An apparatus comprising: an array of pixel sensors (21, 31), each pixel sensor (21, 31) comprising a photodetector, a charge conversion circuit and a gate transistor (34, 35), said charge conversion circuit generating a voltage signal that is a function of a charge on said photodetector, and said gate transistor coupling said voltage signal to a bit line in response to a first signal; a column processing circuit (28, 70, 90) connected to said bit line, said column processing circuit converting said voltage signal to a plurality of output digital values; the column processing circuit configured to generate an output control signal based on said voltage signal, wherein said column processing circuit comprises a variable gain amplifier having a plurality of gain values that amplifies said voltage signal to generate an amplified signal that is digitized by an analog-to-digital converter having a fixed number of bits; wherein said variable gain amplifier comprises a capacitive transimpedance amplifier; wherein the column processing circuit further comprises a double sampling circuit configured to store a first voltage that represents a measurement of a reset voltage measured at a first gain value of the plurality of gain values and a second voltage that measures said voltage signal, characterised in that said capacitive transimpedance amplifier having a feedback capacitance that is determined by said output control signal; and said double sampling circuit configured to output a measurement of said amplified signal minus said first voltage divided by a factor that depends on which of said plurality of gain values was used to generate said amplified signal, and wherein the column processing circuit is configured such that for lower light levels, the first gain value is the same as the gain used to generate the amplified signal and for higher light levels, the first gain value is different to the gain used to generate the amplified signal. an array of pixel sensors (21, 31), each pixel sensor (21, 31) comprising a photodetector, a charge conversion circuit and a gate transistor (34, 35), said charge conversion circuit generating a voltage signal that is a function of a charge on said photodetector, and said gate transistor coupling said voltage signal to a bit line in response to a first signal; a column processing circuit (28, 70, 90) connected to said bit line, said column processing circuit converting said voltage signal to a plurality of output digital values; the column processing circuit configured to generate an output control signal based on said voltage signal, wherein said column processing circuit comprises a variable gain amplifier having a plurality of gain values that amplifies said voltage signal to generate an amplified signal that is digitized by an analog-to-digital converter having a fixed number of bits; wherein said variable gain amplifier comprises a capacitive transimpedance amplifier; wherein the column processing circuit further comprises a double sampling circuit configured to store a first voltage that represents a measurement of a reset voltage measured at a first gain value of the plurality of gain values and a second voltage that measures said voltage signal, characterised in that said capacitive transimpedance amplifier having a feedback capacitance that is determined by said output control signal; and said double sampling circuit configured to output a measurement of said amplified signal minus said first voltage divided by a factor that depends on which of said plurality of gain values was used to generate said amplified signal, and wherein the column processing circuit is configured such that for lower light levels, the first gain value is the same as the gain used to generate the amplified signal and for higher light levels, the first gain value is different to the gain used to generate the amplified signal.