Apparent Magnification

"Apparent magnification" in virtual microscopy is a tricky thing.

On a standard microscope, there are two lenses, the objective and the eyepiece. The eyepiece is nearly always 10X, so the objective magnification determines the magnification of the system. A 20X objective results in 200X magnification, a 5X objective results in 50X magnification, etc. Because of this, people who use microscopes generally say "at 20X" when they really mean "using a 20X objective which results in 200X magnification".

The measure of magnification is non-trivial. Magnification refers to the extent to which an object is "enlarged". Enlarged in this context means an increase in the angular field of view occupied by an object, as perceived by your eye. If you move an object closer so it subtends twice the angular field of view, it will be magnified twice as large. By convention in optical systems 1X magnification means "the angular field of view at a distance of 10 inches".

Digital scanning devices like the Aperio Scancope are a whole different ballgame. When data are captured with a 20X objective lens, it results in images with .46 microns/pixel, because the device has a 20X objective, a 1.5X tube lens, and is projecting data onto a linear sensor array with a spacing of 14 microns/pixel. (14 / 30 = .46) This information is then displayed on a computer monitor with a dot pitch of about 250 microns (good LCD panels are around .25 mm/pixel). The original information was at .46 microns/pixel and it is being displayed at 250 microns/pixel, so the real magnification is about 550X. A computer screen is usually more than the conventional 10 inches away from your eye, more like 24 inches, so that reduces the magnification to about 230X. (550 / 24 * 10)

So - this is why it is called "apparent magnification". Following the convention of microscope usage (people say "at 20X" when they mean "using a 20X objective which results in 200X magnification"), we call the images produced by a ScanScope with a 20X objective lens "at 20X". Looking at a computer screen with ScanScope information, most experienced observers say it looks "better than 20X". It may look better because computer screens are brighter and easier to view than microscope images, and the system is actually magnifying about 230X instead of 200X.

In the same way, when a ScanScope has a 40X objective, the resulting images are magnified by about 460X, but we say "at 40X" and observers agree that what they see appears to be what they expect for "40X".