Tools and techniques for vision testing alter refractive characteristics of rendered digital imagery and use feedback from a test subject to calculate information about errors in the subject's vision. Some embodiments produce a rendered digital image with vision-realistic rendering, present the test subject with the rendered digital image, accept input from the test subject and change the refractive sharpness of the rendered digital image in response to the input. Then they calculate refractive errors or other information about the test subject's vision based at least on: input from the test subject leading to a refractive sharpness change made during the changing step, and a refraction model which relates visual acuity to changes in the refractive sharpness of rendered digital imagery.