Pinhole Camera Problem! I was reading slides of a university course and in one of the slides it says

1 Eyes/cameras can’t have VERY small holes because that limits the amount of entering light 
2 and diffracts/bends the light

Bending of light(diffraction) only takes place when light goes near the edge of the surface of the object(e.g. water droplets in cloud) but in the case of the camera or eye light passes through the eye lens or camera lens(not near the surface of the lens), refraction takes place instead of diffraction in the case of eye/camera, so my question is how they are saying that if eyes/cameras have very small holes they will diffracts/bends the light?
 A: You say:

Bending of light(diffraction) only takes place when light goes near the edge of the surface of the object

and this is broadly correct. Light diffracts when it is near an edge, and the nearer it is to an edge the more it diffracts.
And of course a circular aperture has an edge. The light passing through the centre of the aperture is well away from the edge and doesn't diffract much. However the light that passes near the edge of the aperture will be deflected. With a big aperture most of the light is far from the edge so the percentage of the light that is diffracted by a significant angle is negligable.

However with a very small aperture a lot of the light is near enough to the edge to be significantly deflected.

In the far field limit the diffraction due to the aperture produces an Airy disk. The angular divergence of the light produced by diffraction is given by:
$$ \sin\theta \approx 1.22\frac{\lambda}{d} $$
where $\lambda$ is the wavelength of the light and $d$ is the diameter of the aperture. You ask about the pupil of the eye. The diameter of the pupil is around 2 to 8 mm depending on the brightness of the light. If we take the smaller size and the wavelength of light to be around 550nm that gives us:
$$ \theta \approx 0.0003 \,\text{radians} \approx 0.02 \,\text{degrees} $$
