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Can we improve brightness of eye by changing f-stop

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#1 cambridgeobserver

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Posted 11 April 2021 - 05:06 PM

Hi All, I am getting back into astronomy and thinking about optics. I am wondering if it is possible to change the f-stop of the eye to improve brightness. I understand there are theoretical limits of brightness for a passive optical system. I.e. the brightness of the source. The eye, however, has an f-stop at best of 2, and is not at the theoretical limit.

 

Is there anyway to use lenses to improve the f-stop from 2 to say 1.5? I am thinking of a high NA lens followed by a diverging lens might work. 

 

Thanks, Alex



#2 ButterFly

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Posted 11 April 2021 - 05:58 PM

The pupil stops up and down.  There are limits to both ends.  It's a muscle, so use it or lose it. 


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#3 TOMDEY

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Posted 11 April 2021 - 06:22 PM

As long as the exit pupil of the system fills the fully-dilated pupil of your eye --- that's as good as it can get. The only legitimate way to get more is to have an identical telescope feeding your other eye. That will give you another 0.7 magnitude of legitimately perceived brightness! I know from experience --- have tried that and now mostly observe that way. What I have just described there is (of course) the True Binoscope. Here's a picture of my sixteen-inch Binoscope. >>>    Tom

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#4 freestar8n

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Posted 11 April 2021 - 06:46 PM

There is a way to make objects brighter on the retina with a given max pupil size of 5mm or so - but I wouldn't recommend it.

 

If you surgically insert lenses within the eye itself and behind the pupil you can reduce the focal length and thereby make a reduction in f/ratio and an increase in brightness - though the objects will appear smaller.

 

This isn't too far-fetched because there are surgeries not too different from that.

 

You could also cut a larger hole in front so the pupil is much larger and replace the lens with a bigger one.

 

But if you don't change the eye itself and the only thing you do is place lenses or mirrors in front of the eye and look through them - there is no way to make extended objects brighter than as seen by the naked eye.  You can make them bigger while staying the same brightness - but you can't make them brighter.

 

Frank



#5 cambridgeobserver

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Posted 11 April 2021 - 08:31 PM

Hi, Thank you for the replies. I suppose more fundamentally I am wondering if a "pupil" can be improved upon. Hypothetically (and clearly very hypothetical), the lens of the eye were removed. Could one put a large lens with a large NA, ie very convergent in front of the pupil. And then replace the old eye lens with a slightly divergent lens. The combined focal length of the two lenses would be a little longer than the original eye lens but the diameter of the lens would be much larger and the F-stop would be lower. Is this theoretically possible?

 

Thanks, Alex 



#6 Jon Isaacs

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Posted 11 April 2021 - 08:50 PM

Hi, Thank you for the replies. I suppose more fundamentally I am wondering if a "pupil" can be improved upon. Hypothetically (and clearly very hypothetical), the lens of the eye were removed. Could one put a large lens with a large NA, ie very convergent in front of the pupil. And then replace the old eye lens with a slightly divergent lens. The combined focal length of the two lenses would be a little longer than the original eye lens but the diameter of the lens would be much larger and the F-stop would be lower. Is this theoretically possible?

 

Thanks, Alex 

 

Alex:

 

It's very rare when the F ratio of the eye is the limiting factor. Most observing is done at exit pupils of 2 mm or less, that would be about F/8 or slower. 

 

Even for galaxies, a 2 mm exit pupil is about optimal. 

 

Brightness is rarely the problem.. 

 

Jon


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#7 AstroDog77

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Posted 11 April 2021 - 08:51 PM

I think this needs to be moved to the Cybernetics forum... er... maybe Eyepieces?


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#8 cambridgeobserver

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Posted 11 April 2021 - 08:57 PM

@Jon, Agreed. My question is more theoretical than practical. It is just bothering me. It seems odd that I cannot improve my eyes brightness to that of the source. 

 

Thanks, Alex



#9 freestar8n

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Posted 11 April 2021 - 09:19 PM

It's an advanced topic to explain so I won't attempt it here - but it's good for beginners to know that telescopes don't make things 'brighter' - they just make them bigger while keeping the brightness at most the same.  Stars are very different and large aperture can let you see fainter stars - but extended objects like the moon, planets and galaxies don't get brighter - in terms of how many photons are landing on a patch of the eye.

