arXiv:2610.41531v1 Announce Type: cross Abstract: Image priors can dramatically improve phase retrieval reconstruction, but their effectiveness strongly varies between imaging modalities. We introduce the generalized twin image, which extends the classical twin-image ambiguity from Fourier phase retrieval to any invertible sensing operator, to understand this effectiveness. The ambiguity extends to undersampled and oversampled measurements, remaining exact under undersampling and for oversampled coded Fourier measurements. We show that it provides a simple criterion to assess regularization effectiveness: the image prior should favor the ground truth over its generalized twin image. We propose the total-variation ratio as a simple measure of this preference, and derive closed-form estimates for structured random phase retrieval models. Experiments with a pretrained denoiser demonstrate that the ratio predicts regularized reconstruction performance across different sensing models.