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Why does the Nikkor AI 50/1.4 produce excellent Sunstars?
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PostPosted: Mon Jul 11, 2022 1:52 pm    Post subject: Why does the Nikkor AI 50/1.4 produce excellent Sunstars? Reply with quote
* Why does the Nikkor AI 50/1.4 produce excellent Sunstars? *

For a long time, I’ve been fascinated by the sun-stars produced by the Nikon Nikkor AI 50/1.4 which are always beautifully defined and make for strong elements in a photograph?

Besides the fact that odd-numbered non-rounded aperture blades (7 in the AI 50/1.4’s case) yield double the number of rays, what could be the other reasons for this great asset of the lens?

Are there other lenses, be they short, medium, or long focal length, that produce sunstars as good (“good” or not is a matter of personal taste here) as the Nikkor AI 50/1.4?

Also, as I’ve never used a Nikkor AIs 50/1.4, I suppose it does the same thing?

Below is a comparison crop between the Minolta MC ROKKOR-PG and the Nikon Nikkor AI 50/1.4 lenses @ f/8, ISO 200, jpeg straight out of the Fujifilm X-Pro2 with sharpening set at 0 and noise reduction at -4 (the lowest):




PostPosted: Mon Jul 11, 2022 3:24 pm    Post subject: Reply with quote
It is a diffraction effect (edge diffraction), caused by the edges of the aperture blades. Curved edges produce slightly more diffuse sunstar "points/rays", whereas straight aperture blade edges produce the most well-defined sunstars. A fully round aperture produces no discernable sunstars, but rather a general loss of contrast in the immediate area surrounding the point source of light.

Each straight(~ish) aperture blade edge produces a diffraction pattern (line) perpendicular to the blade's edge, which extends in both directions (i.e. each diffraction line has 2 "points" pointing in opposing directions). So 6 aperture blades give 6 diffraction lines (which implies 12 "points"), but because there are three opposing pairs of blades for each opposing pair the directions of their diffraction lines overlap/coincide so you see only half the number of distinct diffraction lines (= 3 "lines" = 6 "points"). For the 7 aperture blades none are exactly opposite each other, so you see 7 distinct diffraction lines at 7 different angles, which means 14 "points" (each diffraction line has two "point" pointing in opposite directions).

Note that as mentioned for even-numbered aperture blades each pair of diffraction lines caused by opposing blades coincide, thus the resulting half number of individually visible diffraction lines do appear a bit brighter (for even-numbered aperture blades each visible diffraction line is essentially two diffraction lines coinciding and therefore double the intensity/brightness).


PostPosted: Mon Jul 11, 2022 3:42 pm    Post subject: Reply with quote
RokkorDoctor wrote:
It is a diffraction effect (edge diffraction), caused by the edges of the aperture blades...


Mark, thank you very much for your very educational answer. I've been enlightened by your rays of wisdom! 🙏


PostPosted: Mon Jul 11, 2022 3:54 pm    Post subject: Reply with quote
Also a useful article on this:

https://petapixel.com/2018/05/19/the-physics-behind-sunbursts-and-how-it-can-help-you-focus-your-photos/


PostPosted: Mon Jul 11, 2022 5:22 pm    Post subject: Reply with quote
RokkorDoctor wrote:
Also a useful article on this:

https://petapixel.com/2018/05/19/the-physics-behind-sunbursts-and-how-it-can-help-you-focus-your-photos/


Thanks for the link, Mark. 🙏 Will read it now!


PostPosted: Mon Jul 11, 2022 5:44 pm    Post subject: Reply with quote
Many contemporary Voigtlander lenses have excellent sunstars.


PostPosted: Mon Jul 11, 2022 7:53 pm    Post subject: Reply with quote
caspert79 wrote:
Many contemporary Voigtlander lenses have excellent sunstars.

Thank you very much for chiming in, Caspert. 🙏


PostPosted: Mon Jul 11, 2022 9:07 pm    Post subject: Reply with quote
One tradeoff you'll often see is between sunstars and near-wide-open bokeh., as the straight blades that give great sunstars can make less attractive bokeh balls. Modern lenses have aperture leaves specially shaped to be near round at close to open settings, and then straight or near straight when stopped down to sunstar territory. I don't know how the Nikon lens is in this regard, even though I have a copy...


