Smartphone Projector Calculator
Calculate magnification, focal length, and throw distance for DIY smartphone projector setups using lens optics.
How Does a DIY Smartphone Projector Work?
A magnifying lens placed in front of a phone screen forms a real, enlarged image on a wall or projection surface. The lens focal length and the distance from the phone to the lens determine magnification and throw distance.
Projector Optics Formulas
$$M = \frac{f_{\text{lens}}}{d_{\text{object}} - f_{\text{lens}}}$$ $$\frac{1}{f} = \frac{1}{d_{\text{object}}} + \frac{1}{d_{\text{image}}}$$Object distance must exceed the focal length to project a real image. Image distance tells you how far to place the screen from the lens for a sharp picture.
Building a Smartphone Projector
Use a convex lens with a focal length suited to your box depth. Move the phone until the image is sharp on your screen. A longer throw ratio means the screen must sit farther from the lens for the same phone position.
Related tools: Focal Length Calculator and Lens Magnification Calculator.
Frequently Asked Questions
Why must object distance exceed focal length?
When the object is farther than one focal length, the lens forms a real, inverted image that can be projected onto a screen.
What lens focal length works best?
Reading-glass lenses around 100 mm focal length are common in DIY builds. Shorter focal lengths give higher magnification but require closer alignment.
Is the projected image inverted?
Yes. A single convex lens inverts the image. Rotate the phone or mirror the display to view it correctly.
What is throw ratio?
Throw ratio is image distance divided by object distance. It describes how far the screen must be relative to the phone-to-lens spacing.
Can I use the screen distance to check focus?
Yes. Enter your target screen distance and compare it to the calculated image distance. Match them for a sharp projection.
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