Why can't the human eye focus blue light?

Question

I recently noticed that it is hard to focus on blue light sources, especially at night. When observing a blue light source, e.g. a neon sign, it looks somewhat blurry. A sign with a different colour right beside it looks sharp.

I already know about the three kinds of cone cells in the human eye (I'm not a biologist) with their spectral sensitivity peaks in in short (S, 420–440 nm), middle (M, 530–540 nm), and long (L, 560–580 nm) light wavelengths [1]. But does the spectral sensitivity correlate with focus? Or does our eye lens refract blue light in a different way?

When I screw up my eyes looking at a blue light, it becomes less blurry, but then all the other colours are blurred.

[1] http://en.wikipedia.org/wiki/Cone_cell#/media/File:Cones_SMJ2_E.svg

Answer 1

The same thing happens in photography when you see an image with colorful shadows of objects on it - this is called chromatic aberration. Check this wiki page if you're not familiar with. Now this happens because the lens in the objective has different refractive properties for different wavelengths or if go the other way around different wavelengths refract differently within the same material. This is why the famous triangular rainbow prism is capable of separating the sunlight to rainbow. So back to the question: in our eye there is the eye lens that is responsible for focusing. Let's assume that it is homogenous enough. So when light enters into our lens it refracts to separate colours and our lens is not complex enough to fully compensate for this. The second picture is a good approximation of the human eye lens in shape.

Edit: so when you focus one color perfectly the others will be blurry a bit since those will be out of focus due to different refraction.

taken from: http://commons.wikimedia.org/wiki/File:Prism-rainbow-black.svg

taken from:http://en.wikipedia.org/wiki/Chromatic_aberration#/media/File:Chromatic_aberration_lens_diagram.svg

Answer 2

Combination of factors:

1. Red and green (which are, from the evolutionary standpoint, the same frequency, as the red and green cones are recent variants of one another while the blue cones are distinct, they use a different photoreactive enzyme) are the dominant resolution source for human vision, while blue cones are much more important for expanding our ability to detect color (chrominance). It's why we have more red and green cones than blue ones (IIRC the ration is 10:1, though that may be way off).

Since focus is much more important to spatial resolution (details) rather than color resolution, the human "autofocus" is biased to favor the red-green band. Blue gets short shrift because of its relatively minimal importance to resolution. An experiment: if you mess around with the three color channels of an image in a photo program like Photoshop, messing up the blue channel affects the final color image the least, given the same amount of "messing" (like noise or deresolution).

Last but not least: as noted, chromatic aberration means that the different colors won't quite match up. It's not normally visible, likely because our brains auto-compensate when combining the different cone inputs into the final image (Canon digital cameras recently acquired a similar ability.) Not everyone is going to notice the effect: this is probably because blue focusses "shorter" in terms of lens focal length than red. So, you are much more likely to notice "blue fringing" if your unaided vision tends towards the nearsighted side (like mine does), given the focus center on red/green -- and you'll see it worst with distant blue sources than near ones. If you know someone who has worse nearsighted vision than yours and wears glasses (and neither of you have astigmatism) try using their glasses and see what happens.

Answer 3

Light is scattered by gas molecules in air. The shorter the wavelength of light, the more it is scattered by the atmosphere. Because it has a shorter wavelength, blue light is scattered ten times more than red light. One reason is that blue light has a frequency that is closer to the resonant frequency of atoms than e.g. red light (Exploratorum). Due to this scattering the eye has difficulty focussing as object edges get fuzzy.

The scattering of blue light degrades vision in normal day light, especially in foggy conditions when light scattering is at its worst due to the droplets of water in the air. Yellow filters (for example yellow sports sunglasses) improve visual acuity by filtering out the aberrant blue light (Laramy-K Optical).

Fun fact
The predominant scattering of blue light makes that the sun is clear in the sky as a yellowish circle, while the sky is blue due to the fact that it is the blue light predominantly being scattered away from the sunlight.

Answer 4

As well as natural shortsightedness to blue, due to chromatic aberration in the lens of the eye, there are also no S cones at the centre of the fovea. This means that focussing on a point source of blue is difficult and can be uncomfortable for some observers. Hope this helps.