A camera , in particular 3D time-of-flight camera, for acquiring three-dimensional image data from a detection area , comprising an illumination unit for transmitting transmitted light into the detection area , the transmitted light being modulated with at least a first modulation frequency, an image sensor for receiving received light from the detection area , the image sensor comprising a plurality of receiving elements for generating a respective received signal, a plurality of demodulation units for demodulating the received signals at the first modulation frequency to obtain sample values, a reference illumination unit for transmitting reference light modulated at the first modulation frequency and guided within the camera onto the image sensor , and a control and evaluation unit configured, for a distance measurement, to control the illumination unit and/or the demodulation units for a first number of partial measurements with a respective different phase offset between the first modulation frequency for the transmitted light and the first modulation frequency for the demodulation, and to determine a distance value from the sample values obtained by the partial measurements per receiving element , and, for a function test, to control the reference illumination unit and/or the demodulation units for a second number of reference partial measurements with a respective different phase offset between the first modulation frequency for the reference light and the first modulation frequency for the demodulation, and to determine a reference distance value from the sample values obtained by the reference partial measurements per receiving element , characterized in that the control and evaluation unit is further configured to distribute the reference partial measurements for a function test over a plurality of distance measurements. an illumination unit for transmitting transmitted light into the detection area , the transmitted light being modulated with at least a first modulation frequency, an image sensor for receiving received light from the detection area , the image sensor comprising a plurality of receiving elements for generating a respective received signal, a plurality of demodulation units for demodulating the received signals at the first modulation frequency to obtain sample values, a reference illumination unit for transmitting reference light modulated at the first modulation frequency and guided within the camera onto the image sensor , and a control and evaluation unit configured, for a distance measurement, to control the illumination unit and/or the demodulation units for a first number of partial measurements with a respective different phase offset between the first modulation frequency for the transmitted light and the first modulation frequency for the demodulation, and to determine a distance value from the sample values obtained by the partial measurements per receiving element , and, for a function test, to control the reference illumination unit and/or the demodulation units for a second number of reference partial measurements with a respective different phase offset between the first modulation frequency for the reference light and the first modulation frequency for the demodulation, and to determine a reference distance value from the sample values obtained by the reference partial measurements per receiving element , characterized in that the control and evaluation unit is further configured to distribute the reference partial measurements for a function test over a plurality of distance measurements. <EOS>