shapes of single points of light are caused by comatic aberration https://en.wikipedia.org/wiki/Coma_(optics)

Single spots like stars cannot reveal distortion caused by a lens. You‘ll need a pattern of parallel lines to find out whether your lens is able to show these lines in parallels or whether they will bend. There are lenses which show almost no distortion, but a lot of coma: the spherical 35mm Summilux (fully opened) is a typical example. Modern lenses which are electronically corrected by the camera (or the software for post-production) may show almost no coma but huge distortion (if you don‘t use the correction). 

Edited August 25 by UliWer

[JOE58BC]

I get your point but I guess I made the mistake of referring "generally" to distortion/aberration and not exclusively or specifically about barrel or pincushion distortion. Having numerous top level telescopes I appreciate the distinction. Abberations at the corners/ periphery of the field can be due to the lens focusing the cone of light at different planes and due to focusing wavelengths of light differently as pointed out in the Wikipedia article you looked up and quoted below. 

However coma usually manifests radially toward the edge and the distortion displayed 

Quote

In optics (especially telescopes), the coma (/ˈkoʊmə/), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is also a form of chromatic aberration.

 

However coma usually manifests radially toward the edge and the distortion displayed is morphing the Airy Disk tangential to the optical axis. The distorted stars at the corners of my images do not look like the coma shape and are more classically "seagull" shape so maybe some astigmatism, field curvature plus some defocus.?