Light diffusion by colour

Canon Bob

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This isn't specific to photography but I thought this might be the best place to get an answer.

If I look at our LED "fairy lights" then the red, yellow, green and blue LED's all appear to give the same size pool/point of light. If I stick my reading glasses on then they're blurred and the red is smaller than the yellow which is smaller than the green and the blue is the largest (approximately 4x the area of the red). If I take a camera lens and defocus it then all the "bokeh" circles are the same size...as perhaps one would expect. Various pairs of reading glasses all show the same size increase by colour but the camera lenses all maintain a uniform size circle when defocused. I understand the principle of lower wavelength light scattering more but this doesn't seem to fit the experiment.

What gives?

Bob
 
The diffraction index of the cheap lens material in the reading glasses?

It's possible but if I stand close enough to focus the LED's then they all appear to be the same size.
 
Isn’t that how diffraction works? That’s why rainbows are bigger than chromic aberrations, no?

Could be, Ned. Your suggestion has alerted me to another little experiment so I'll report back later.
 
Camera lenses have correction for chromatic aberration, i.e. that simple single element lenses focus the different colours at different places.
 
Camera lenses have correction for chromatic aberration, i.e. that simple single element lenses focus the different colours at different places.

Wouldn't that apply when the subjects were in focus too? If I view the LED's at "reading distance" then they're all the same size.
 
Wouldn't that apply when the subjects were in focus too? If I view the LED's at "reading distance" then they're all the same size.

I would guess that's because that is the focal point, at which they should all be the same...

It's been a long time since I did A-Level physics.
 
Camera lenses are designed to counter all sorts of aberrations. Spectacle lenses (generally) aren't. They are calculated assuming "White" light...which is made up of all the colours (wavelengths) of the visible spectrum, all of which focus at slightly different points.
When you de-focus a spectacle lens these anomalies become more apparent. You mentioned the red and blue lights as being at opposite ends apparent size wise...they are also at opposite ends of the spectrum.
This factor is pretty much ignored in spectacle lens manufacture...until you start discussing Anti-Reflection coatings.
 
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Well thanks Chris, Ned and Graham.....I thought that there was something more interesting going on.

Bob
 
Glasses don't actually focus the light at all... they manipulate the light so that your eyes can focus it. I.e. the difference is with your eyes and their ability to focus the different wavelengths... the camera lens doesn't have the same issue (to the same extent).

There is a condition called night myopia which is a form of nearsightedness for blue light (most prominent in low light environments)... There is also a condition called digital eye fatigue which is due to the prominence of blue light emitted by LED screens and the eye's ability to focus it.
Another interesting thing about blue light is that it is of shorter wavelength and greater energy than the other end of the spectrum. So it retains more energy after diffraction/diffusion, which is why shadows/cloudy days are blue; only scattered blue light is making it to those areas (or more-so).
 
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