Smartphone Projector Calculator
Estimate how big (and how dim) your DIY phone-and-magnifying-lens projector image will be at a given projection distance.
How a smartphone projector works
A simple DIY phone projector uses a single magnifying (convex) lens placed between the phone screen and a wall or screen. The lens bends light rays so the small, bright image on your phone is projected as a much larger, dimmer image at a distance. The relationship between object distance, lens focal length, and magnification follows the basic thin-lens optics used in simple projector and camera systems.
How this calculator works
Enter your phone’s screen diagonal and aspect ratio to get its physical width and height, then enter your lens’s focal length and how far you plan to place the screen or wall from the lens. The calculator estimates magnification as (distance – focal length) / focal length, then scales the screen dimensions by that factor to estimate the projected image size. It also estimates relative brightness, since a projected image’s brightness falls off roughly with the square of the magnification — bigger images from the same light source are always dimmer per unit area.
This is a simplified geometric-optics estimate for hobby/DIY projects and general educational purposes. Real results vary with lens quality, room darkness, and screen distance, and are not a substitute for testing your actual setup.