 

The only way to make the object brighter is by operating on the eye and either making the pupil larger or inserting lenses in it to make the focal length shorter.

 

No amount of fast or slow or big or small will help - if it is in front of the eye and the eye is looking into it.

 

Frank


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#10 cambridgeobserver

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Posted 11 April 2021 - 09:32 PM

@Frank, Thank you for the reply and understood on the topic. Is it possible to move the thread to a different part of the forum? I would like to read your answer on inserting lenses into the eye with shorter focal lengths. Particularly if you left the pupil the same size. 

 

Thanks, Alex



#11 TOMDEY

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Posted 11 April 2021 - 09:37 PM

Hi, Thank you for the replies. I suppose more fundamentally I am wondering if a "pupil" can be improved upon. Hypothetically (and clearly very hypothetical), the lens of the eye were removed. Could one put a large lens with a large NA, ie very convergent in front of the pupil. And then replace the old eye lens with a slightly divergent lens. The combined focal length of the two lenses would be a little longer than the original eye lens but the diameter of the lens would be much larger and the F-stop would be lower. Is this theoretically possible?

 

Thanks, Alex 

That's what Frank described up there. Theoretically yes, but for myriad practicable reasons no. One is that the human eye simply would not respond gracefully to a feeding F# faster than is ~natural~ This has actually been measured and is quite similar to the same phenomenon governing films and solid state detectors. As the feeding cone becomes faster, the resposivity drops rather precipitously. It's a kinda no free lunch diminishing returns dynamic, where you really can't improve all that much on what Mother Nature has already optimized. I'd put the scalpel away for now, and just but a better quality telescope --- and a Night Vision eyepiece. That's actually what I use!    Tom

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#12 ButterFly

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Posted 11 April 2021 - 09:45 PM

This took a sharp turn to the left!

 

 

@Jon, Agreed. My question is more theoretical than practical. It is just bothering me. It seems odd that I cannot improve my eyes brightness to that of the source. 

 

Thanks, Alex

Just let your brain do the lying for you!  Seeing is more than just sensing and perceiving.  The brain shows you only what it can conceive of.  A and B are the exact same in every way, except where they are:

 

316px-Checker_shadow_illusion.svg.png


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#13 Don W

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Posted 11 April 2021 - 09:55 PM

Pilots in WWII were sometimes given Bilberry which supposedly increased night vision. I've heard of some observers that saw an improvement, others that did not.

 

https://www.puritan....wE&gclsrc=aw.ds



#14 cambridgeobserver

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Posted 11 April 2021 - 10:43 PM

@Cosmos, I am not sure what Frank and I are saying the same thing. I am suggesting ways of using lenses to increase the f-stop of the eye, but purely externally. I mentioned replacing the lens of the eye as a thought experiment. Instead of removing the lens, one could put a divergent lens to cancel it, and then put the optical system I suggested in front of that. The crux of my idea is to have a large lens, then a Barlow, where the combination has a short focal length. The Barlow would be the size of the eye's pupil, but the aperture would be much larger. 

 

Thanks, Alex 



#15 KBHornblower

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Posted 11 April 2021 - 11:07 PM

As has been said over and over, nothing external to the eye can increase the surface brightness of the image on the retina.



#16 TOMDEY

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Posted 12 April 2021 - 12:08 AM

@Cosmos, I am not sure what Frank and I are saying the same thing. I am suggesting ways of using lenses to increase the f-stop of the eye, but purely externally. I mentioned replacing the lens of the eye as a thought experiment. Instead of removing the lens, one could put a divergent lens to cancel it, and then put the optical system I suggested in front of that. The crux of my idea is to have a large lens, then a Barlow, where the combination has a short focal length. The Barlow would be the size of the eye's pupil, but the aperture would be much larger. 

 

Thanks, Alex 

Hi, Alex!

