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by Aditya

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I noticed on a photo I did of a client that only one of their eyes turned red on a photo. I remembered reading that this could be a sign of eye cancer (see http://www.chect.org.uk/cms/index.php/signs-and-symptoms)

I wanted to double check what other things could cause this before I worry a client. It has never happened in my work before, but I don't know if this is a common occurrence.

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3  
Would you take medical advice from a random stranger? –  S Vilcans Feb 3 at 12:54
1  
@SVilcans If that person said to you, "Hey, it looks like you may have a symptom of retinoblastoma," it might be enough to get you to ask your doctor. Better safe than sorry. –  ESultanik Feb 3 at 18:37

4 Answers 4

If you take a lot of flash photos, with the flash pointed directly at the subjects, you'll notice that eyes only sometimes go red: it depends on the angle of the eye with respect to the flash & lens. If the eye is facing off to one side, or outside the main area painted by the flash, it may not appear red.

Having one red eye could happen if, for example:

  • the person is quite close to the camera
  • off to one side of the frame
  • not looking directly at the flash/lens
  • has a lazy eye (or is otherwise pointing their eyes in different directions)

I would recommend checking more photos of the person (if you have more), to see if they consistently have just the one red-eye (the same one each time).

If in doubt, I would suggest to the client that 'it can be a symptom of eye problems, you might want to get it checked by a GP', rather than suggesting cancer straight up. The link you provided doesn't suggest it's unequivocal evidence, and pretty vaguely suggests that (for one red one black eye) "This can also be a sign that something is not right".

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The optical axis of eyes change with the distance the person focuses at. So they do not look in the same direction most of the time. If the person is facing you completely, then there is better chance that the two optical axis have the same angle to you and your camera. Still, one eye may be a tiny bit off, that is natural. Or it can be waaay off - those people still see well.

So the only information that you can deduct from only one red eye is: only one eye was directed towards the camera. But even you can do that. E.g. have someone photograph you from 5 meters when you have a pencil between your right eye and the camera, you look over the top of the pencil and directly to the camera, and focus on the pencil with your both eyes. The pencil should be 50 cm far. Then your right eye will look directly into the camera, and the left will definitely not look into the camera.

However, you might as well spotted some eye problems, and a quick check with an eye doctor is not a bad idea. (Usually 10 minutes, they just check if the two eyes are properly aligned). My bet is: eye cancer? Nah. Some minor axis error? Small chance.

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There's a famous rule taught to med students: "If you hear hoofbeats outside the window, don't think 'zebra' when it's almost certainly a horse." (reverse the species if you live in Africa, I guess :-) ). The point is that observing a single possible symptom which might correlate with a disease has very little to do with proper diagnosis. You need to be able to Rule Out the myriad other conditions which also correlate with the symptom in question, as well as weighing the relative probabilities.

It's like having a coughing attack and thinking lung cancer. There are lots more likely and common reasons to cough, and several other symptoms which a lung cancer victim would be presenting.

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I quite regularly end up with red-eye in only one eye. I do a lot of casual, candid shots where people don't know I'm taking a photo and it happens all the time. While red eye most typically happens when people are looking directly at you, it can also occur regularly when they are not. If someone is looking to the side, the side of one eye will still reflect enough light back while the other will not.

Note that this principal is the same as the tidbit about cancer being detected through red-eye. If cancer is causing a deformation of the eye, then it won't reflect light back in the expected way and you won't get red-eye. This should occur even when directly facing. It may still be possible to have the same occur for many other reasons though to, so it is just a very small possibility to be the cause.

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