Digital Coin Photography – H. Craig Hamling – October 2001

Technical Details and Image Manipulation

Images can be “enhanced” to make them look more like the coin and to show more detail and I regularly do this to images I get on-line to get a better idea of a coins appearance but that is beyond the scope of this presentation.  When I make coin pictures I do my best to capture a good coin image with the camera and negate the need for enhancement.  Here I want to discuss basic technical issues about how the camera makes image files and how to use them.

The FD-71 has two imaging modes, “Fine” and “Standard”.  In both cases the image size is 640 x 480 pixels or “dots”.  Also in both cases the resolution is 300 dots per inch (dpi).  The difference between the two modes is the compression level used to save image as a jpeg file.

All of the images shown so far in this presentation have been made in the Fine mode.  Here is another one:

This file size is 64K bytes.

Compare it to one made in Standard:

This file size is 32K bytes

There is some loss of crispness from fine to standard that is hard to see unless you look very closely.

The camera saves the file with a name like mvc-xxxf.jpg for the fine pictures and mvc-xxxs.jpg for the standard pictures.  If you can’t stand these files names it is better to just rename them than it is to open them up in an image program and save them as another name.  Every time a jpeg image is opened up and saved it is uncompressed and then re-compressed.  Each time this is done some definition is lost.

I use MS Photo editor for manipulating my image files.  When you go to save an image you have the opportunity to select the compression ratio for the jpeg file.  I have found that 80% results in a file size that is nearly the same as the original file size.  Boosting the ratio higher vastly inflates the file size with no apparent increase in image quality.

Here is a screen shot of the save as dialog showing the compression selection:

Lower compression percentages save file size but compromise image quality.  It is hard to see the effect on the computer screen above about 40% but the differences in printed pictures is huge.

Here is the same image saved at 20%:

You can start to see some degradation of the color.  There is some pink in the right obverse field.

And at 10%:

This image is now very much degraded.

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