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If lenses cause chromatic aberration, how come we don't see it with our own eyes?

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#26 dscarpa

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Posted 28 June 2019 - 12:46 PM

  The Duncan Idaho robot eyeballs I'm saving up for are APOs.  David



#27 Vla

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Posted 28 June 2019 - 01:56 PM

 Thanks to lylver for all that science that is, unfortunately, way over my head.  I'll have to read it again.  And again.

 

It isn't all that complicated. Unlike some other aberrations, which can vary widely individually, chromatism of the eye varies within a relatively narrow range. Nominally, average eye has about 0.9 diopters of defocus between the red C and bleu F line. That is about 1/66 of the eye's focal length in diopters of ~59, i.e. 0.26mm of its optical focal length of ~17mm. Since this is primary spectrum, with the two colors on the opposite side of the green focus, it can be, somewhat roughly compared to half as much of secondary spectrum. That gives ~0.13mm of it, or ~1/130 in units of the focal length, which is 13 times more than in a doublet achromat. How is it then that we don't see it?

 

The answer is simple: primarily due to eye's small size, and its small magnification. At its daily pupil size of about 2mm, eye is optically an f/8.5 system. If we'd have a 100mm f/8.5 achromat with this much of secondary spectrum, it would be showing horrendous chromatism. But scaling down to 2mm lower chromatism as aberration too, so it is now actually 50/13, or about four times smaller than in the achromat (about f/33 standard achromat level). Magnification is unit magnification vs. that in any telescope. On top of that, everything in it is now 2500 times (nominally) fainter.
 


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#28 helpwanted

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Posted 28 June 2019 - 02:01 PM

I have a follow up question for this thread, would a person that is color blind see chromatic aberrations?



#29 lylver

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Posted 28 June 2019 - 03:55 PM

I have a follow up question for this thread, would a person that is color blind see chromatic aberrations?

Yes, as some blur like a black & white sensor would when defocus occur, with truncated effect maybe when a cone is inactive or his absorption frequency moved. Go back to post #16, diagram about color focusing.

Vla would be better than me about blur equivalent diameter versus dioptrie defocus amount.



#30 Alan French

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Posted 28 June 2019 - 04:08 PM

I find it fascinating that our "view" of the world around us exists entirely in our mind, created from information extracted from the image on our retina and sent to the brain.

 

It's no wonder our visual system is subject to illusions and the unusual pursuit of viewing through a telescope (seeing the finest detail possible in images or seeing things just barely bright enough to be perceptible, usually with one eye) improves considerably with regular practice.

 

Clear skies, Alan 


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#31 ericthemantis

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Posted 28 June 2019 - 04:14 PM

I mean the human eye is only a single lens system, how is it that we don't see any color fringing at all in our vision but we see it easily with an artificial lens?

It is actually closer to a 2 lens system - Cornea + "lens". And our brain translates what is projected onto the retina into an image. What we see is actually projected upside-down onto the retina, but our brain flips it back over. People have done experiments around this to see how quickly our brains can adapt to changes to the norm, like wearing goggles that make everything appear upside down - https://www.theguard...ing-upside-down. Our brains do crazy things to keep our senses from driving us crazy.



#32 Vla

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Posted 29 June 2019 - 04:02 PM

Vla would be better than me about blur equivalent diameter versus dioptrie defocus amount.

It's actually quite simple. Optical focal length of the eye is about 17mm, or 1/0.017~59 diopters (lens power 1/f, the inverse of lens' f.l. in units of 1m). So if longitudinal aberration is, say, +2 diopters, that implies aberrated focus at (2/59) times 17mm, or 0.58mm behind retina. If it is defocus, needed accommodation is -0.58mm, or -2 diopters.



#33 Redbetter

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Posted 29 June 2019 - 07:27 PM

It's actually quite simple. Optical focal length of the eye is about 17mm, or 1/0.017~59 diopters (lens power 1/f, the inverse of lens' f.l. in units of 1m). So if longitudinal aberration is, say, +2 diopters, that implies aberrated focus at (2/59) times 17mm, or 0.58mm behind retina. If it is defocus, needed accommodation is -0.58mm, or -2 diopters.

What is the expected longitudinal difference for the eye for 650nm (red) vs. 550 (green)?  I am wondering about the impact of wavelength vs. focus for reading at night, e.g. having to hold charts further away to read.



#34 lylver

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Posted 30 June 2019 - 05:15 AM

What is the expected longitudinal difference for the eye for 650nm (red) vs. 550 (green)?  I am wondering about the impact of wavelength vs. focus for reading at night, e.g. having to hold charts further away to read.

It depends on your own eye pupil, when observing bright target you will remain near 3mm. The Moon doesn't pose problem, but in fact... there are few color to distinguish on the moon.

I think about Jupiter, Venus, Mars (opposition), that are often at the upper mesopic/lower photopic limit.

A 2,5mm eye pupil you won't accommodate the entire color range : the defocus is not wide enough ~.1mm @575nm (yellow perceived as the white point)

I would say the main planetary range is more like 555 to 615nm to embrace so around a .25 diopter shift, something like half the maximum defocus for the eye best sharpness. You can shift this range to lower/upper frequency by moving best focus of the instrument.

Increasing magnification will make your pupil increase : no good.

 

Pupil%20size%20chart.png

About a transition to read chart : red color is not mandatory to preserve your night vision.

orange near or after the D ray works with me. Some low intensity neon lamp for example

I use 2*0.5 W lamp that I put in the corridor to avoid bumping on child toys.

Glimmlampe_spektrum.jpg.0109114e224bc87a




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