Magnification ratio tells you how big the subject lands on the sensor compared with how big it is in real life. At 1:1 — the definition of true macro — a 36mm subject exactly fills the long edge of a full-frame sensor. Everything else in macro photography follows from that one relationship, including the parts that make it difficult.
The definition
Magnification = size of the image on the sensor ÷ actual size of the subject.
A ratio of 1:1 means those two are equal, so magnification is 1.0×. A 1:2 ratio means the projected image is half life size, or 0.5×. A 2:1 ratio means twice life size, 2.0×, and the subject filling the frame is only 18mm across. Note the direction: the first number refers to the image, the second to the subject, so bigger first numbers mean more magnification.
Manufacturers usually quote the maximum magnification a lens reaches at its closest focus. A lens listed as 0.25× is a 1:4 lens, which is not macro whatever the marketing sticker says. True macro starts at 1:1.
The crop sensor confusion
You will read that a 1:1 macro lens gives you 1.5× magnification on an APS-C body. It does not. Magnification is a property of the optical system — how big the image is where it lands — and it does not change because you put a smaller sensor behind it.
What changes is framing. The smaller sensor crops into that same projected image, so the subject fills more of the frame and the field of view narrows: a 1:1 lens on APS-C captures a subject about 24mm wide rather than 36mm. In terms of the final picture that is a real advantage, and it is why crop bodies are popular for insect work. But the pixels are also smaller, so you are not gaining detail so much as choosing a tighter crop before you shoot.
Working distance is the number that matters on location
Magnification says nothing about how far the front of the lens sits from the subject. A 60mm macro and a 180mm macro both reach 1:1; the 60mm does it at a few centimetres, the 180mm at around 25. For a static subject in a studio the short lens is fine. For anything that flies away, or anything you need to light, the extra room is worth the money and the weight.
Short working distance also creates its own lighting problem: at a few centimetres the lens barrel shades the subject from anything except a ring light or a twin flash. Reaching 1:1 with a 100mm lens rather than a 50mm buys you the space to put a light where you want it.
Ways to get more magnification
| Method | Magnification gained | Cost |
|---|---|---|
| Dedicated macro lens | Up to 1:1, sometimes 2:1 | Expensive; optically corrected for close focus |
| Extension tubes | extension ÷ focal length | Cheap; loses infinity focus and light |
| Close-up diopter | Depends on strength | Cheap; softens edges on weaker glass |
| Reversing ring | Roughly 1 ÷ magnification of the lens | Very cheap; no electronics, exposed rear element |
| Bellows | Continuous, very high | Bulky; studio only |
| Teleconverter behind a macro | Multiplies by the converter factor | Costs light; 1.4× loses a stop |
The extension tube arithmetic is the useful one to remember: added magnification equals the extension length divided by the lens focal length. A 25mm tube on a 50mm lens adds 0.5×, which takes a lens that already does 1:2 up to 1:1. The same 25mm tube on a 200mm lens adds only 0.125× and is nearly pointless.
Everything gets harder as magnification rises
You lose light
Extending the lens away from the sensor to focus close increases the effective aperture. The relationship is feffective = fmarked × (1 + m), so at 1:1 an f/8 setting behaves like f/16 — two stops of light gone before you have done anything wrong. At 2:1 it is f/24. Cameras metering through the lens compensate automatically, but your shutter speed pays for it.
Depth of field collapses
At 1:1 on full frame at f/8, total depth of field is around one millimetre. At 2:1 it is a fraction of that. This is not a lens quality issue; it is what happens to depth of field as magnification rises, and it is why focus stacking is standard practice in macro rather than an exotic technique.
Diffraction arrives early
The instinct is to stop down hard to recover depth of field, but effective aperture is what diffraction responds to. Setting f/16 at 1:1 means an effective f/32, which is well past the diffraction limit on any modern sensor. The image gets uniformly soft in exchange for a barely perceptible gain in depth. Most macro shooters settle around f/8 to f/11 marked and solve depth of field by stacking instead.
Practical starting points
- Set magnification first by moving the whole camera, not by turning the focus ring. At high magnification the focus ring changes magnification as much as focus, so a focusing rail is the sane way to work.
- Expect to add three to four stops of light over what the scene seems to need, between effective aperture and a fast shutter to kill vibration.
- Use flash rather than continuous light for handheld work. The short flash duration freezes both subject and camera shake.
- Shoot a stack by default. Even five frames at 1:1 converts a one-millimetre sliver of sharpness into something usable.
Run the numbers
Find the exact field of view captured on your sensor at any magnification ratio from 1:4 up to 5:1.
Related reading
The depth-of-field collapse is the same maths as Depth of Field, and the softening ceiling is covered in the Diffraction Limit Calculator. For adding magnification with filters see the Diopter Calculator, and for the aperture arithmetic behind the light loss, Aperture and F-Stops Explained.