Macro Magnification Calculator
Magnification, extension, working distance and light loss — where 1:1 costs exactly two stops and brings the diffraction limit forward by the same two.
Distances are measured from the lens's principal plane, not from the front element, so the real gap between glass and subject is shorter than the figure here by roughly the length of the barrel. Internally-focusing macro lenses also lose focal length as they approach life size, which shortens working distance further — so treat these as the thin-lens ideal rather than a spec sheet. Nothing is uploaded.
Life size costs exactly two stops — and moves diffraction two stops with it
The effective aperture is N(1 + m), so at 1:1 it is exactly 2N — set f/8 and the light sees f/16, two full stops, not approximately two. The part that gets forgotten is that diffraction follows the same number. On full frame the Airy disc fills the circle of confusion at a nominal f/10.8 at life size, against f/21.6 at infinity. So the f/16 people reach for when the depth turns out to be 0.93 mm is already 1.48× past the limit: the depth of field grows and the detail goes.
| Magnification | Ratio | Stops lost | f/8 becomes | Subject across | Depth (100 mm) |
|---|---|---|---|---|---|
| 0.1× | 1:10 | 0.28 | f/8.8 | 360.0 mm | 51.07 mm |
| 0.25× | 1:4 | 0.64 | f/10.0 | 144.0 mm | 9.28 mm |
| 0.5× | 1:2 | 1.17 | f/12.0 | 72.0 mm | 2.78 mm |
| 1× | 1:1 | 2.00 | f/16.0 | 36.0 mm | 0.93 mm |
| 2× | 2:1 | 3.17 | f/24.0 | 18.0 mm | 0.35 mm |
| 5× | 5:1 | 5.17 | f/48.0 | 7.2 mm | 0.11 mm |
The penalty is front-loaded: half life size has already spent more than half of the two stops, because the cost grows with the logarithm rather than the magnification. And at 1:1 the subject across the frame is exactly the sensor width, which is what "life size" literally means.
| Set aperture | Effective at 1:1 | Airy disc | Against the circle |
|---|---|---|---|
| f/4 | f/8 | 0.0107 mm | 0.37× |
| f/5.6 | f/11.2 | 0.0150 mm | 0.52× |
| f/8 | f/16 | 0.0215 mm | 0.74× |
| f/11 | f/22 | 0.0295 mm | 1.02× |
| f/16 | f/32 | 0.0429 mm | 1.48× |
| f/22 | f/44 | 0.0590 mm | 2.04× |
A tube is worth eight times as much on a short lens
Added magnification is extension over focal length, so the same 25 mm tube adds 1.000× on a 25 mm and 0.125× on a 200 mm — exactly eight times as much. Reaching life size always needs a tube as long as the lens, which is why tubes are a short-lens trick and a long-lens disappointment.
| Lens | 25 mm tube adds | Ratio | Tube for 1:1 |
|---|---|---|---|
| 25 mm | 1.000× | 1:1 | 25 mm |
| 50 mm | 0.500× | 1:2 | 50 mm |
| 100 mm | 0.250× | 1:4 | 100 mm |
| 200 mm | 0.125× | 1:8 | 200 mm |
Working room runs the other way, and exactly proportionally: at life size the subject sits at twice the focal length, so a 100 mm macro puts you precisely twice as far from the insect as a 50 mm at the same framing. That proportionality is the whole argument for a long macro lens.
| Macro lens | Subject distance at 1:1 |
|---|---|
| 50 mm | 100 mm |
| 60 mm | 120 mm |
| 90 mm | 180 mm |
| 100 mm | 200 mm |
| 180 mm | 360 mm |
How to use
- Enter the focal length of the lens.
- Enter the magnification you want.
- Enter the aperture you plan to set.
- Read the working distance, extension, light loss and depth.
Frequently asked questions
How much light does macro cost?
The effective aperture is N times one plus the magnification, so at 1:1 it is exactly double — two full stops, not approximately two. Set f/8 and the light sees f/16, which is why macro work runs out of light long before it runs out of depth.
Should I stop down to get more depth of field?
Only so far. Diffraction follows the effective aperture, so at 1:1 the Airy disc fills a full-frame circle of confusion at a nominal f/10.8 — against f/21.6 at infinity. By f/16 the disc is already half again too big.
What does 1:1 actually mean?
The subject is projected at life size, so what fills the frame is exactly the width of the sensor — 36 mm on full frame. At 1:2 it is 72 mm, at 2:1 it is 18 mm. The ratio is the image size over the subject size.
How much extension do I need?
Focal length times magnification, so life size always needs a tube as long as the lens is. That is why tubes are a short-lens trick: 25 mm on a 25 mm lens reaches 1:1, while the same tube on a 200 mm adds only an eighth.
Why buy a longer macro lens?
Working room, and it is strictly proportional. At a given magnification the subject sits at the focal length times one plus the magnification, so a 100 mm puts you exactly twice as far from the insect as a 50 mm at identical framing.
How shallow is the depth of field?
At 1:1 on full frame at f/8 it is under a millimetre. That is the whole difficulty of macro photography — the depth is measured in fractions of a millimetre and stopping down to fix it costs sharpness to diffraction.
Are these distances measured from the front of the lens?
No, from the principal plane, so the real gap between glass and subject is shorter by roughly the length of the barrel. Internally-focusing macro lenses also lose focal length as they approach 1:1, shortening it further.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.