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In this paper, we propose a noise level evaluation method for real captured photos. Different from conventional noise removal methods that assume noise follow a simply additive Gaussian distribution, noise distribution in our method is supposed to be a more sophisticated intensity-dependent distribution, which has a better fit with actual noise model. Follow the definition of noise level function (NLF) which represents the variation of the standard deviation of noise with respect to image intensity. After exposing the close relationship between NLF and camera response function (CRF), we fit the curve of NLF with the constraints imposed by the shape of CRF. Index Terms— image noise estimation, noise level function, camera response function