Image credit: James St. John
In this simplified treatment, observation of grain size is made without magnification (e.g., hand lens, microscope). However, digital assets may be imaged and enlarged on a computer display. So a physical examination of the grain size of a hand sample of an unknown rock may not provide as much detail as on a computer display.
An assessment of grain size, whether viewed by hand sample or computer display, remains the same - a judgment is made about the visibility and sizes of grains (crystals).
Here are examples for training (calibrating) your eye and mind.
Comparison of grain size in two metamorphic rock samples.
Comparison of grain size in two metamorphic rock samples.
Evaluation of grain size in one metamorphic rock sample.
Evaluation of grain size in one metamorphic rock sample.