Red eyes and yellow eyes in flash photography
The « red-eye effect » is a well known phenomena in photography. It only appears in specific configurations: the subject’s pupils must be wide open, which happens when the flash takes the eye by surprise in a dark environment. Additionally, the flash/eye/lens angle must be small enough, this is why we never observe the red-eye effect when the camera is near the eye.
These red reflections are not as simple as they seem, and two incorrect ideas are very often diffused in forums and websites. The first incorrect idea is that the red-eye effect only appears when the subject looks directly at the lens: on the contrary, it happens with any direction of the eyes. The second incorrect idea is that a person with a red eye and a yellow eye on a photo is most probably suffering a malignant tumor of the retina (retinoblastoma) or at least has strabismus. In fact, leukocoria (white glow in people’s eyes) is indeed a medical sign for several diseases and it should be reported when it happens consistently, but one yellow eye can be observed with everyone in a very specific configuration. This article aims at explaining all this in detail and at raising some questions about the red-eye effect that lack satisfactory explanations.
|Red eyes that do not look directly at the lens||Yellow-eye effect with perfectly healthy eyes|
Anatomy of the eye
Our eyes, like all photographic lenses, focus the light they receive to create sharp images on a layer of photoreceptors, the retina. The light that enters the eye goes through the cornea, the aqueous humour, the pupil, the lens and the vitreous humour, before reaching the retina and then the choroid, a layer of colored cells and blood vessels. The drawings of Netter provide more detail:
The color of the choroid seems to be brown or orange, with dark areas (including the fovea in the center) and lighter areas (the optic disc, aka the « blind spot », closer to the nose).
A dissection of the human eye by Jessica M. Skeie and Vinit B. Mahajan allows for visualizing the appearance of the eye, cut in a butterfly pattern for the occasion:
However, the color of this eye is certainly impacted by the lack of blood irrigation during the dissection.
The lens has a special property: its shape can change thanks to the ciliary muscles,
this is called the accommodation. This allows the « focus » on an object depending on its distance from the eye, in order to obtain a sharp image on the retina. The iris can be more or less open and controls the amount of light that reaches the retina.
A simple eye model
The eye behaves roughly like a converging lens with a diaphragm. When the eye focuses on an object, the light emitted or scattered from this object converges to one point on the retina:
|One eye||A simple eye model|
Therefore, the light emitted from a flash far from the eye converges on a small area on the retina. For the same reason, when we look at a pupil, we only see a very small part of the retina, like in a magnifying glass. Reciprocally, if only a small part of the retina and the choroid is enlightened by a flash, then the light of the flash reflected by the eye goes back towards the flash, forming a red light beam (because the interior of the eye is more or less red).
If everything was perfect, this light beam would go back precisely onto the flash and it would not be possible to capture it in a photo. But if for some reason the system is not perfect, then this light beam can slightly spread and it becomes visible in the neighborhood of the flash. To give an idea of the order of magnitude of this neighborhood, I observed that the camera can capture the red beam when the angle flash/eye/lens is smaller than 1.5 or 2 degrees, which means in practice that the flash must be close from the lens of the camera, or that the photo must be taken from a distance. This reflected beam exists for any position of the flash and it is always directed towards the flash, this is why the red-eye effect can be seen for any orientation of the eyes.
I conducted a small experiment with a virtual flash and a simple eye model, with Tantalum, a simulator of light beams in 2D created by Benedikt Bitterli. My test scene is here: simulation of flash and red-eye effect. In this test, the light does not focus perfectly on the retina (a diffuse half-sphere), which results in a red light beam that can be seen behind the flash. Here are some screenshots:
|Diaphragme très ouvert
|Diaphragme presque fermé
Among the open questions, there is the question of what is exactly the imperfection in the system that creates a wider red beam. One hypothesis is that a wide open iris decreases the sharpness of the image on the retina because of optical aberrations. A second hypothesis is that the light could be locally scattered in the choroid, like in the skin (e.g. a hand looks red when it is placed in front of a strong light source). I don’t think that multiple reflections of the light in the eye has something to do why the red-eye effect.
The yellow-eye effect
We have seen that the interior of the eye is not uniform, and in particular there is a lighter region called the optic disc, where the optic nerve connects to the eye. When the flash falls exactly on this region, we observe a yellow-eye effect instead of a red-eye effect. This optic disc is situated slightly on the side of the eye, closer to the nose. This is why it is impossible to observe two yellow eyes at the same time:
Here are some photos I captured to show that the color can vary depending on the direction of the eyes. We can observe various colors, from yellow to purple. I used a smartphone, which is well adapted for this experiment since the flash is often very close to the lens of the camera, such that we can capture the red-eye effect without a selfie-stick.
A lot of additional information can be found about ophtalmology at opticiancertification.org.