Archived from groups: rec.photo.digital (More info?)
>If I have a color image and convert to grey, do I get 4 x the spatial
No. You'd get a greyscale image with the same resolution as the colour
Actually, if you look closely, not all colour images are the same.
Uncompressed colour images from a 3-CCD camera, or a flatbed scanner,
or a film scanner, can have full-resolution red green and blue
components. If you convert this into a luminance plus colour
difference space (i.e. LUV, or Lab), the two colour channels have the
same resolution as the luminance channel. When you convert to
greyscale, you're basically just keeping the luminance and discarding
the two colour difference channels.
If your images are in JPEG format, they've already been converted to a
luminance/colour form internally, and the colour information has almost
certainly been downsampled by a factor of 2. So the colour information
is already only half the resolution of the luminance information. This
works well in practice for images destined for viewing, because the
human eye has much lower resolution for colour changes than luminance
If you have a digital camera with a Bayer-type filter, it inherently
generates images where the colour resolution is half that of the
luminance information. This matches JPEG output pretty well, but even
if you use RAW output files the colour information is lower resolution
But in all of these cases, the actual spatial resolution of the image is
comparable to the spatial resolution of the image file.