[Shiraz (Ray)]

well depth is 4x with 2x2 binning, but the transfer and output electronics usually cannot handle that much signal - I think that the makers generally turn down the gain by about 0.5 to protect the output stages, so that you get a compromise of up to about 2x the effective well depth - seems that it varies with the chip design. Still quite usable if the read noise is low - but sometimes the read noise also suffers with binning. Overall, binning is a useful extra facility, but not quite the same as a "real" larger pixel. As Greg points out, the icx694 comes pretty close though.

Software binning is possibly a better option overall - if you expose the 1x1 subs for long enough to bury the read noise you get all the advantages (and disadvantages) of bigger pixels - 4x the sensitivity, 4x the well depth and 1/2 the linear resolution.