PostPosted: Mon Jul 11, 2022 11:17 pm    Post subject: Reply with quote
RokkorDoctor wrote:
It is a diffraction effect (edge diffraction), caused by the edges of the aperture blades. Curved edges produce slightly more diffuse sunstar "points/rays", whereas straight aperture blade edges produce the most well-defined sunstars. A fully round aperture produces no discernable sunstars, but rather a general loss of contrast in the immediate area surrounding the point source of light.

Each straight(~ish) aperture blade edge produces a diffraction pattern (line) perpendicular to the blade's edge, which extends in both directions (i.e. each diffraction line has 2 "points" pointing in opposing directions). So 6 aperture blades give 6 diffraction lines (which implies 12 "points"), but because there are three opposing pairs of blades for each opposing pair the directions of their diffraction lines overlap/coincide so you see only half the number of distinct diffraction lines (= 3 "lines" = 6 "points"). For the 7 aperture blades none are exactly opposite each other, so you see 7 distinct diffraction lines at 7 different angles, which means 14 "points" (each diffraction line has two "point" pointing in opposite directions).

Note that as mentioned for even-numbered aperture blades each pair of diffraction lines caused by opposing blades coincide, thus the resulting half number of individually visible diffraction lines do appear a bit brighter (for even-numbered aperture blades each visible diffraction line is essentially two diffraction lines coinciding and therefore double the intensity/brightness).


Just an illustrative picture of the sun that confirms what RokkorDoctor said above. The photo was taken with a Tamron SP 17mm F3.5 lens at F16. The diaphragm has 5 blades, so the sunstar has 10 rays. I would say the quality of the sunstar is good, but not exceptional like the Nikon AI 50mm F1.4.



This is a good article on what are the best lenses for sunstars: https://phillipreeve.net/blog/best-lenses-for-sunstars/
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PostPosted: Tue Jul 12, 2022 2:44 am    Post subject: Reply with quote
The 'sun-stariest' photo I ever took was using a Minolta MD 200/2.8 which has 8 fairly straight blades. This crop is zoomed in slightly, but the length of the stars from the double spot lamp fittings on the vessel is impressive. I guess it's a function of just how bright they were against the ambient background. I also wonder if there is some optical property of certain lenses that governs things like this, but have no idea what it might be.



PostPosted: Tue Jul 12, 2022 10:25 am    Post subject: Reply with quote
Gerald wrote:

This is a good article on what are the best lenses for sunstars: https://phillipreeve.net/blog/best-lenses-for-sunstars/


Thanks for the link, Gerald. Quite a good read with many example photos. 👍 🙏


PostPosted: Tue Jul 12, 2022 10:43 am    Post subject: Reply with quote
Alun Thomas wrote:
The 'sun-stariest' photo I ever took was using a Minolta MD 200/2.8 which has 8 fairly straight blades.


Indeed, Alun, the aperture blades of the MD era seem to be less rounded than those of the MC era. Below is a non-crop photo with my 1978~79 Minolta MD-II(1) 135/2.8 @ f/8. Its sunstar spikes are definitely better-defined than those yielded by my MC-II & MC-X lenses. From what I've digested from the articles shared by Mark and Gerald, the main factor for how spiky sunstars get is how straight-edged the aperture blades are (while smaller aperture values produce longer spikes). It seems that optical scheme design, glass material and coatings don't seem to matter that much.



Last edited by Sakyaputta on Tue Jul 12, 2022 10:50 am; edited 1 time in total


PostPosted: Tue Jul 12, 2022 10:44 am    Post subject: Reply with quote
IMO the late Minolta Zooms (35-70/3.5 and 70-210/4) produce pretty good sunstars too for a vintage lens! Not incredibly defined but pleasing to the eye and subtle with their 14 stars. They have 7 blades opposed to just 6 on most minolta lenses.

Here's a crop (about 1:2) of a shot made with the 35-70/3.5 (last version), Minolta xd7 with Portra 400

//forum.mflenses.com/userpix/20227/big_11702_sunstars_min3570_1.jpg][/url]


PostPosted: Tue Jul 12, 2022 4:24 pm    Post subject: Reply with quote
Just out of curiosity, NASA has just presented a photo of the first galaxies ever seen. Note the strong sunstar effect in the brightest stars. Can we conclude that the James Webb Space Telescope uses a diaphragm with 6 blades? Or would it be 8 blades? Or...?



