Quote:
Originally Posted by Larry.Manuel View Post
[...] Close-up lenses don't change the focal length of the camera's lens, [...]
That is exactly what they do: they shorten the focal length of the lens (or rather: the combined focal length is shorter), so that with the same amount of extension, you focus closer.
Respectful disagreement follows:
The close-up lens -"filter" in OP - makes a more-strongly converging cone of light enter the camera's lens, thus the [macro-esque] image will focus on the film, using the same focus travel. If the lenses' combined focal length was shorter [or longer] the image scale - size of objects on film - would be affected, and it's not.
The close-up lens [Rolleinar for Rollei users] is like a reading glass for you eyeball. Without it, the image of the newspaper /now Facebook/ you're trying to read would focus behind your retina. Ergo: blurry vision. The correct eyeglass lens begins converging the light before it enters the eye. Ergo: clear vision.