Originally posted by GoldMountain Apologies for my naivete, but does "focused at infinity" mean?
The lens attached to your camera can focus from infinity down to somewhere close, three feet, eighteen inches, depending on the lens. The "strength" of a close-up filter is by convention determined by the distance to an in-focus object when the lens attached to the camera is focused at infinity = the farthest away the basic lens can focus. Also by convention, the "strength" of a close up filter is called the "diopter value." To get the distance from the front of a lens to the object that is in focus when a close-up filter is attached AND the lens is focused at infinity, you divide 1,000mm (= one meter) by the diopter value of the close-up filter. So if the diopter value of a close up filter is +1, then when it is attached TO ANY LENS REGARDLESS OF FOCAL LENGTH, an object 1,000mm in front of the lens will be in focus IFTHE LENS IS FOCUSED AT INFINITY. By rotating the focusing ring, you can focus closer, but you cannot farther away. For a +5 diopter close-up filter, the focus distance would be 200mm; for a +0.25, the focus distance would be 4,000mm (= four meters). etc..... The vast majority of close-up filters have diopter values from +1 to +5, but there are some that have higher or lower values. There are some +10 diopter close-up filters, but these are generally plagued by poor IQ in a variety of forms.