Post by: remotemass on 21/01/2021 14:03:57
(https://i.imgur.com/5eESzXa.png)
Post by: Colin2B on 21/01/2021 14:41:16
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Post by: remotemass on 21/01/2021 16:15:32
Does it involve only diffraction or also reflection, refraction, and diffusion?
Post by: evan_au on 21/01/2021 21:52:30
Quote from: remotemass
Does it involve only diffraction or also reflection, refraction, and diffusion?In the images posted by Colin, the orange-colored circle higher and to the left of the Sun is probably caused by a reflection of the Sun's bright disk within the lens of the camera.
- With complex lens systems (eg with a zoom lens), you can often get a row of such blobs near a bright light
- Good cameras use coated lenses to reduce reflection, but the Sun is so bright that even a small amount of reflection produces a blob, if the Sun is slightly off-center,
- The fact that this reflected disk has some colored fringes is a result of refraction (different colors are bent by different angles: chromatic aberration)
See: https://en.wikipedia.org/wiki/Chromatic_aberration
"Diffusion" is not a term we often use for light.
- Diffusion is a term that applies to electrons: electrons in the camera's silicon sensor can diffuse away from the site of photon impact, providing a signal on pixels near a bright object.
- "Diffuse light" is a term we use for light; the kind of light you get in fog, or on an overcast day. The light has been reflected and refracted so many times by tiny water droplets that it comes from all directions.
- Someone who knows more grammar than I can advise if one of these usages is a verb or a noun or an adjective (they have different pronunciation, even though the spelling is the same)...
Post by: RD on 22/01/2021 18:17:53
- The fact that this reflected disk has some colored fringes is a result of refraction ...
The colour in lens-flare (https://en.wikipedia.org/wiki/Lens_flare) is due to anti-refection coatings (https://en.wikipedia.org/wiki/Anti-reflective_coating#Applications) on the surfaces of the lens elements ...
(https://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Voigtlander_lenses_75mm%2C_50mm%2C_and_15mm.jpg/800px-Voigtlander_lenses_75mm%2C_50mm%2C_and_15mm.jpg)
(https://upload.wikimedia.org/wikipedia/commons/thumb/8/8f/A_Lens_flare.jpg/800px-A_Lens_flare.jpg)
So that means it's due to interference, rather than refraction.
Post by: charles1948 on 22/01/2021 20:20:22
Your image of lenses. in your post, shows them with a red coating. Isn't this a relatively new phenomenon.
In the past, lenses on optical instruments such as telescopes and binoculars, if they were coated, always had a blue coating. I noticed that in recent years, lenses have been coated with different colours, such as pale yellow, and in a pair of Russian 7 X50's that I owned, even green!
This I found puzzling. The human eye, as is well known, is most sensitive to light in the yellow-green part of the spectrum. Therefore the eye will be best when using transmitted yellow-green light to form images.
But yellow-green coatings on the lenses, don't transmit such light - they actually reflect it away!
As do, possibly, red-reflective coatings on lenses. Red, yellow, and green are in the same low-frequency part of the spectrum, to which the eye is best adapted. Therefore such light should not be reflected away from the eye, but rather transmitted to it !
This can achieved by blocking out the higher-frequency blue light. As is done by the traditional blue coatings.
Do you think that the appeal of red-coatings is only this:
They look pretty in advertisements, and encourage more people to buy them?
Post by: Petrochemicals on 25/01/2021 20:47:28
Buuuuut, that looks like anti reflection coatings are about as much use as a dragon at an oil refinery. I have been informed some binoculars have a coating to enable night use, is the light level a factor?- The fact that this reflected disk has some colored fringes is a result of refraction ...
The colour in lens-flare (https://en.wikipedia.org/wiki/Lens_flare) is due to anti-refection coatings (https://en.wikipedia.org/wiki/Anti-reflective_coating#Applications) on the surfaces of the lens elements ...
[
So that means it's due to interference, rather than refraction.
Post by: Petrochemicals on 25/01/2021 20:55:37
This I found puzzling. The human eye, as is well known, is most sensitive to light in the yellow-green part of the spectrum. Therefore the eye will be best when using transmitted yellow-green light to form images.Rod, Red light for movement identification , cones colour and detail perception.
https://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html#:~:text=Rods%20%26%20Cones&text=There%20are%20two%20types%20of,light%20levels%20(scotopic%20vision).&text=Cones%20are%20active%20at%20higher,responsible%20for%20high%20spatial%20acuity.
It could be probable what you see through the view finder could be different to the picture that is taken.
https://www.amazon.co.uk/Neewer-Starburst-Twinkle-Effect-Filter/dp/B00425XUGA
Guaranteed to give you a six pointed burst from any light source.
Post by: evan_au on 25/01/2021 21:54:02
Quote from: evan_au
The fact that this reflected disk has some colored fringes is a result of refraction ...
Quote from: RD
So that means it's due to interference, rather than refraction.I agree that the overall color of the blob comes from the anti-reflection coating.
However, I was referring to the narrow rainbow of colors around the edge of the blobs, shown in this closeup of Colin's f/5.6 example:
Fringes.png (108.98 kB . 340x275 - viewed 4800 times)Quote from: charles1948
lenses have been coated with different colours, such as pale yellow, ... even green!Anti-reflective coatings are always a compromise - they require exotic materials, a complex method to lay down multiple thin layers of precise thickness, and all this costs money.
The main goal is to reduce reflection over the entire visible band, for light striking the lens straight-on.
- That is why the blobs are seen most clearly when the camera is pointed away from the Sun.
- The picture of lenses posted by RD is taken from about a 45° angle, which highlights the color
Typically, if several layers are applied, different layers will be optimized to block reflections over different parts of the visible spectrum.
- These particular wavelengths will be blocked almost completely
- Reflections at the in-between wavelengths are reduced in intensity, but not cancelled completely.
- Whatever mix of wavelengths are left over forms the colored reflection that you see - different designs using different materials, different combinations of thickness and different number of layers will produce a different reflected color.
Quote
This I found puzzling. ...coatings on the lenses, don't transmit such light - they actually reflect it away!Although you may see this reflected light from an anti-reflective coating as "wasted", in fact, even more light would be reflected from uncoated lenses.
So, rather than making the image dimmer, the anti-reflective coating makes the image brighter.
- What draws attention to it is that it has a distinctive hue; the brighter reflection from uncoated glass covers the entire visible spectrum, and doesn't draw attention to itself (unless you are a spy, and the bright reflection from your uncoated lenses gives you away...)
- The hue of these anti-reflective coatings used by the fashion industry to produce distinctive sunglasses
See: https://en.wikipedia.org/wiki/Anti-reflective_coating#Interference_coatings