 

But alas... that's impossible. It violates Emmy Noether's theorem. A passive optical system cannot increase étendue; at best it can only maintain it. So, once you preclude modifying the eye itself, it is restricted to its native max étendue, no more than that. That invariance appears under different guises in different contexts (Radiance, Lagrange Invariant, Abbe Sine Condition, distortion conditional, paraxial vs real ray-trace, entropy laws...). What you are inventing is the Free Lunch --- but Mother Nature refuses to serve up loaves and fishes. That's what Emmy realized and formalized, in a context that is so generalized that it applies to all gauge bosons and their associated probability density distribution functions.... much to the astonishment and relief of her contemporary mathematicians and scientists. In our context here, the bosons are photons and the field is electromagnetism. It's that fundamental and that unavoidable.   Tom

 

PS: Here's my Arcane T-Shirt celebrating her achievement. When my son gifted it to me, I immediately recognized it and commented, "Excellent, in integral form!" and then waxed lyrical regarding the implicit conditions under which it can seem to be violated, but is not. Things like del n, where the photons slow down so are bunched up, etc. etc. So, we chatted theoretical and applied physics and I blew out the candles on my Birthday Cake.    Tom

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#17 Redbetter

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Posted 12 April 2021 - 03:41 AM

 

It's very rare when the F ratio of the eye is the limiting factor. Most observing is done at exit pupils of 2 mm or less, that would be about F/8 or slower. 

 

Even for galaxies, a 2 mm exit pupil is about optimal. 

 

Brightness is rarely the problem.. 

 

Jon is hitting at the key point for visual observing:  there are only two cases where a wider eye pupil is helpful.  Those cases are naked eye observation and telescopic/binocular observing at large or maximum exit pupil to present the brightest possible image.  In both cases we are talking about relatively large objects (1x for naked eye, or some low telescopic power) where scale is less important. 

 

Most astronomical objects are best observed through a telescope at small or moderate exit pupil.  You will see fainter stars and smaller/fainter galaxies, etc. this way.  So even if a person had 10mm pupils, it wouldn't improve the level of detail or how bright the image would be through a scope at the best magnifications (exit pupil) to show the target.  A 3, 2, 1 or 0.5mm exit pupil will all fit comfortably through any observer's dark adapted pupil.

 

The main thing you can do is to maximize the sensitivity of your eye(s) at all exit pupils:

  • Maintain maximum dark adaptation while observing, and give your eye a few minutes to adapt when increasing power (smaller exit pupil means dimmer image, so your eye will re-adapt for the lower brightness level.)
  • Protect your vision from glare at night, and even reflections from the sky, bright planets, bright stars onto your face and back onto the surface of the eyepiece.  This improves contrast detection.
  • Breathe.  Yes, it sounds like an unnecessary suggestion, however it is easy to find yourself unintentionally holding your breath while concentrating on an object.  Ironically, this will reduce your blood oxygenation level and dim the image.
  • Maintain good cardio vascular fitness.  This should contribute to good eye health and also assure good oxygenation level at night, particularly if you observe at altitude.  If a person is getting winded while observing, it likely will impact what they can see.  
  • Get periodic eye and medical exams.  On the medical side, if you have any sort of vitamin A deficiency, that will hurt your night vision from what I understand.  From what I have read, once you have sufficient vitamin A level, more doesn't add any benefit.  Don't go crazy with high levels of supplemental vitamin A, it is fat soluble and eventually vitamin A toxicity can be an issue.  [Note:  I am not medically trained.]
  • Abstain from alcohol and smoking before and while observing. 
  • Stay hydrated.  I have had visual migraines before from being dehydrated.
  • Snack/take breaks as needed.
  • Protect your eyes during the day.  Wear sunglasses etc. while out in bright sunlight.  Enough exposure to bright sunlight during the day can impact your night vision that evening.  Longer term the lens yellows over time anyway, with cataracts potentially forming as we age.  


#18 freestar8n

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Posted 12 April 2021 - 03:42 AM

@Frank, Thank you for the reply and understood on the topic. Is it possible to move the thread to a different part of the forum? I would like to read your answer on inserting lenses into the eye with shorter focal lengths. Particularly if you left the pupil the same size. 

 

Thanks, Alex

Hi-

 

The explanation Tom gave does point to the fundamentals of what's going on - but the overall topic has been discussed in many ways in many different threads and I don't know a way to give a good 'feel' for how it works.

 

But since you gave the subject of this thread a reference to f-stop - I will go from there and keep it brief.  Your question refers to f-stop and speed - which suggests you feel f-ratio by itself tells you something about how bright an image is.  Well the f-ratio can be very fast with a small lens and it can be slow with a big lens - but if you believe it relates to brightness of the image - that's great because it does.

 

What is the f-ratio?  Ultimately it is just how wide the cone of light is that arrives and focuses on the image.  If that angle is wide as it comes to focus, you have a fast system and a bright image.  If it is narrow you have a slow system and a faint image.

