FIELD / ROCK IDOne rock. A closer look.

PHOTO SETUP

How to photograph a rock with a scale without hiding the evidence

Short answer: Take one full-specimen photo with a ruler or other known scale beside, not on, the rock. Then take a separate close photo of the texture or edge. A scale records size; it does not add detail that the camera did not capture.

Make size visible, then make texture visible

A photo without a scale leaves an important question unanswered: is the visible grain a fraction of a millimeter or several millimeters across? A scale object gives a later viewer context for size. But when a ruler, coin, or hand covers the surface, it can hide the very grains, holes, banding, or edge shape that a photo-based rock identification needs.

Use two simple views instead of trying to make one photo answer every question: one frame for the whole specimen and its scale, and one close frame for the surface. USGS publications commonly show rock specimens with a scale bar placed to one side, while field documentation treats photographs and written observations as complementary records.

A simple two-photo setup

  1. Place the specimen on a plain, non-reflective background. Keep the background different in colour from the rock so its outline is readable.
  2. Put the scale alongside an edge. Leave a small gap so it does not cover a crystal face, layer, hole, or natural break. A ruler or a marked scale is clearer than an object whose size varies.
  3. Photograph the whole rock straight on. Keep the camera parallel to the face you want to document, then make sure the scale and the specimen are both in focus.
  4. Remove or move the scale for the close view. Photograph a representative patch of grains, banding, or an existing edge. Do not make a fresh break solely for a picture.
  5. Add a short note. Record whether the photographed surface is dry, wet, weathered, polished, or an existing broken edge.

What a scale can and cannot tell you

A known scale helps describe apparent grain size, clast size, and specimen size. It cannot make a blurred photograph sharp or reveal minerals that are below the camera’s resolution. USGS notes that visible detail depends on image resolution; enlarging an image makes it larger, not more detailed.

Do not use the size of one photographed feature as proof of a mineral or rock name. Size is supporting evidence. Combine it with texture, layering, luster, and the setting where the specimen was found.

When a ruler is not available

If you are documenting a specimen in place and have no ruler or scale bar, first take a clean, unobstructed photograph. Then take a second frame with a common object for scale, keeping it beside rather than over the feature. The National Park Service uses the same basic field-documentation principle for fossils: photograph the specimen and record associated observations, with a familiar item used for scale only when a ruler is unavailable.

For a loose specimen at home, retake the photo later with a ruler. A clear scale is more useful than guessing from a coin, a hand, or a background object.

Use the images deliberately

Submit the clearest whole-specimen view to the rock identification tool, then keep the scale photo and close view together with a location note. If a tentative result conflicts with the visible evidence, follow the AI rock identification checklist rather than forcing the scale or the photo to fit a name.

Sources for this guide

A scale improves a field record. It does not turn a photograph into a laboratory identification.