Calculate the setup for your DIY smartphone projector, including lens placement and image size.
Last updated: March 2026 | By ForgeCalc Engineering
Typical magnifying glass: 100-200mm
A DIY smartphone projector uses a simple convex lens (like a magnifying glass) to project the light from your phone's screen onto a wall. The lens bends the light rays so they converge on the wall, creating an enlarged image.
The key to a clear image is the "Thin Lens Equation." By adjusting the distance between the phone and the lens, you can find the "focus point" where the image is sharpest for a given distance to the wall. Note that the image will be projected upside down and mirrored!
Where:
• f is the focal length of the lens
• d_o is the object distance (phone to lens)
• d_i is the image distance (lens to wall)
• Magnification (M) = d_i / d_o
A single convex lens inverts the image. To fix this, you need to turn your phone upside down and disable screen rotation. The image will also be mirrored horizontally.
Hold the lens under a light (like the sun or a ceiling light) and measure the distance from the lens to the point where the light focuses into a tiny, sharp dot. That distance is the focal length (f).
Check your phone-to-lens distance (d_o). Even a small error of a few millimeters can cause blur. Also, ensure your phone brightness is at maximum and the room is completely dark.
Yes! Move the projector further from the wall (increase d_i). You will then need to move the phone slightly closer to the lens (decrease d_o) to refocus. The image will be larger but dimmer.
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