 

Well - look at the inside of the eyeball.  You have a pupil that can only get so big - and then you have the retina.  How big can that cone get?  It's completely limited by the pupil diameter and the distance to the retina.  Putting a big lens in front won't do anything to make that angle bigger - it is stuck.  Unless you do some cutting...

 

So that's a simple way to get a feel for it.  f/ratio does matter - but the only one that matters is the one finally landing on the image.  That cone angle is limited by the pupil of the eye - and there is nothing you can do to make it bigger - and faster.

 

Frank


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#19 spereira

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Posted 12 April 2021 - 07:25 AM

Moving to General Observing.

 

smp



#20 cambridgeobserver

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Posted 12 April 2021 - 07:34 AM

@Frank, I am not sure I am violating étendue, but maybe. If you had an optical system, like a describe. An ultrafast lens feeding a negative lens to slow it down, the cone would be very wide to the first lens and then narrow from the second lens to the detector. Is the f-stop of that system the narrow cone, or is it the cone of the combined system? 

 

Thanks, Alex



#21 GShaffer

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Posted 12 April 2021 - 07:42 AM

Where are the Borg when you need them? Little doubt they would be whom to ask given their penchant for clinical detachment as well as they appear to have experimented widely with modifying a single eye among members of the collective smile.gif



#22 Jon Isaacs

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Posted 12 April 2021 - 10:07 AM

@Frank, I am not sure I am violating étendue, but maybe. If you had an optical system, like a describe. An ultrafast lens feeding a negative lens to slow it down, the cone would be very wide to the first lens and then narrow from the second lens to the detector. Is the f-stop of that system the narrow cone, or is it the cone of the combined system? 

 

Thanks, Alex

 

Ok.. 

 

The goal is to increase the relative aperture of the eye, i.e. decrease the focal ratio. 

 

I believe we are assuming any surgery will not be used to increase the aperture so the only avenue available is to shorten the focal length. 

 

A relay lens plus a focal reducer.. 

 

Jon



#23 freestar8n

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Posted 12 April 2021 - 04:40 PM

Ok.. 

 

The goal is to increase the relative aperture of the eye, i.e. decrease the focal ratio. 

 

I believe we are assuming any surgery will not be used to increase the aperture so the only avenue available is to shorten the focal length. 

 

A relay lens plus a focal reducer.. 

 

Jon

But the reducing lens would need to be inside the eye - hence surgery.  It's the only way to make the arriving cone of light wider when it hits the retina.

 

Frank



#24 ButterFly

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Posted 12 April 2021 - 06:39 PM

@Frank, I am not sure I am violating étendue, but maybe. If you had an optical system, like a describe. An ultrafast lens feeding a negative lens to slow it down, the cone would be very wide to the first lens and then narrow from the second lens to the detector. Is the f-stop of that system the narrow cone, or is it the cone of the combined system? 

 

Thanks, Alex

There is a virtual exit pupil in that system.  It's just like the 2x Galilean astro binos.  The "right" way, they are 2x, so your pupil looks 2x bigger through the front aperture.  One only uses 2 * pupil size of the aperture at any one time.  Cue Tom's picture ... here.

 

How about the "wrong" way?  They are a 1/2 x system that way.  So how much area of the aperture does you pupil take up at one time in that case?  It's much harder to take a picture of that one.  Things don't appear much brighter at 1/2 x.  Try your Keplerian binos backward as well.  10x binos backwards don't make things 10x brighter.

 

The fundamental issue is that the eye's lens acts so that it focus on the retina, a set distance away from your pupil.  The lens won't change that distance, and it can't always curve enough to reach the retina.  Because I can't do that, I need glasses.  My glasses in no way change that set distance, nor do they affect the largest my pupil can get.  That set distance and the largest my pupil can get sets the minimum f/ratio, just because I can't fit a larger cone into it.  They are negative lenses, however, so things appear noticably smaller than in real life, but in focus.



#25 Dave Mitsky

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Posted 12 April 2021 - 08:47 PM

This took a sharp turn to the left!

 

 

Just let your brain do the lying for you!  Seeing is more than just sensing and perceiving.  The brain shows you only what it can conceive of.  A and B are the exact same in every way, except where they are:

 

316px-Checker_shadow_illusion.svg.png

That's a great illusion!




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