A lens forming an image that cannot be caught on a screen has an image distance that is:

1. The sign convention follows from the type of image. 2. A real image is one that can be projected on a screen. 3. An image that cannot be projected is a virtual image. 4. The image distance is positive for a real image and negative for a virtual image. 5. So this image distance is negative. _Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 25 'Geometric Optics', sections 25.1-25.7_