It was necessary to obtain and process the relationship between the physical input and digital output of digital imaging equipment for its proper application in optical experiments. Based on ISO 14524:2009 standards, the focal plane opto-electronic conversion functions of digital single lens reflex camera Canon EOS 500D were measured and the data was analyzed. By comparing the effects of different pixel area sizes and different picture formats on the measurement results, it was found that the measurement deviation of the focal plane opto-electronic conversion function increased with the decrease of the pixel area size, and the relationship between the digital output value in RAW formats and the focal plane exposure was linear, which was different from the nonlinear relationship between the digital output value in JPEG formats and the focal plane exposure. When the DSLR camera was used as an instrument to record and measure light distribution, the RAW format that directly reflected the irradiance information received by the sensor can be used to obtain a large dynamic range, theoretically avoid the impact of subsequent processing of camera such as nonlinear transformation, and represent the scene information more accurately after necessary corrections.