Calculating magnification of objects viewed through an optical microscope. How to calculate the magnification using the information in this question: STEP 1: From the question measure the length of the starch grain as viewed through the optical microscope. STEP 2: Convert this value into the same units as the stated actual length of the starch grain. STEP 3: Divide the value from step 2 (length as seen through eyepiece) by the actual length. STEP 4: Round off this value to the nearest whole number and add the X after the number. (eg. 40X) NOTE: Check your final answer – This question was taken from a past exam paper and the examiner accepted any value from 479X to 521X magnification. I hope that your value fell within this range. If not, recheck your calculations and determine where the error was made.