Glenis Stevens of Amberley asks :-

In a family flash photo our eyes appeared red but our old dog's were yellow. Why the difference?

Gordon Sanderson, an ophthalmologist at Otago University's School of Medicine, responded.

Firstly the reason you get a red pupil in your photo, which is called the 'red eye effect', is because the flash of the camera and the lens of the camera are what is referred to as paraxial ie, the rays are parallel to each other. This means that the light from the flash which hits the retina is reflected back along an almost identical ray path and into the lens of the camera. This effect can be avoided if the flash is held further away from the camera, for example if you have your photograph taken in a professional studio.

On the other hand this can be a useful technique as it can be used to diagnose eye problems. In normal eyes you should get two nice equally bright red reflexes. If one eye is not focussing correctly or if it deviates relative to the other eye then it will have a lighter or darker coloured reflex. This technique is called Photoscreening. What you have done in effect is photoscreen your family and your dog.

There are various reasons why the dog's eyes might appear yellow. Firstly the dog could have a retinal problem, which would mean that the retina was not its normal colour. Secondly if your dog were elderly it might have cataracts, which would certainly alter the appearance of the red reflex although it may not make it go yellow. Thirdly dogs have an additional membrane called the tapetum which reflects light inside their eye. This is one of the reasons that dogs and other similar animals can see so well at night because the tapetum multiplies the effect of light inside their eye and acts as an image intensifier. Fourthly the dog may not have been looking directly at the camera in which case the light would be falling on a less sensitive part of the retina which may give rise to a different coloured reflex.