al effects we never see!
Second, optical accommodation (the focusing of the lens) is an extremely weak depth cue (note Luke's use of "slightly" and "small"). In studies that have looked at various depth cues, optical accommodation to object distance (even though it is a "larger change" in the lens) has an inconsequential effect on depth judgments for objects more than a few feet away.
Finally, in everyday experience we see red and blue objects together, as part of a unified, three dimensional world. Luke's "relaxation sensation to invisible aberration" could only be experienced by focusing on monochromatic lights viewed in isolation first a pure "red" pattern and then a pure "blue" one. And in that context humans are nearsighted to blue light.
chromatic aberration and focal length
humans are nearsighted to "blue" light, which means an object comes into focus as it moves closer to the eye
The difference in the angle of refraction between "red" and "blue" light is equivalent to an extremely large difference in the distance of two objects, assuming no accommodation occurs in the eye (diagram, above). That is, if you cover one eye and focus on a line grating of monochromatic "red" light 100 yards away, a line grating of monochromatic "blue" light just two feet away would be in focus at the same time. Wherever we looked, our experience of the world would be of near, in focus blue objects or refractions popping out of a field of view anchored on distant, in focus warm objects.
This separation is visible in rare situations, such as an ultraviolet light viewed in darkness: the physical source, which appears in shades of pink or red, is surrounded by an unfocused, but apparently closer, violet nimbus. But this peculiar effect is clearly unusual, so it too cannot provide the hypothetical associations claimed to produce warm/cool contrast effects.
Lightness and Chroma. OK, so what does explain the qualitative differences between warm and cool colors? As with any complementary color contrast, lightness and saturation strongly influence our color judgments, and in my view lightness and saturation explain the depth and mood effects associated with warm or cool colors. In fact, Alfred Munsell noted the "advancing" effect of light valued and saturated colors and developed principles of color harmony around them.
The proof is that when the lightness and/or chroma of warm colors are made less than the lightness and/or chroma of their cool complements, those special warm color depth and mood effects either disappear or shift over to the cool colors!
warm color effects caused by lightness and/or chroma
cool colors can easily appear "advancing" or "arousing" if they are lighter and/or more intense than the warm colors around them; a white disk appears "closer" than a black disk
But what makes lightness and saturation "attention getting" or "advancing"? The luminance: they are the most luminous or chromatic objects in the visual field, and our visual system perceives extreme brightness or chroma as similar.
Vision continually isolates contrast or novelty wherever it appears, as a primitive strategy to identify unusual (attention getting) features in the environment and bring them (advancing) to our attention. Even in an urban consumer environment, intensely saturated colors are infrequent in our routine visual experience. We occupy a fairly gray world, colorimetrically speaking: saturated colors stand out because they are unusual.
I think the conventional wisdom about warm colors evolved primarily from comparisons between pure pigment paints. In general, warm colored pigments achieve a higher chroma and lighter value than their cool complementary equivalents. (Ultramarine blue is a spectacular exception.) A saturated yellow or orange paint is much lighter valued and more intense than any green or blue paint. But if the yellow or orange is mixed with black so that its lightness and chroma are matched to a complementary iron blue, the color is transformed into a raw umber or burnt umber. And I have never heard anyone claim that brown is "advanc