PostPosted: Tue Jul 12, 2022 5:17 pm    Post subject: Reply with quote
You can get stars with lenses that have no aperture, so there are other factors at play.


PostPosted: Tue Jul 12, 2022 5:24 pm    Post subject: Reply with quote
Or you read this: https://bigthink.com/starts-with-a-bang/james-webb-spikes/


PostPosted: Tue Jul 12, 2022 5:50 pm    Post subject: Reply with quote
D1N0 wrote:
Or you read this: https://bigthink.com/starts-with-a-bang/james-webb-spikes/

Like 1 Thank you!

The support struts works matters!


PostPosted: Tue Jul 12, 2022 6:21 pm    Post subject: Reply with quote
D1N0 wrote:
Or you read this: https://bigthink.com/starts-with-a-bang/james-webb-spikes/

Great find! Like 1


PostPosted: Tue Jul 12, 2022 6:32 pm    Post subject: Reply with quote
Back to the Nikkor 50 1.4 for a second...does the D series version make the same cool sunstars as the AI, I wonder?


PostPosted: Tue Jul 12, 2022 6:47 pm    Post subject: Reply with quote
Gerald wrote:
Just out of curiosity, NASA has just presented a photo of the first galaxies ever seen. Note the strong sunstar effect in the brightest stars. Can we conclude that the James Webb Space Telescope uses a diaphragm with 6 blades? Or would it be 8 blades? Or...?


Those are attributed to mirror support struts. Oh! I see D1N0 has already provided a link -- Thanks!

Also, from the link Mark provided:

Quote:
Like many space photos, some bright stars appear with 4 spikes. While this might suggest a 4-bladed aperture, the spikes actually result from diffraction caused by struts that hold the secondary mirror in place in reflecting telescope designs like Hubble.


PostPosted: Wed Jul 13, 2022 8:58 pm    Post subject: Reply with quote
Stars from a lens that has no aperture blades...



PostPosted: Wed Jul 13, 2022 10:49 pm    Post subject: Reply with quote
The tree's leaves function as aperture blades.


PostPosted: Thu Jul 14, 2022 12:02 am    Post subject: Reply with quote
But what about this one, again the lens has no aperture blades and there are no leaves or anything else that could affect things.



PostPosted: Thu Jul 14, 2022 8:43 am    Post subject: Reply with quote
Gerald wrote:
The tree's leaves function as aperture blades.


Yes, the leaves act as a field-stop here (localised to the bright light spots, not an overall field stop), which can also cause diffraction. A clue here is that both sunstars have totally different "point"-patterns; if it were aperture blades inside the lens causing this, you would expect both sunstars to have a similar appearance.

iangreenhalgh1 wrote:
But what about this one, again the lens has no aperture blades and there are no leaves or anything else that could affect things.



Here the three sun-stars do have a very similar appearance, which makes it likely to be a lens artefact rather than something inherent in the streetlights.

Ian, does this lens have any pressed aspherical elements? This could be resulting from the interference caused by ridges left behind in the pressing mould, which are usually a result of the diamond-cutting process used in making the mould. Another well-known effect this causes is the onion-ring bokeh of aspherics. On such pressed aspherics the very out-of focus point sources of light give the onion-ring bokeh-balls, but in-focus bright point sources of light can cause these streaks & stars. Not really diffraction, but a phenomenon much closer to what you see with the old star-filters, that have etched parallel lines in the filter. In this image, I simply suspect a poorer quality surface finish of a pressed aspheric to be the cause.

Reticulation in the cement of a cheaper doublet could also be a cause (poor cement or poor cementing procedure).

EDIT: micro-scratches on an element (particularly the front element due to cleaning), as well as grease-stains on a lens are also a very likely cause, probably even more likely than my other two suggestions.


PostPosted: Thu Jul 14, 2022 10:03 am    Post subject: Reply with quote
The lens in question is a Computar f1.4 4mm with missing aperture blades.

No idea what the formula is, if it has any aspherics etc.
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