Smartphone Projector
DIY smartphone projector lens calculator using the thin lens equation. Enter screen size and any two of object distance, image distance, or focal length for magnification.
Fill any two of the three below:
About This Calculator
Building your own smartphone projector is an easy and affordable DIY project — you only need a shoebox, a magnifying glass, and your smartphone. The Smartphone Projector Calculator helps you determine the perfect optical setup by applying the thin lens equation (1/f = 1/u + 1/v) to find the ideal distances between your phone screen, lens, and projection wall.
This calculator is designed for anyone building a DIY smartphone projector, whether for a movie night, classroom demonstration, or hobby project. Simply enter your smartphone's screen diagonal in inches, then fill in any two of the three optical parameters — object distance (phone to lens), image distance (lens to wall), or focal length of your magnifying glass — and the calculator instantly computes the missing value.
The results include the magnification factor (how many times larger the projected image is compared to your phone screen) and the projected image diagonal, giving you a clear picture of what to expect before you even start building. The breakdown table shows all five parameters at a glance, and the interactive chart helps visualize the relationship between distances.
For the lens to produce a real, projectable image, the object distance (u) must be greater than the focal length (f). If u equals or is less than f, the image becomes virtual and cannot be projected onto a wall. A standard magnifying glass with a focal length of 10–20 cm works well for most smartphones. Shoeboxes typically measure 30–35 cm in length, which is sufficient for most setups.
Once you have your calculated values, build your projector by cutting a hole in a shoebox for the lens, mounting your phone on a makeshift stand inside the box, and adjusting the distances as calculated. Remember to paint the inside of the box black for better contrast and dim the room lights for the best viewing experience.
Frequently Asked Questions
How does a smartphone projector work?
A DIY smartphone projector uses a convex lens (magnifying glass) placed in front of a smartphone screen. The lens magnifies and projects the screen image onto a wall. The thin lens equation 1/f = 1/u + 1/v determines the relationship between the object distance (phone to lens), image distance (lens to wall), and focal length of the lens.
How far should my smartphone be from the lens?
The distance between your smartphone and the lens must be greater than the focal length of the lens to produce a real, projected image. Use the formula v = u×f/(u−f) where u is the object distance and f is the focal length. Most DIY projectors work well with object distances of 15–25 cm when using a 10–15 cm focal length lens.
What focal length lens should I use for a smartphone projector?
A magnifying glass with a focal length between 10 cm and 20 cm works best for a DIY smartphone projector. Shorter focal lengths produce higher magnification but require the phone to be very close to the lens, while longer focal lengths produce lower magnification but are easier to set up. A 15 cm focal length lens is a good starting point.
How big will the projected image be?
The projected image diagonal equals your smartphone screen diagonal multiplied by the magnification factor m = v/u, where v is the distance from lens to wall and u is the distance from phone to lens. For example, a 6.1-inch smartphone placed 20 cm from a 15 cm focal length lens projects a 43-inch image on a wall 60 cm away.
Why is my projected image blurry?
A blurry image usually means the lens-to-wall distance or phone-to-lens distance is not properly set. Adjust the distance between your smartphone and the lens (u) or move the entire projector closer to or farther from the wall (v). The image is sharpest when the thin lens equation 1/f = 1/u + 1/v is satisfied. Also ensure the lens is clean and the room is dark.
Do I need a special lens for a smartphone projector?
No special lens is required — a standard magnifying glass with a convex lens works well. You can find them for as low as $3 online. Fresnel lenses (thin plastic sheets with concentric grooves) are also popular for DIY projectors as they provide a larger aperture and shorter focal length in a flat form factor.
Can I use any box for a DIY smartphone projector?
Yes, any sturdy box like a shoebox works well. The box should be long enough to accommodate the distance between your phone and the lens (object distance u). If the box is too short, the phone may need to be placed closer than the lens focal length, which won't produce a projected image. A typical shoebox of 30–35 cm length is sufficient for most setups.
How do I find the focal length of my magnifying glass?
To measure a lens focal length, go outside on a sunny day and hold the lens above a piece of paper. Move the lens up and down until the sun's rays focus to the smallest, brightest point on the paper. Measure the distance from the lens to the paper — this is the focal length. Alternatively, use our calculator by entering measured u and v values from a working projector setup to compute f.