Does taking a picture with 8 megapixel resolution with my Canon IXUS with a 1/2.3" sensor and 12M max resolution provides the same or worse image quality (particular, noise) as if the picture was taken by another camera of the same sensor size and but with 8 megapixels maximum resolution?
I believe Flimzy replied to exactly what you're asking:
This probably depends entirely on your camera, and how good a job it does at down-sampling. To answer for a specific camera model, you'll probably just have to try it and see the results for yourself. Flimzy
Additionally, fyi: when you select a lesser megapixel count mode (16:9 or something), that usually just crops the image, you get the same image quality "per pixel". When you select some of the modes with less megapixels within the same proportions, down-sampling comes in, and "per pixel quality" may vary.
I refer you to Flimzy's answer again, downsampling is entirely camera-dependent.
That being said, i bet photoshop or lightroom probably downsample as well or even better than in-camera algorithms.
When you reduce a noisy picture from its original sensor size to a smaller size you will always find that some of the noise disappears. This is not a defect in the camera, you will see the same thing if you shoot 12MP and then reduce the picture in Photoshop.
The reduction algorithm takes a bunch of noisy hi-res pixels and averages them down into less pixels in the reduced image. You can define noise as pixels that have a higher or lower value than the correct value. When you average a few pixels together some of the noise will cancel out, since you will be mixing pixels that are too high with pixels that are too low. The end result is that the smaller image has less noise.
If you want less noisy 12MP pixels, then switch to P mode (or any other manual or semi-manual modes) and use as low ISO as you can.
Another option to get less noisy images is to mount the camera on a tripod and shoot 2 or 3 or as many pictures as you want at 12MP, all identical. Take all those pictures into Photoshop or GIMP and average them out. This averaging has the same effect on noise that a size reduction has without actually reducing the picture.
In a general way, a higher-resolution sensor, with the same lens, is more sensitive to optical problems like chromatic aberrations. So if you have a lens that makes those aberrations, they should be more visible on the higher-resolution sensor, even after downsampling.
You should also care about the dynamic range: the higher the resolution is, the lower the dynamic range should be (for the same sensor size).
For noise, it cannot be compared theoretically between 2 cameras. The noise reduction depends on the camera itself, most of all if pictures are taken in JPG.
Image quality (especially noise, especially when not using max resolution) is so affected by the camera technology (noise reduction algorithm, down sampling algorithm, low ISO performance) and the lens (aperture) that the effect of the number of pixels is negligible.
If you could swap your camera's sensor with a sensor with similar technology but fewer pixels you would have gotten better quality — but since you can't, the camera with better optics and technology would always perform better regardless of megapixels.
That means that, if you are using good P&S cameras, the newer model (that is likely to have more pixels) will produce better pictures.
This is not always true for DSLRs, but is still usually true for cameras in the same "family" with the same lens. For example the 550D 18MP sensor performs better in low light than the 500D 15MP sensor because it handles high ISO better even with the smaller pixels.
Yes, you can trade off spacial resolution for signal to noise ratio. Each camera will have its own tradeoff between sensitivity and noise. One way to look at sensitivity is the point at which you get a certain level of noise.
Since spacial resolution and noise can be traded off, that leaves the question of how to compare sensitivity between cameras with different resolutions. To be fair, it would have to be done at the same resolution. For example, what is better, a 12.5 Mpix camera with good sensitivity or a 25 Mpix camera with less sensitivity. The 25 Mpix image may have more random noise on each pixel than the 12.5 Mpix image with all else held equal. But the 25 Mpix image can be shrunk to 12.5 Mpix and some of the noise reduced in the result. So the real question is which one makes the better 12.5 Mpix picture, and how in general should the noise (or sensitivity) of cameras with different resolutions be compared.
This was one of the things I was wondering when I made some tests with my camera (Nikon D3S, reputed to have top notch sensitivity). I don't have other cameras to compare it to, so published the result of my tests at www.embedinc.com/d32s. I did this mostly to see what the tradeoffs were with my camera, but it would be interesting to see similar tests done with other cameras, especially higher resolution cameras (like the D3X) and what the results would look like filtered down to the same resolution. Anyone with a D3X or other high end hi-res camera want to try and show the results?