[GeoffMc (Geoff)]

What you're experiencing is mesopic (twilight) vision: things are visible but blurry and monochromatic. This contrasts with photopic (day time, high resolution colour) and scotopic (night time) vision.

As you already know, the inability to see colour in low light is due to the insensitivity of the cones in the retina which are responsible for colour vision. With these out of action, vision is only possible with the rods, which are more sensitive to light but cannot detect colour. You probably know that the eye is less sensitive to red light under mesopic and scotopic conditions and so doesn't respond when you use a red light at the telescope. An ordinary white light would trigger contraction of the pupil, etc so you'd lose your dark adaptation.

The low resolution (you call it 'noise') comes from the fact that, aside from the rods being less numerous than the cones (i.e. lower density means lower resolution), they are distributed around the retina away from the fovea. The fovea is a small (~2mm) depression at the back of the eye responsible for high resolution, such as reading this post. The eye cannot form sharp images very far away from the fovea: a few degrees away from the fovea visual acuity has dropped to ~50% and continues to fall off sharply. You can convince yourself of this by focussing on this mark - * - and trying to read the letters on the ads on the right of the screen, or in fact elsewhere in this message. So, even if the rods were densely packed they'd still "see" a distorted blurry image produced by the eye. The only sharp vision we have is at the fovea. This is where the problem starts.

The fovea is crammed with cones, but with these out of action due to insufficient light they cannot provide the high resolution needed to, say, read fine print under low light levels. While an image can fall on the retina away from the fovea, the lower density of rods in this part of the retina and the optical distortions away from the fovea means much lower resolution. The main problem is therefore the lack of rods in the fovea where the sharpest image is formed. So, no cones (due to low light), no rods (they're not there): no resolution hence a blurry image.

Adding to the loss of resolution is the dilation of the pupil in low light conditions, which allows more peripheral rays to enter the eye, amplifying the eyes many optical aberrations. In other words, even if cones were sensitive to low light levels, they would be receiving a distorted image from the dilated pupil. (Interestingly, this is why albinos often have poor vision: the lack of pigmentation in the iris means, in effect, the aberrations in the eye are amplified by an essentially non-existent pupil.)

Hope this answers the question. Let me know if I'm off track or you want to know more.

Cheers,

Geoff Mc