Numismatics​.org (Spring 2003)

by Sebastian Heath

In this column I will describe one method for making one to one images of coins using a digital camera. This is very important for the ANS because we frequently fill orders for black and white prints. We use a digital camera to take the pictures but the customers expect the images to be printed at the original size of the coins they are interested in.

Before I begin step-by-step instructions, some background is necessary. As I have mentioned before, the ANS is building up an archive of digital images of its objects. The default resolution of these images is 600 dpi, or “dots per inch.” “600 dpi” means that for every square inch of an object, 360,000 dots — roughly equivalent to the computer graphics term ‘pixel’ – are used to represent it.

To put these numbers in context, an image of a U.S. quarter, which is approximately .95” in diameter, would be 570 by 570 pixels in size, for a total of about 255,127 pixels devoted to the representation of the coin. One conclusion that stands out from this number is that the resolution of today’s multi-megapixel cameras is easily able to meet the needs of most numismatic photography.

It is also important to remember, however, that cameras have no inherent resolution in terms of the real world. At the ANS, our Nikon D1x takes pictures that are 3008 wide and 1960 pixels high. If I were to fill a frame with a quarter, the resolution would be 2063 dpi (1960/.95). If I pull back so that the quarter fills only half the height of the image, the resolution would be just over 1030. When I am taking pictures of coins, medals and other objects, I am constantly moving the camera up and down so that the resolution of the resulting images is often changing for each object I shoot. The rest of this column will show how I resize every image to 600 dpi.

The first step in this process is to determine the resolution of the image that comes off the camera. Figure 1 — a screen dump from Adobe Photoshop running under MacOS X – shows how to do this. You can see that there is a scale in the image and that I have selected a part of that scale one inch high. Just above the coin is the “Info” box which tells me that the selection is 705 pixels high. This establishes that the real world resolution of this image is 705 pixels.

Figure 1

Figures 2 and 3 illustrate how to resize an image. The “Image Size” menu-item is on Photoshop’s “Image” and shows the dialog box that is in the middle of these figures. The first step is to tell Photoshop that this image is 705 pixels. We do this by unclicking the “Resample Image” check-box, typing “705” into the “Resolution” field, and clicking “OK”.

Figure 2

Figure 3

The next step is to change the resolution to the ANS standard of 600 dpi. As figure 3 shows, we bring up the “Image Size” dialog box again. This time the “Resample Image” check-box is checked and we have entered “600” into the resolution field. Clicking “OK” will cause Photoshop to resize this image from 705 dpi to 600. By way of confirmation, figure 4 shows that the rectangle selecting an inch on the scale is now 600 pixels high. Photoshop, as well most of other graphics programs, will use this information to determine the size of the image on the printed page.

Figure 4

The end result of this process is seen in figure 5, which shows the coin, a late fifth century B.C. Syracusan decadrachm, printed at its original size. Of course, color versions of these images are stored in the ANS’ digital archive as 600 dpi TIFF files and the coin can also be seen on the Society’s web-site.

Figure 5