focusing in monochromatic light

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

>Anaglyph filters bug my eyes which is why I don't like anaglyph.
>Sometimes (not often) I wear yellow filters over my glasses to
>enhance contrast....  I
>saw something the other day (in P3D?) about the eye using its own
>lateral chromatic aberration as a focussing aid. - I guess the idea
>is that it balances the size of the red image point disk against 
>the size of blue image point disk.  Is this true?  If it is, this 
>could be part of the problem with anaglyphs.  Maintenance of focus
>may be awkward.    
>John B

Microscopists often use monochromatic light.  This eliminates chromatic
aberration,  and higher resolution is attained by using blue light, because
resolution is measurably higher with shorter wavelengths.  When using phase
contrast, monochromatic light is necessary because many wavelengths of light
would muddy the 'contrast', which is between images that are identical
except one has the light slowed or retarded by 1/4 wavelength.

 However, it is very difficult to focus the eye on these monochromatic
images, and although my information is anecdotal there must be studies of
this in the microscope journals.

Those yellow glasses that target shooters wear have been very popular for
many years, and I have to assume there is something to them...but I haven't
seen any enhancement of an image when using them.

Many 3d'ers find analglyphs unsatisfying, and not just from the loss of
color and lowering of contrast.  I hadn't thought of the  difficulty in
focusing.  The limited depth information given by natural color (purplish is
distant...how does that go?) and the highly saturated colors given to the
eyes by analglyphs would add to the confusion.
I have posted several articles to p3d on the eye's use of chromatic
aberration to focus, it is accepted by vision scientists as part of the
eye's arsenal of focusing tricks.  
I found it hard to visualize the process, but through e-mail I was
enlightened by John Roberts, who is being flagrantly quoted without notice:

 The key is that the lens has multiple focal lengths -
in the case of red/blue, that's two specific focal lengths at the same time.
The objective of the focus is to put a sharp image of the object being viewed
on the retina, which is essentially a fixed distance from the lens. If the
focus of the eye is too far for the object you want to look at, then the
focus (foci) of the lens will be further from the lens than the retina, so the
object will appear to be out of focus. *However* since the blue focal point is
closer to the lens than the red focal point, the blue part of the image will
appear not quite as badly out of focus when projected on the retina.
   If the lens is focused too close, than the foci will be somewhere between
the lens and the retina, so the object will again appear out of focus.
The blue focal point is still closer to the lens, but in this case that
puts it *further* from the retina than the red focal point, so the blue
image will appear *more* out of focus.
   To give it a simple 2-dimensional geometric model, imagine a horizontal line
representing the optical path, with the lens at some point over to the left.
The point F (for fovea) on the line represents the surface of the retina,
always at a constant distance from the lens. The point R represents the
focal point of the red light *from the object you're looking at*. The point B
represents the focal point of the blue light from the object you're looking
at. R and B can be shifted side to side along the line by changing the focus
of the lens or by moving the object you're looking at closer to or further
from the eye, but since the blue focal length is always shorter than the
red focal length, point B is always to the left of point R on the line.
   If the focus is too distant for the current object, then points B and R will
be to the right of point F, and since B is always to the left of R, this
means that the FB (the distance from F to B) is less than FR (the distance
from F to R), so the blue image is more nearly in focus.
   If the focus is too close for the current object, then points B and R will
be to the left of point F, and since B continues to be to the left of R,
this means that FB > FR, so the blue image is more badly out of focus.
ASCII diagrams (let L be the lens):

   L------------------F------BR--   (FB < FR:   blue image sharper than red
                                                focus too far 
                                                decrease focal length of lens)

   L------------BR----F----------   (FB > FR:   blue image fuzzier than red
                                                focus too near 
                                                increase focal length of lens)

   L-----------------BR----------   (B and R are very close to F - image
                                                in focus)

And this from photo3d:
suppose you're looking at a scene using one eye, and an object
you wish to look at is out of focus with the current focal setting of the eye.
To get a good view the focus of the eye must therefore be adjusted, and the
most efficient way to do this is to know whether you should move the focus
closer or further. If your eye picks up both red and blue from the object,
and the red is fuzzier than the blue, then the object is closer than the
current focal distance of the eye, and you need to adjust the focus closer.
If the blue is fuzzier than the red, then the object is further than the
current focus, and you need to adjust the focus further away. If the light
from the object is monochromatic, then chromatic aberration is not available
as a focal cue, and other methods (possibly trial and error) must be used.

According to my source,
"The most current and compelling work (including the effects of
doublets that reverse chromatic aberration) has been done by 
Phil Kruger at SUNY Schnurmacher Vision Research Institute in Manhattan.
Mostly  published in the journal Optometry and Vision Science."

I found nothing on this in my vision books or optics references, except this
bit of amateur science:
 1920, R. Clay, Practical Exercises in Light, brief section on chrom. ab. of
the eye.  'view light filament through cobalt blue (transmits blue & red),
so near that you can focus blue but not red, filament appears blue with red
border.  Back off, until can focus red but not blue, appears red with blue
border.  Over ~20 ft., can't focus violet, still can see red image of
filament, but each point of fil. will form large diffusion circle of violet.  
At 10 inches, can focus white, but not red light.'

I will be looking for the articles by Kruger when I get out to the school of
optometry library.  It is a fascinating phenomenon.
\\\\\\\\\\\\\\\\\\\\////////////////////
  Peter Abrahams    telscope@xxxxxxxxxx
the history of the telescope, the microscope,
   and the prism binocular


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