Can dirt on a camera's lens cause graininess, without causing large-scale blurring of the picture as a whole?

Question

I have a simple point-and-shoot camera that takes satisfactory pictures when the picture is filling the computer screen. The pictures are shot with auto ISO and full daylight. But when I look at the picture at 100% - it's grainy. At first I thought of just cleaning the lens before posting this question, but several answers on this site suggest not cleaning a lens unless there's a good reason, so as not to scratch it.

So my question is: is this possible at all, that a lens that isn't completely clean (or is scratched?) will degrade the image quality without a large-scale degradation of the picture, such as a fingerprint seen on the image, or dirty smudges etc.? (Assuming it's not the ISO etc. at fault)

Answer 1

Yes, it's possible — but it's more likely that that's just how it is. You can't expect miracles from a point and shoot camera, and fine detail with no noise when "pixel peeping" would be a miracle. You say that the results are satisfactory for viewing as a whole on a computer screen. They'll also be fine when printed at reasonable sizes.

In any case, it's unlikely that the result of this would be something you'd describe as grainy. Instead, there might be a slight loss of sharpness and contrast, and (especially if it's grease and smudges rather than just dirt) glare/flare/ghosting. If you're seeing noise, that's probably just the natural result of a lot of pixels in a tiny sensor.

There may be some changes you can make to get somewhat better results, but I bet the pixel-level detail will still disappoint. See How can I make a very sharp photograph with a point & shoot camera? for more — but, mostly, don't set your expectations too high.

Answer 2

Dirt can never cause grain. It's more likely you are seeing digital noise. Switching to lower ISO should help.

Answer 3

Every type of contamination on a camera lens is out of focus so the answer is that it would most definitely not cause 'graininess'. You may, however, notice a reduction in brightness or contrast of the image as less light will be going through the lens.

All camera sensors produce visible 'noise', which used to be called grain in the days of film and which can be seen if the image is magnified. The amount depends on the ISO rating being used (another film term) and is always greater at higher ISOs.
The situation is bad with point-and-shoot cameras because they have small lens apertures which let in less light resulting in the use of the noisier higher ISO settings.
More expensive cameras tend to have bigger lenses and use cunning software to reduce the apparent levels of noise in the image.

A scratch or big blob of dirt on the lens might cause other issues as suggested above.

Incidentally, the auto-cue equipment used in television works by relying on the fact that the text being read is so close to the camera lens that it is invisible to the audience.