Macro magnification and subject coverage

What 1:1 actually means

A reproduction ratio of 1:1 means a 10 mm subject is projected as a 10 mm image on the sensor. On a 36 mm-wide full-frame sensor, the horizontal subject field is therefore about 36 mm. On a roughly 24 mm-wide APS-C sensor at the same magnification, only about 24 mm fits across the frame.

At 2:1 the image on the sensor is twice life size, so the covered subject field halves. At 0.5× it is half life size, so the field doubles. This geometric definition is more dependable than marketing labels such as close-up mode.

Pixels per millimetre and useful detail

Dividing the image’s horizontal pixel count by the calculated subject-field width gives a theoretical pixels-per-millimetre figure. It is useful for document capture, film digitisation and specimen records, but it is not the same as resolved detail.

Lens performance, diffraction, focus accuracy, vibration, sensor sampling and subject contrast all reduce real resolution. At high magnification, effective aperture becomes smaller and depth of field becomes extremely thin, so stable support and focus stacking often matter more than adding pixels.

Working distance matters when buying

Two lenses that both reach 1:1 can leave very different space between the subject and the front of the lens. More working distance helps with lighting and skittish subjects; a compact short lens may be easier for copy work. Check the manufacturer’s working-distance specification, not only minimum focus distance, which is measured from the sensor plane.