Focal Length Calculator: Lenses and Mirrors
Calculate focal length, image distance, and object distance for convex/concave lenses and mirrors using the thin lens equation. Essential for optics in physics an...
What is Focal Length?
Focal length (f) is the distance from a lens or mirror to its focal point — where parallel rays of light converge (convex) or appear to diverge from (concave). It determines the magnification and image position produced by an optical element and is critical for cameras, telescopes, microscopes, and eyeglasses.
🔭 Focal Length Calculator
Use our free calculator for instant, accurate results.
📐 Formula
1/f = 1/v + 1/u
Thin lens equation. f = focal length, v = image distance, u = object distance. Sign convention: distances measured from lens, + is real/on same side as light exit.
📝 Worked Example
Object 30 cm in front of convex lens (f=10 cm):
1/v = 1/10 − 1/30 = 3/30 − 1/30 = 2/30
v = 15 cm (real image on other side)
Magnification = −v/u = −15/30 = −0.5 (inverted, half size)
📝 How to Use the Calculator
❓ FAQ
What is the difference between a real and virtual image?
Real images form where light rays actually meet (can be projected on a screen). Virtual images form where rays appear to diverge from (like in a mirror — cannot be projected).
How does focal length relate to magnification in cameras?
Longer focal length = narrower field of view + more magnification. Shorter = wider angle. A 50mm lens approximates human eye perspective.
Veer Kumavat
Founder & AuthorVeer is a 14-year-old student from Nashik, Maharashtra, who built SciFi Calculators to help students worldwide master STEM subjects. He is passionate about making complex science and math problems accessible through intuitive digital tools.