Magnification and field of view
How to estimate size by how much of the field an organism takes.
The key asks about size relative to the field of view. You can estimate the real width of your field from two numbers.
The field number
Most eyepieces have a field number engraved on them, often 18 or 20 (millimetres). Divide it by the objective magnification: that is the diameter of the field at the slide.
- 18 mm eyepiece, 4× objective: about 4.5 mm across.
- 18 mm eyepiece, 10× objective: about 1.8 mm.
- 18 mm eyepiece, 40× objective: about 0.45 mm (450 µm).
So an organism that crosses about a tenth of the field at 100× total magnification (10× objective, 10× eyepiece) is roughly 180 µm long: Paramecium size.
Start low
Always find things at the lowest power and move up. At 400× the field is small and the depth of focus shallow, so fast swimmers leave it in a second.