Hold your CD up against the screen, then use the slider (or scroll) to resize
until both CDs are exactly the same size.

Hold your note (any value will do) up against the screen, then use the slider (or scroll) to resize until both notes are exactly the same size.

Hold your bank card or driver's license up against the screen, then use the slider (or scroll) to resize until both cards are exactly the same size.

Hold your ruler or tape up against the screen. Then use the slider (or scroll) to resize the image until the numbers line up exactly with your ruler.