Eye sight oddity

[Tim Hall]

Watching a band in Thee Pubbe last weekend I noticed that the thing on the wall behind them was slightly out of focus. The sort of out of focus you might see when an auto focus camera hasn't quite got to grips with its surroundings. Then it was in focus. And out.  And in again.  A bit of thinking and experimentation (while listening to them bash out Tainted Love) isolated it to being slightly out of focus when their "stage" lighting effect was shining green.

What's all that about then?

[T42]

The iris changing f-stops as the lighting varied.

[Kim]

Eyes aren't expensive camera lenses molished from materials designed to minimise chromatic aberration, and will have different focal points for different wavelengths.  It's usually most noticeable as an apparent depth effect when primary red and blue are adjacent, and is presumably why the optiquack does that A-B testing of lenses to try to balance the focus of a black symbol on a red blackground (TBAGO) against a black symbol on a green background.

https://en.wikipedia.org/wiki/Chromostereopsis  refers (the article on chromatic aberration is mostly concerned with photography).


It seems unintuitive that you wouldn't be able to resolve with green light, as that's in the middle of our visible spectrum.  It's usually blue that makes focusing difficult (exacerbated by lack of blue receptors in the fovea, presumably in order to correct chromatic aberration).  Of course stage lighting may have weird spectral peaks so who knows...

[Kim]

The iris changing f-stops as the lighting varied.

Ah, hadn't thought of that.  GPWM.

[T42]

The iris changing f-stops as the lighting varied.

Ah, hadn't thought of that.  GPWM.

I only learnt that lesson after buying new spectacle lenses because I had trouble reading signposts towards the end of a long drive, forgetting that it was dusk by then.  And me a supposedly-competent photographer.

[rogerzilla]

How much beer had you consumed?

[hellymedic]

I often CAN’T focus blue lights on a white background at night.

[Kim]

The iris changing f-stops as the lighting varied.

Ah, hadn't thought of that.  GPWM.

I only learnt that lesson after buying new spectacle lenses because I had trouble reading signposts towards the end of a long drive, forgetting that it was dusk by then.  And me a supposedly-competent photographer.

I've got to that age, too.

I seem to spend half my life bickering with barakta (who is a migrainous troglodyte) over light levels.

[FifeingEejit]

I tend to go to the opticians on the way to work, this means:
1) I'm there about 60 mins after falling out of bed
2) I'm at morning hydration levels (i.e. not)

My optician has reassured me in the past that the stuff I just read going out of focus is normal because of the above.

What I always find worrying is THEY ask ME questions about my bilateral coloboma as if they've never seen it before (they haven't, ever, well except possibly in textbooks)

[hellymedic]

Isn’t that what Maddy McCann had?

[barakta]

Unilateral I believe.

I get people looking for it in my eyes a lot as one of the similar-to-my syndromes has it as a key symptom (I don't have it, I have other freaky eye stuff instead).

[FifeingEejit]

some sources now claim it was a freckle,



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[fruitcake]

Prompted by this thread, I've just been reading an article where an optician notices a similar effect to the OP when reading red vs green illuminated signage at night (the signage showed prices at petrol stations). The fact he couldn't read the green indicated slight shortsightedness.

https://www.optometrytimes.com/view/poor-mans-redgreen-test

[hellymedic]

When I stopped wearing hard contact lenses I had rapidly changing astigmatism so new specs were unusable after the 10 day interval between the eye test and the new specs arriving.
I bought specs with circular lenses and tuned the axis for correction by looking at the red and green lights on my hi-fi...

[Kim]

https://www.optometrytimes.com/view/poor-mans-redgreen-test

I'd wondered about this...

Blue and red wavelengths are the furthest apart dioptrically with longitudinal chromatic aberration. Why do we use red and green for the duochrome test instead of logically using red and blue?

First, blue light is inherently darker in comparison to green light; therefore, the color green is interpreted as “brighter” than blue by most patients.2 If blue were used for the duochrome test, interpretation of the test may be difficult because some patients might inadvertently choose which side is brighter instead of which side is clearer.

Presumably the red and the green used for the duochrome test are balanced so that they appear to be of equivalent brightness to you trichromats?  I find some effort is required to judge the clarity of the red part, because it's so much darker.  It sounds like I'd be better off with blue rather than green...