# Camera Lens Magnification Calculator (Macro and Extension Tubes)

URL: https://speedycalculator.com/calculators/other/camera-lens-magnification/
Last updated: 2026-10-30

> The camera lens magnification calculator finds the magnification and reproduction ratio of a photographic lens from its focal length and focus distance, splits that distance into object and image distances, and shows how much an extension tube raises the magnification for macro shots.

## Inputs

- Focus distance (sensor to subject) (m, cm, mm, ft, in, km, yd, mi)
- Lens focal length (mm, cm, in)
- Extension tube length (0 = none) (mm, cm, in)
- Sensor width (for the area covered) (mm, in, cm)
- Subject size (optional) (m, cm, mm, ft, in, km, yd, mi)

## Results

- Magnification
- Reproduction ratio 1 : x, x =
- Object distance (lens to subject)
- Image distance (lens to sensor)
- Width of the scene covered
- Size of the subject on the sensor
- Magnification with the extension tube
- Subject distance from the lens with the tube
- Tube magnification with the lens at infinity (e ÷ f)
- Closest possible focus distance for a simple lens (4f)

## Formula

```
r = √(d²/4 − f·d) · image distance h = d/2 − r · object distance g = d/2 + r · m = h ÷ g · with an extension tube e: h′ = h + e, m′ = h′/f − 1, g′ = f·h′ ÷ (h′ − f)
```

## Explanation

Magnification in photography is the ratio between the size of a subject's image on the sensor and its real size. A frame-filling bird photo might have a magnification of only 0.01 (1:100), while a true macro lens reaches 1.0 (1:1) – a 10 mm insect becomes 10 mm wide on the sensor. It is different from "zoom": zoom compares focal lengths, magnification compares sizes.

Cameras and lenses specify the focus distance from the sensor plane (the ⦽ mark on the body) to the subject. That distance is the sum of the object distance g (lens to subject) and the image distance h (lens to sensor). For a thin lens these satisfy 1/f = 1/g + 1/h, so a known focus distance gives both and the magnification h/g. The closest focus is 4f for a simple lens, where magnification reaches 1:1.

Extension tubes are hollow spacers that move the lens farther from the sensor. They add nothing optical, but the extra image distance raises the magnification – by e ÷ f at infinity focus – which is why a 25 mm tube on a 50 mm lens gives about 0.5× even when the lens is set to infinity.

### How to use the Camera Lens Magnification Calculator

- **Focus distance (sensor to subject)**
- **Lens focal length**
- **Extension tube length (0 = none)**
- **Sensor width (for the area covered)**
- **Subject size (optional)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the focus distance (sensor to subject) and the focal length. The quadratic from the thin-lens equation gives the object and image distances and the magnification. An extension tube adds to the image distance; the new subject distance and magnification follow from the same equation.

### Worked example

A 50 mm lens focused at 1 m: r = √(1000²/4 − 50 × 1000) = √200,000 = 447.2 mm, so the image distance is 500 − 447.2 = 52.8 mm and the object distance 947.2 mm. Magnification m = 52.8 ÷ 947.2 = 0.0557 (about 1:18), and a full-frame sensor covers 36 ÷ 0.0557 ≈ 646 mm of the scene.

Add a 25 mm extension tube: the image distance becomes 77.8 mm, so m′ = 77.8 ÷ 50 − 1 = 0.556 (about 1:1.8) and the subject must now be 50 × 77.8 ÷ 27.8 ≈ 140 mm from the lens.

### Magnification and reproduction ratio

MagnificationRatioTypical use0.01×1:100Full-length portrait0.1×1:10Flower, product shot0.5×1:2Close-up / "half macro"1×1:1True macro – insects, coins5×5:1Extreme macro, e.g. the Canon MP-E 65 mm

### Inputs explained

FieldWhat to enterFocus distance (sensor to subject)Units: m, cm, mm, ft, in, km …Lens focal lengthUnits: mm, cm, inExtension tube length (0 = none)Units: mm, cm, inSensor width (for the area covered)Units: mm, in, cmSubject size (optional)Units: m, cm, mm, ft, in, km …

### What each result means

- **Magnification** = [h]÷[g]
- **Reproduction ratio 1 : x, x =** = [g]÷[h]
- **Object distance (lens to subject)** = [g]
- **Image distance (lens to sensor)** = [h]
- **Width of the scene covered** = [Sensor width (for the area covered)]÷[Magnification]
- **Size of the subject on the sensor** = [Subject size (optional)]×[Magnification]
- **Magnification with the extension tube** = ([h] + [Extension tube length (0 = none)])÷[Lens focal length] - 1
- **Subject distance from the lens with the tube** = [Lens focal length]×([h] + [Extension tube length (0 = none)])÷([h] + [Extension tube length (0 = none)] - [Lens focal length])
- **Tube magnification with the lens at infinity (e ÷ f)** = [Extension tube length (0 = none)]÷[Lens focal length]
- **Closest possible focus distance for a simple lens (4f)** = 4×[Lens focal length]

## Worked example

Inputs — Focus distance (sensor to subject): 1 m; Lens focal length: 50 mm; Extension tube length (0 = none): 25 mm; Sensor width (for the area covered): 36 mm; Subject size (optional): 0.01 m.

Results — **Magnification = 0.05573 ×**; Reproduction ratio 1 : x, x = = 17.94; Object distance (lens to subject) = 0.9472 m; Image distance (lens to sensor) = 52.786 mm; Width of the scene covered = 0.646 m; Size of the subject on the sensor = 0.557 mm; Magnification with the extension tube = 0.55573 ×; Subject distance from the lens with the tube = 0.14 m; Tube magnification with the lens at infinity (e ÷ f) = 0.5 ×; Closest possible focus distance for a simple lens (4f) = 0.2 m.

## Key terms

- **Magnification**: Image size on the sensor divided by the real subject size.
- **Focus distance**: Distance from the sensor plane to the subject.
- **Extension tube**: A hollow spacer that moves the lens away from the sensor to focus closer.
- **Working distance**: Space between the front of the lens and the subject.

## FAQ

**What magnification counts as macro?**
1:1 (1.0×) or more is true macro. Lenses that reach 1:2 (0.5×) are often sold as "macro" too.

**How do I get 1:1 with a normal lens?**
Add extension equal to the focal length: about 50 mm of tubes on a 50 mm lens gives roughly 1:1.

**Why is the magnification of a 55 mm lens at 100 m so small?**
The image distance is about 55 mm and the object distance 100 m, so m ≈ 0.00055 – a 2 m subject is only 1.1 mm tall on the sensor.

**Does an extension tube reduce light?**
Yes. The effective f-number rises by (1 + m), so at 1:1 you lose two stops; through-the-lens metering compensates automatically.

**Is focus distance measured from the front of the lens?**
No – from the sensor plane mark on the camera body. The distance from the front of the lens is the working distance, which is shorter.

**Why does the calculator say no result for very close distances?**
A simple lens cannot focus closer than four times its focal length (2f object distance plus 2f image distance).


## Sources

- [Wikipedia – Macro photography](https://en.wikipedia.org/wiki/Macro_photography)
- [Wikipedia – Extension tube](https://en.wikipedia.org/wiki/Extension_tube)
- [Wikipedia – Magnification](https://en.wikipedia.org/wiki/Magnification)