As an example for a typical explanation about red-eye reduction pre-flashes, the description in the Metz SCA3202-M7 flash adapter manual states

Red eyes are always the result of a physical effect. This arises whenever a person looks more or less straight into the camera, the ambient light is relatively dark and the flash unit is mounted on or directly next to the camera. The flash unit illuminates the back of the eyes, revealing the blood filled retina through the pupil. This is recorded by the camera as a red spot in the eyes.

The red-eye reduction function brings about a significant improvement in this respect. When this facility is used the flash unit triggers, prior to shutter operation, a few weakly visible preflashes which are followed by the main flash. These preflashes induce the pupils to close down, thereby diminishing the red-eye effect."

So here are three flash photographs of my eye taken with one self-timer, distance probably 8m. Focus to approx half the camera distance

Focus on camera

Focus quite near

There is no significant difference in iris size: if any, there is a small overall reduction throughout the (chronologically sorted) sequence with photographs taken with a distance of about 1sec.

Nevertheless the last image not even looking in the direction of the camera (with the reflexes from the flash being almost or entirely off the pupil) clearly has the strongest red-eye, followed by the first image.

The middle image, in spite of having the flash reflex basically straight in the pupil, clearly has a minimal amount of red-eye.

And PostScriptum, to counter theories that some sort of reflection law ignoring the presence of the lens is responsible or that the area conversing with the blind spot is, here another photograph where I am focusing quite closer than flash and camera are but in a completely different direction:Looking sideways and downwards, focus comparatively short distance

What gives? The typical description of the red-eye effect strangely does not take into account that the eye is an optical system designed to create a sharp image of what you are looking at on the retina. If there is no overlap of the image of the flash on the retina and the image of the camera's entrance pupil (the aperture as viewed through the front lens), no retina area lit by the flash will be visible from the camera's entrance pupil and consequently the sensor.

So the main objective of a pre-flash to me appears to be making the eye bringing flash and camera in focus so that their respective images on the retina are sharp and disparate. While reducing the iris size would also help by increasing the eye's depth of focus (and thus decreasing blurring of the retina images), the effect seems minor compared to what the accommodation reflex can achieve in separation even with comparatively wide pupil.

Interestingly, in the fourth image the right eye on the left side of the image is considerably more red. Looking closely, the eye on the right side of the image has its vertical aperture narrowed by a drooping eye lid. This reduces the vertical blurring of the images of flash and camera on the retina, making them overlap less.

So contrary to conventional wisdom, staring focusedly at camera/flash (or a birdie waved at same distance or some other object that shares focus with camera/flash) will do most of the work even without preflash. An urgently blinking self-timer lamp should be almost equally effective for red-eye prevention as a pre-flash.

Now the people writing about cameras and designing red-eye reduction systems know their place inside out optics. It seems preposterous to assume that they of all people would not consider the implications of the eye being an imaging device after which cameras have been modeled complete with lens, aperture, and sensor surface. And money is riding on it: basically every camera review tests for red-eye.

What am I overlooking?

  • We're all Vulcans and it is our second eyelid that closes behind the retina that prevents the flash from bouncing off the retina and producing red-eye? – Michael C Feb 23 '19 at 2:54
  • I thought I mentioned this in another question here recently, but I can't find it now. Maybe it was at another forum. It seems to me that the worst red-eye would result from an eye that is pointed halfway between the center of the lens and the center of the flash, so that the angle of incidence/angle of reflection maximizes the amount of the flash's energy falling upon the retina being reflected back through the pupil to the camera. Your examples seem to prove this out. – Michael C Feb 23 '19 at 2:59
  • @Michael C: My examples prove this out? The worst red-eye is when I am looking at the ceiling above which is about as far from half-way between lens and flash as it gets, camera and flash center being less than 20cm apart at a distance of 8m at eye level. The camera clearly is looking straight at the eyes in the first two images which have flash reflex almost in the center of the pupil. The third image with the worst red-eye has the eye looking completely elsewhere. How do you figure this supports your reflection angle theory? – user82185 Feb 23 '19 at 3:37
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    This doesn't appear to be a real question, but some strange rant about a poorly defined hypothesis about the nature of red eye. – xiota Feb 23 '19 at 16:02

What am I overlooking?

You are overlooking physiology. The visibility of the red reflex varies when you focus at different distances because you are changing the size of your pupils, as well as the direction of your gaze. The accommodation reflex is the change in pupil size when looking at objects at different distances.

Consider taking anatomy and physiology classes.

The multiple flashes of light likely affects both the size of pupils and the direction of gaze.

The pupillary response begins almost immediately upon changes in lighting, but takes longer to complete. It also takes longer for pupils to fully open than it does for them to constrict. (Shine a light in your eyes while looking in a mirror if you're interested.) The direct response is the change in the pupil through which light is shone. The consensual response is the change in the other pupil. Both eyes should respond equally because neural pathways cross over connecting both eyes to the controlling regions of the brain.

