As a result, scanned photos will lose some image quality in the highlights and shadows. But the average user with a regular printer should find 24 bits adequate for most documents and web graphics. If you plan on getting a scanner for slides, negatives or transparencies you need at least 30 bits, and 36 bits would be preferable. Now you might be wondering what happens if you get a scanner with 30 bits, but your monitor or printer can only handle 24 bits of colour data. The extra information at higher bit depth will still produce smoother colour gradations and better overall image reproduction.
More pixels mean better resolution and sharper image quality.
The number of pixels a scanner can generate is based on how many CCDs are arranged horizontally and vertically in the scan head. Remember one CCD produces one pixel. When comparing resolutions always use the smaller number. If you need to pick up fine details, small fonts or complex lines or edges you'll want more optical resolution. The interpolated resolution for a scanner is always higher than its optical resolution.