[Fozzie (OP)]

We all know not to look directly at the sun for any length of time as it will cause eye damage. The lens of the eye forms a point image of the sun on the retina, so why is it that when you look say, ten degrees away from the sun at the blue sky, you don't get damage to the retina? The suns image is still falling on the back of the eye, just not at the central point...

[JnA]

well yes.. you do damage the retina, the compensation is that you have more retina (though I suspect that particular part will be damaged) you only have one spot where the optic nerve sends signals to the brain though.. so once that's fried all the retina in the world isn't going to help you.

[chris]

Excellent question.

The key is in the focus. The eye focuses on the thing that it is looking at. This brings the light falling on the retina from the source to a point. So, as you say, if you stare at the Sun, the full power of the light will be focused on the retina, producing a localised heating / burning effect with potentially irreparable consequences.

But if you look off to one side then, although the Sunlight is still entering the eye, the Sun is not the object of interest and hence is actually out of focus on the retina. Consequently the energy is dispersed over a much larger area and hence remains below the threshold required to injure the tissue.

The situation is analogous to using a magnifying glass to burn a leaf. Only when the sunlight is focused directly onto one point does it produce a sufficient heating effect to cause burning. When the lens is pointed off-target, Sunlight still passes through the lens (equivalent to looking "off to the side" at the Sun) but is not focused to a point and so the heating effect is reduced.

Chris