Does this help? href=”https://www.bing.com/ck/a?!&&p=4bf8ca1abbda9c4bJmltdHM9MTcxOTg3ODQwMCZpZ3VpZD0wOGY4MmQ1Yi0wYzUxLTY5OTUtMWQ2NC0zOWViMGQ3NjY4MTQmaW5zaWQ9NTIxNw&ptn=3&ver=2&hsh=3&fclid=08f82d5b-0c51-6995-1d64-39eb0d766814&psq=measuring+luminance+with+a+digital+camera&u=a1aHR0cHM6Ly93d3cuYXRlY29ycC5jb20vYXRlY29ycC9tZWRpYS9wZGZzL2RhdGEtc2hlZXRzL3Rla3Ryb25peC1qMTZfYXBwbGljYXRpb24ucGRm&ntb=1″>atecorpSee page 8 first. Callibration starts at page 6.
British Astronomical Association
Supporting amateur astronomers since 1890