Some models for super-resolution restoration assume that low-resolution (LR) images are formed from high-resolution (HR) ones by a warping first, blurring second (followed by decimating) imaging process; whereas others adopt a blurring first, warping second imaging constraint. These models are abused to some extent and respective conditions for usability of them are not identified. Such ambiguities are analyzed and cleared up in this letter. Conclusions are: The warping-blurring model coincides with the general imaging physics, but it is usable only if the motion among HR images is known a priori. When the motion is estimated from LR images, this model may cause systematic error, but the use of the blurring-warping model is more appropriate, and leads to better performance.