A white background for object photography

The ground an object lies on ends up in every photograph of it, and it changes how the object is seen. PHOTARCH photographs onto white and keeps the shadow the object casts on it. This guide explains why, how to expose a white ground without losing the object, and why a photograph on white is not the same thing as a cut-out.

The ground is the PHOTARCH board, or without one a sheet of glossy or semi-gloss white card. Not matt: a matt card has a texture of its own, and it shows in the shadow.

Why photograph objects on white?

The eye takes the brightest thing in a scene as white and judges every other tone against it. On a white ground, that reference is the same in every photograph: the ground. On grey, the brightest thing is often part of the object itself — a glint, a pale glaze — and it moves from object to object, so two pictures are judged on different scales before anyone compares them.

For a collection meant to be compared across thousands of pictures, a fixed reference decides it. A grey ground could serve, but grey is a scale, and a grey can land anywhere on it. Even if every collection agreed on the same card, say an 18 % grey, each photographer's exposure would put it at a slightly different tone in the picture. White adds no such doubt: the ground is exposed just under the point where it clips, and on the copies it is taken to full white, so white always ends up white.

White has practical virtues as well. Clipping shows at once in the histogram, nothing needs cutting out for the object to stand clear of the ground, and a uniform ground is what software needs to find an object reliably. The PHOTARCH app finds the object against the white ground and masks it. The mask is kept as an alpha channel: a fourth channel beside red, green and blue that says how much of each pixel belongs to the object, from fully to not at all. With it, objects can be shown side by side on any ground, each without its own. Large recording schemes have built their collections on the same rule: the Portable Antiquities Scheme asks for objects on a white background, and natural-history digitisation tends to the same.

White or grey?

Grey has one real argument behind it. A white surround makes an object look slightly darker and less saturated — simultaneous contrast — and that is why ISO 3664, the standard for viewing conditions, specifies the surround in a viewing booth — a neutral one, in its 2009 edition. That standard governs the room and the frame around a picture, not the picture's own background. No international standard sets a background for photographs of objects. In the PHOTARCH app the objects can be shown against a grey ground instead of white, its level set exactly.

Some collections have chosen differently: some photograph on a light grey, others on grey or black with the shadow removed. PHOTARCH chooses white with the shadow kept. Because every PHOTARCH picture has the same white, the effect grey would avoid is the same in all of them and cancels out in comparison. Grey remains the reference for making the picture; white is the ground for showing it. The full case is under Principles & Setup.

What should not happen is a mixture: grey for some objects and white for others makes two collections that cannot be laid side by side. That holds for F, the plumb view, which is the picture every object is compared with. Other views may differ, and so may pictures made for another purpose: a picture for an exhibition, on a black ground and from an angle, can be added to an object's record beside the view it shows, never in F's place. An object can also have more than one F — F1, F2 and so on — with other lighting or another ground, as long as one of them is made the way the method describes.

How to expose a white background without clipping the object

The exposure is for the object, and the white shows where it lies. The brightest part of the ground should sit just under the point where it clips — in an 8-bit view, around 250 of 255, which leaves a little room above it. The camera's highlight warning or a zebra shows where that point is.

Do not leave it to the camera's automatic exposure. A camera's meter aims to make what it sees a middle grey, so in a frame that is mostly white ground it darkens the picture until the white turns grey, and the object darkens with it. Photographers have long set the exposure with a grey card instead: an 18 % grey card laid where the object will be and metered, so that the middle tones come out right whatever surrounds them. It works, and with film it was the safest way. The PHOTARCH method reads the white instead. The white is in every picture, the camera's own displays show where it stands, and set just under the point where it clips it puts the brightest part of the picture where the raw file holds the most, with no reading in between to go wrong. Why middle grey is under How to photograph artefacts.

Two mistakes are common:

• Leaving the ground grey. Trusting the meter and letting the white fall two or three stops below its ceiling wastes the top of the file, and the correction that lifts the picture afterwards lifts the noise with it. More under Bit depth.
• Clipping the object. Polished metal and glaze can be brighter than the ground. Check the highlight warning on the object as well; a clipped area on the object holds nothing that can be recovered.

The light comes from the upper left, so the ground is a little brighter on that side than on the other. That fall-off comes with a light that has a direction. In the published copies, taking the ground to white removes it, while the light on the object stays as it fell — see Lighting archaeological objects.

Setting the exposure with the zebra and the highlight warning

Two displays on the camera do the work. The highlight warning — also called highlights, blinkies or overexposure warning — makes the parts of a picture that have clipped blink when it is played back: it says whether anything has turned pure white. The zebra lays stripes over the parts of the live view that are brighter than a level you choose: it shows the same thing before the picture is taken, and it can warn a little below the top.

Set the zebra to 95 %, or the nearest step the camera has, and photograph in raw. Then:

1. Put the camera on manual exposure, so that nothing changes between pictures.
2. Raise the exposure until the brightest part of the ground — the side nearest the key light — shows stripes. The ground is then above 95 %.
3. Take a picture and look at the highlight warning. The ground must not blink. If it does, close down a third of a stop.
4. Check the object as well. Stripes or blinking on polished metal, glaze or a pale object mean it is clipping there, and a clipped area holds nothing that can be recovered.

The window to stay in is stripes on the ground, no blinking anywhere. Between the two, the ground sits at about 250 of 255: white, with a little room above it. The gap between the two levels is the margin. The ground is not evenly lit, and both displays read the camera's processed preview rather than the raw file, which usually holds a little more at the top.

Keep the exposure for the whole sitting, and set it again only when the light changes.

Some settings decide whether the displays can be trusted. The highlight warning and the zebra read the camera's own processed preview, not the raw file, so whatever changes that preview changes what they show. The picture style does not touch the raw data: it changes the preview, and the default look in the maker's own raw software. Choose the camera's neutral or standard style, with contrast left at its middle setting, so that the preview follows the raw file as closely as it can. Turn off the settings that lift shadows or protect highlights; makers call them dynamic range, highlight priority, lighting or range optimisers. Some change only the preview. Others make the camera expose the raw file darker and brighten the picture afterwards, so the exposure no longer matches from one sitting to the next. The zebra in PHOTARCH for iPhone, and in the viewfinder of the PHOTARCH app on a Mac, warns at 97 %.

Keep the shadow

The shadow where the object meets the ground is part of the picture. It places the object on its surface and shows how high it stands; remove it, and the object seems to float. Leave room around the object so that the shadow ends inside the frame; a shadow that runs off the edge cannot be put right afterwards.

Cutting out versus photographing on white

A cut-out removes the background altogether and puts the object on a white made by software. The shadow goes with it, and so does anything near the edge that the selection misjudged: a frayed rim, a thin projection, a translucent edge of glass. A background that drops out completely can take faint detail with it.

Photographing on white keeps everything the camera saw. The ground can still be taken to an even white in the copies that are published, as long as three things hold: the change stops at the object rather than at a brightness threshold, the shadow stays, and the master — the picture as photographed — is kept untouched beside the copy. That is a correction of the ground, not a retouching of the object.

1. CUT OUT — no shadow: it floats; the edge is where the selection put it
2. PHOTOGRAPHED ON WHITE — the shadow places it; the edge is where the camera saw it

The PHOTARCH app takes the ground to white through an object mask on the copies it exports, leaves the object's own pixels and its shadow alone, and never whitens the master.