The accommodation reflex is the change in pupil size in response to distance. When doctors check eye movements, they also look for normal pupillary responses. Making you look at your nose is not just because it's fun to make people look cross-eyed.

During a fundal exam, doctors use the red reflex to align the ophthalmoscope during approach. Given the proper angle of approach, the red reflex can still be seen when the pupil is small, but it is more difficult to "lock" onto. That is why dilated eye exams are much easier to perform, but it's still possible to examine the retina without dilation.

Even a small change in the size of the pupil can significantly change the angle at which the red reflex is visible. Notice that the light reflex is located in different positions with respect to your pupils in each of the pictures. Also notice that the size of your pupils in the middle picture is slightly smaller and the red reflex is still present, but less visible.

As for why you can look away from the camera and still have a strongly visible red reflex, not only are your pupils especially dilated in that photo (they look smaller than they are because of the direction in which they are pointed), but there is also a bundle of vessels within the eye that corresponds with the blind spot. This forms the optic disc, which is brighter because of the presence of additional vessels in the area.


  • There is no reason for the pupils in the third image to be larger since they just received two subsequent flashes from taking the first two images while looking in the direction of the flash. An opthalmoscope's entrance pupil, as opposed to a camera lens you fixate, is not in eye focus. If it were, you could not view anything except the retina area containing the ophtalmoscope's entrance pupil, so the optics of ophtalmoscopy are not comparable to the red-eye situation. – user82185 Feb 23 '19 at 4:20
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    The pupils in the third image are clearly larger. It doesn't matter whether you "think" there's a "reason" for them to be or not. Your pupils do not respond to your conscious demand. – xiota Feb 23 '19 at 4:28
  • The last half of your comment about being in "eye focus" makes no sense. – xiota Feb 23 '19 at 4:31
  • In optics, there usually is reciprocity. A can see B if and only if B can see A. If the eye can see an object sharply, each point of the object can only see its own image point on the retina regardless of where it looks through the pupil. – user82185 Feb 23 '19 at 5:11
  • The core of this answer is "The multiple flashes of light likely affects both the size of pupils and the direction of gaze" and that that is enough to think about whereas the question was about the possible importance of the focusing distance the eye accommodates to in response to the optical stimulus. I don't see that a discourse between the two of us will deliver additional insights to either so maybe it would make sense to wait for some other contribution more likely to resonate with either of us. – user82185 Feb 23 '19 at 17:35

What am I overlooking?

Answering myself: let us assume for argument's sake that your stipulations have merit. In that case you are dealing with confirmation bias of significant size (for example, this is the version in Wikipedia). The eye is a biological mechanism and the reactions you consider relevant require conscious attention. As such, they are not reliably repeatable even with a single subject. Statistics are also tricky to do since actively focusing on a certain item does not just cause the lens to accommodate but also causes narrowing of the pupil exactly because that leads to a sharper image.

So if you are going to convince anybody after a long history of different generally accepted reasoning, anecdotal evidence that cannot be quantifiably reproduced under laboratory conditions and is gathered from a single subject themselves affected by confirmation bias (since they obviously want to believe their own reasoning) is not going to do the trick.

A mechanical model of a human eye complete with adaptable iris, focusable lens and retina would be required for reproducibility. Fortunately, those models are readily available and are called "camera". Your best bet is to take a reasonable fast and long camera, likely a mechanical one that does not mind being operated without proper film and can keep its shutter open for long, and replace the film with bright red paper (white paper would deliver more return light but for one thing you want to convince humans regarding the red-eye effect and for another that makes it easier to discriminate from lens flare). Because of lens flare, you want to aim the "eye simulator" camera such that the flashing camera is not in the center of its picture but rather close to a corner. A fast long lens will distribute a maximum of light from the flash on the sensor substitute.

This should allow you to make a series of photographs with different focus distances and different apertures of your "eye simulator". Of course you can just use the formulas for depth of focus and circle of confusion and its ilk and do the same just in theory and on paper. But without impressive pictures and reproducible experiments, nobody will be interested in the formulas some crackpot turned out in the attempt to disprove a century of common knowledge.

Of course, you will still have to make people accept applying the logic and physics of a dead object like a camera to a live thing like the human eye, particularly when its interior outside of its optical operation is common knowledge. But hard, obviously related non-anecdotal evidence and suggestive photographs (after all, a camera's interior outside of its optical operation also is common knowledge) will help a lot.

Good luck, myself.

  • What is your point? I don't see any viable hypothesis in what you've written. – xiota Feb 23 '19 at 16:00
  • This answers the "What am I overlooking?" part of my question. I am overlooking that mere logic will not buy me the way through a firmly established preconception assuming I am right. And of course if I am wrong, logic will not buy me anything either. So checking for prior publications would be warranted, and of course, if simple experimentation as sketched proves me wrong, even that is not necessary. When appropriate publications come to similar conclusions, then one can try figuring out why conventional wisdom has focused on something else and whether that is a problem in practice. – user82185 Feb 23 '19 at 17:07

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