I recommend IDA Pro with Hex Rays ARM dissasembler for reverse engineering.
First get yourself an encrypted firmware image. Pentax host firmware images for a load of models.
Decrypt the firmware with the Pentax Firmware tool available at https://github.com/i-am-shodan/PentaxFirmwareTool
Load the decrypted file in IDA. You'll need to set big endian ARM CPU and a load address of 0xA0000000.