Light microscopes can magnify objects by up to 1,000 times. These objects may be much too small to measure with a ruler, which makes knowing the size of the field of view -- the size of the area visible through your microscope -- a useful piece of information. Calculating the field of view in a light microscope allows you to determine the approximate size of the specimens that are being examined.
Set your microscope to its lowest magnification and note the magnification power. Position your slide scale -- a microscope slide marked with measurements -- or your clear ruler under your microscope. Bring the ruler or scale into focus.
Move the scale over so that a scale bar, or line on the ruler, is at the farthest left edge of the field of view. Estimate the number of scale bars and spaces that go across your field of view. Estimate the fraction of a scale bar remaining at the far right, if necessary. You should now have a measurement such as 1.5 millimeters.
Calculate the size of the field of view for higher powers of magnification for your microscope using the following equation: low power magnification divided by high power magnification times low power field diameter in millimeters.