FYI there are two biconvex lenses in each loupe (from Daiso 100 Yen shop). Each lense is 3cm diameter with focal length of approx 6.5cm, but when spaced 1.5cm apart, they correct to a focal length of 4cm. Trouble is, they zoom into the image too far, and the edges of the picture are lost. By taking a single lense, you can focus it further from the phone screen, but then it takes in a slightly too large a field of view. Is it possible to scale ones phone display down slightly to suit a different focal length?