Gowllands Hand Lens, 20X
Choosing a Hand Lens. Four factors effect the performance of hand lenses: (1) magnification power, (2) width of field, (3) depth of field and (4) focal length. For purposes here, magnification power is a measure of image enlargement. Width of field is a measure of the area that can be seen through the lens. Depth of field is the range of distance through the lens within which an object remains in focus. Focal length refers to the distance from an object at which a magnifier must be held to achieve clear focus and maximum magnification.
Higher powered magnifiers give greater image enlargement. However, they have greater light requirements, narrower width of field, shallower depth of field, and shorter focal length. Three lens (triplet) construction provides high clarification at edges of lens. Two lens construction provides a lesser measure of clarification.