One of the common recommendations when sending digital images for diagnosis of insects, plants or diseases is to include a "size reference" in the photo.
For larger specimens and plants this can be a ruler or even a handy shop caliper! A ruler works for smaller specimens if you can get close.
Frequently, though, if you grab something close at hand it will be a coin from your pocket such as a penny, or a dime or a quarter (for larger specimens).
The Dimensions of Coins
A reason that coins work well for size reference is that the dimensions are known. The table below from the United States Mint gives coin diameters down to the thousandths of an inch! In general, pennies are three-quarters of an inch, quarters are one inch and you can think of nickels and dimes as approximately three-quarters of an inch.
Of course, to get to the exact length of the specimen in your image, we first measure the diameter of the coin in the photo compared to the true diameter; second, measure the length of the specimen in the photo and third, calculate the length of the actual specimen through the miracle of cross-multiplication (the only thing I learned in high school that I still use nearly every day!).
The known diameters of coins can used to determine the size of a specimen in digital diagnosis.
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