At low light levels the charge transfer is sometimes imperfect resulting in slightly deformed images, showing a trailing along the rows and/or columns of the CCD. To increase the performance one can preflash the chip with a fixed amount of light. This is, of course, undesirable because it adds shot noise to the data. All the chips in use on La Palma have good charge-transfer characteristics and do not require preflash. The charge transfer characteristics are somewhat worse at faster readout speeds.