Smartphone Projector Calculator — DIY Lens Projection Distance & Image Size
Find the exact lens-to-wall distance and projected image size for your DIY phone projector — just enter the lens focal length, the phone-to-lens gap, and the screen size.
mm
mm
mm
Place the wall or screen this far from the lens to get a focused image
- 1
Denominator (dO − f)
100 − 80 = 20 - 2
Image distance dᵢ
80 × 100 ÷ 20 = 400 - 3
Magnification
400 ÷ 100 = 4 - 4
Wall distance (rounded)
round(400) = 400
How does this calculator work?
dᵢ = (f × dₒ) / (dₒ − f); magnification = dᵢ / dₒ. Enter the lens focal length and phone-to-lens gap (must exceed f) to get the wall distance and image size. A shorter phone-to-lens gap → farther wall, larger image; move the phone closer to 2f for a life-size image at 2f from the lens.
Formula
How this is calculated
A DIY projector works like a camera in reverse: a convex lens (magnifying glass) takes the bright phone screen as its object and projects a magnified, inverted image onto a wall. The key relationship is the thin-lens equation: 1/f = 1/dₒ + 1/dᵢ, where f is the focal length of the lens (a fixed property of the glass), dₒ is the distance from the phone screen to the lens, and dᵢ is the distance from the lens to the wall. Rearranging gives dᵢ = (f × dₒ) / (dₒ − f).
For a real, focused image to form, dₒ must be strictly greater than f. When dₒ is only slightly larger than f the image is far away and very large but dim; as dₒ increases toward 2f the image distance equals 2f and magnification equals 1 (life-size). Further increase in dₒ brings the image closer and smaller. The chart shows how projected image size scales with wall distance, so you can choose a trade-off between image size and brightness.
Practical limitations: this model assumes an ideal thin lens and a perfectly dark room. Real lenses have aberrations that blur the edges, especially cheap single-element magnifiers. Phone screen brightness and lens diameter (aperture) determine how bright the image appears — a larger lens diameter gathers more light. The image appears inverted and mirrored; rotating the phone 180° and flipping it face-up (in a box) corrects the orientation.
Frequently asked questions
Hold the lens in sunlight and focus the beam to a sharp spot on a piece of paper, then measure the distance from the lens to the paper — that's the focal length. Most cheap magnifying glasses used in DIY projectors fall between 50 and 150 mm.
A convex lens produces a real image on the opposite side only when the object is beyond the focal point. The image is always upside down and left–right mirrored. Place the phone upside-down inside the box so the projected image appears the right way up on the wall.
Single-element lenses suffer from spherical and chromatic aberration — the focal length varies slightly across the lens diameter, causing edges to blur. Using only the central zone of the lens (narrowing the aperture with a cardboard mask) sharpens the image at the cost of brightness.
Also known as
TG we-Calculate Editorial Team. (2026). Smartphone Projector Calculator — DIY Lens Projection Distance & Image Size [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/smartphone-projector-calculator
TG we-Calculate Editorial Team. "Smartphone Projector Calculator — DIY Lens Projection Distance & Image Size." TG we-Calculate. 2026. https://we-calculate.com/calculator/smartphone-projector-calculator.
TG we-Calculate Editorial Team, "Smartphone Projector Calculator — DIY Lens Projection Distance & Image Size," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/smartphone-projector-calculator
@misc{wecalculate_smartphone_projector_calculator, title = {Smartphone Projector Calculator — DIY Lens Projection Distance & Image Size}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/smartphone-projector-calculator}}, year = {2026}, note = {TG we-Calculate} }
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