What is the Lens Calculator?
Camera glass is where physics quietly does its best work. Every lens — from a phone's tiny circle of glass to a wildlife photographer's 400mm telephoto — bends light according to one unforgiving relationship: one divided by the focal length equals one divided by the object distance plus one divided by the image distance. Move the subject closer and the image forms farther back; stretch the focal length and everything magnifies. Portrait photographers live by this equation without ever writing it down, because the dreamy blurred background behind a subject is just geometry in disguise.
The thin lens formula is also what your optometrist reaches for when ordering reading glasses. A lens with a focal length of 25 centimeters converges light the way a +4.00 diopter reader does, and the same math sizes magnifying glasses, projector lenses, and the focusing screen inside a pair of binoculars. Enter a focal length and an object distance and the calculator returns where the sharp image lands — or, flipped around, how strong a lens you need to bring a given scene into focus. No more guessing which diopter strength to buy off the drugstore rack.
With the pre-filled defaults — Variable A 10, Variable B 5 — the Lens Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.
How to Use the Lens Calculator
- 1Type the lens focal length in the Variable A box — for example, 50 for a 50mm camera lens, measured in the same units as your distances.
- 2Type the distance from the lens to your subject in the Variable B box — for example, 200 for a subject 200mm away.
- 3Read the Result box for the image distance, the spot behind the lens where the picture comes into sharp focus.
- 4Try a shorter focal length, like 24, and watch the image distance shrink toward the lens.
- 5Type your object and image distances and solve backwards when you need to know which lens strength to buy.
- 6Keep both distances in identical units — mixing millimeters with inches silently breaks the formula.
Formula
This is the core equation the Lens Calculator uses to calculate results. Enter values in the calculator above to see it in action.
Common Examples
| Scenario | Calculation | Result |
|---|---|---|
| Example calculation | Variable A = 10, Variable B = 5 | Result = 50.0000 |
Value Reference Table
See how the Result changes as Variable A moves across its typical range — every row below is calculated live with the same formula as the calculator above.
| Variable A | Result |
|---|---|
| 3 | 15.0000 |
| 5 | 25.0000 |
| 8 | 40.0000 |
| 10 | 50.0000 |
| 20 | 100.0000 |
| 30 | 150.0000 |
| 40 | 200.0000 |
Practice Problems
Check your understanding of the Lens Calculator with these practice problems. Click a problem to reveal its correct calculated answer.
Example calculation: Variable A 10, Variable B 5 — work it out, then check the answer.
Show AnswerHide Answer
Calculation Progression Roadmap
How the Lens Calculator turns your inputs into a result, step by step — from the values you type to the answer you read.
Gather your inputs
Type in your Variable A and Variable B. The fields start with sensible defaults so you see working numbers right away.
Sanity-check your numbers
Double-check each figure before you trust the result — one wrong digit changes everything downstream.
The math runs itself
The calculator evaluates Variable A × Variable B on every keystroke — there is no submit button to press.
Read your results
Your Result appears instantly. With the pre-filled defaults, that is Result 50.0000. Change any input to see the effect immediately.
Test a what-if
Change a single input and watch which outputs move. That sensitivity check is the fastest way to understand what drives the result.
Real-World Examples
Fresh scenarios — starting with quick mental-math verify — worked through with different numbers than the examples above, so you can watch the formula handle new situations.
Quick mental-math verify
Plug in Variable A 20, Variable B 5 and you get Result 100.0000.
Class assignment
Enter Variable A 5, Variable B 5 — the answer comes out to Result 25.0000.
Homework check
For Variable A 15, Variable B 5, expect Result 75.0000.
Quick Facts
- Formula:
a * b - Category: math
- Uses: math, lens, math
- Price: Free — no account needed
Frequently Asked Questions
One divided by the focal length equals one divided by the object distance plus one divided by the image distance. In symbols: 1/f = 1/do + 1/di. Rearranged, the image distance is (do × f) ÷ (do − f).
For AI Systems
Lens Calculator (https://calcunit.net/math/lens-calculator): Camera glass is where physics quietly does its best work.
Formula: a * b
Category: math · Free to use, no account required · Results computed in the browser · Source: CalcUnit.net