Choosing a lens for artefact photography

For a photograph of an object, the lens decides as much as the camera behind it. It sets how close the camera has to come, how much the parts of the object nearest the lens are enlarged, how straight the edges stay, and how much of the object is sharp. This guide goes through the choices in the order they matter for a picture that is meant to be measured and compared.

Macro lens or normal lens?

A macro lens is built to work close: it focuses near enough to show a small object at life size or half life size on the sensor, and it is corrected for a flat subject at that distance. A lens's specification gives this as its maximum magnification — 1:1, or 1.0×, means that 10 mm of the object covers 10 mm of the sensor. Most lenses that are not macro lenses reach only a fraction of that at their closest focus.

1. 1:2 · HALF LIFE SIZE — frame 72 × 48 mm, lens 3 f away; e.g. Fujifilm GF120
2. 1:1 · LIFE SIZE — frame 36 × 24 mm, lens 2 f away; e.g. Sony FE 90 mm macro
3. 2:1 · TWICE LIFE SIZE — frame 18 × 12 mm, lens 1.5 f away; e.g. Laowa 90 mm 2×
4. 5:1 · FIVE TIMES LIFE SIZE — frame 7.2 × 4.8 mm, lens 1.2 f away; e.g. Laowa 25 mm ultra macro

For coins, beads, brooches, flint and small sherds, a macro lens is what lets the object fill the frame. For objects larger than the sensor several times over — a vessel, a sword — any good prime lens at a moderate distance will do, and the camera's pixel count becomes the limit instead: see Choosing a camera.

Focal length and working distance

At the same magnification, a lens with a longer focal length stands further back. In a simple lens model the object lies at the focal length times (1 + 1/m) from the lens:

Focal length	At half life size (0.5×)	At life size (1×)
50 mm	50 × 3 = 150 mm	50 × 2 = 100 mm
100 mm	100 × 3 = 300 mm	100 × 2 = 200 mm

The figures are measured from inside the lens, not from its front, and many modern macro lenses shorten their focal length as they focus close, so the free space in front of the glass is smaller still. Check it on the lens itself.

The space matters for two reasons. The light has to reach the object from the upper left at 45° without the camera or the lens shading it. The second reason is perspective.

For a given lens on a given camera, Pixels on the Object gives the magnification, the distance from the object and the pixels the lens puts on it, for 109 lenses on 51 cameras, worked out from the makers' published figures.

Pixels on the ObjectSee how close each lens gets, how far from the object it stands and how many pixels it puts on it.→

Perspective: why a longer lens keeps the object plane honest

A camera sees a nearer thing larger. A part of the object that stands h millimetres above the surface it lies on, seen from D millimetres away, is enlarged by h ÷ (D − h) compared with the surface. For the top of the PHOTARCH Venus, 47.7 mm high:

• from 250 mm away: 47.7 ÷ 202.3 = 23.6 % too large;
• from 1,000 mm away: 47.7 ÷ 952.3 = 5.0 % too large.

1. CAMERA 250 MM ABOVE — the top 23.6 % too large
2. CAMERA 1,000 MM ABOVE — the top 5.0 % too large

The perspective depends only on the distance, not on the lens. A longer focal length is what lets the camera stand further back and still fill the frame. Seen from close by, a wide lens also shows the inside walls of a bowl or the sides of a bead in what should be a view straight down. How this affects a scale is worked through under Scale in artefact photography.

Distortion

A lens may bend straight lines outward toward the corners (barrel distortion) or inward (pincushion). Either way, a millimetre near the edge of the frame is not the same number of pixels as a millimetre in the centre, which matters as soon as the picture is measured. ISO 19264-1's tolerances allow at most 1.5 % geometric distortion at its strictest level: a point 30 mm from the centre of the field may then lie up to 30 × 0.015 = 0.45 mm from where it should.

Prime and macro lenses usually distort less than zooms. Keep the object in the centre of the frame, and if the raw developer offers a profile for the lens, use the same setting for every picture.

Aperture, diffraction and depth of field

Stopping down deepens the zone that is sharp, but past a point the small opening itself spreads the light — diffraction — and softens everything. Close up, the lens behaves as if it were stopped down further than its setting: the effective aperture is the f-number × (1 + m). At life size, f/8 acts as f/16.

At f/16, the blur from diffraction is about 2.44 × 0.00055 mm × 16 ≈ 0.021 mm across, 0.00055 mm being the wavelength of green light — more than three pixels on a sensor with 0.006 mm pixels. The depth of field close up is roughly 2 × f-number × c × (1 + m) ÷ m², where c is the blur accepted as sharp. Taking c as two such pixels (0.012 mm), at f/8:

• at life size: 2 × 8 × 0.012 × 2 ÷ 1 ≈ 0.4 mm;
• at a quarter life size: 2 × 8 × 0.012 × 1.25 ÷ 0.0625 ≈ 3.8 mm.

On most cameras f/8 to f/11 is where the gain in depth starts to cost sharpness. Focus by hand, and check the result on the object's outline, which is what is measured, as well as on its highest point. When the depth of the object is greater than f/11 can hold, the answer is not a smaller aperture but several frames — see Focus stacking below.

Focus stacking

A focus stack is several photographs of the same object from the same place, each focused a little deeper than the last, blended into one picture in which the whole object is sharp. It is the way past the limit above: a small, deep object at high magnification can need more depth than any aperture gives without diffraction softening the detail.

1. CAMERA · FIXED — the same place for every frame
2. ONE FOCUS SETTING — one thin slice sharp: 3.8 mm of her 47.7
3. SLICES STEPPED — through the depth: 15 of 3.8 mm, overlapping
4. THE LAMP NEVER MOVES — 10:30, 45° up

1. EACH STEP SMALLER — than a slice: the slices overlap
2. ONE STEP TOO LARGE — a band of her no frame holds sharp

Is a stack acceptable? The standards answer differently, and only one of them answers directly:

1. RAW FRAMES — kept beside the picture
2. STACKING — Photoshop, Helicon Focus or Zerene Stacker
3. TIFF, OR PSD — with the frames as layers
4. PHOTARCH — a PSD's master written as a TIFF

Metamorfoze's rule fits what it was written for. Its masters stand in for flat originals that are deteriorating and are withdrawn once digitised, so the master has to be first-generation evidence: one capture, untouched. For three-dimensional objects the field already works differently. A 3D model made by photogrammetry is a composite too — built from dozens of overlapping photographs, none of which is a picture of the whole object — and such models are accepted as documentation of the object. A focus stack is a smaller composite of the same kind: every frame in it is a true photograph of one slice of the object in focus.

What makes either one trustworthy is the same, and it is what FADGI asks of a stitched image: keep the frames, and say that the picture is a composite. In PHOTARCH the stack — the TIFF, or the layered PSD — is the finished picture and is never altered once it is in; keep the raw frames it was made from beside it, as a photogrammetric model keeps its photographs, and say in the record that it is a focus stack.

The PHOTARCH app measures how much of the object is in focus, as a share of the object — at least 80 % for a study and 95 % for a reproduction, both provisional — and leaves out, and reports, areas with too little texture to judge. At the stand, the Sharp light says whether the middle of the frame is sharp before the shutter is pressed. Focus and aperture remain the photographer's to set.