A test comparing macro photos shot with a Minolta A2 in RAW format with and without a Hoya single element +4 close-up lens (CUL) . In both cases the camera was positioned with a tripod as close as possible to the subject (a page of a phone book) for the camera to still be able to focus. The image shot with the +4 CUL was output from Adobe Camera RAW (ACR) at the nominal A2 resolution of 3264 x 2448 pixels. For the image taken without the CUL the file was sampled up while in ACR to 6144 x 4608 pixels in order to produce a larger image to more closely match the size of the elements in the CUL shot.

I have never found the image quality produced with using single element close up lenses to be acceptable. Corner and edge chromatic aberrations and sharpness have always been terrible no matter how much I stop the aperture down. In this test, sampling up while in ACR produced a far better image even in the center of the frame.

Two element closeup lenses, which aren't shown here, are another matter entirely. They are much more expensive than single element lenses, but produce excellent results and I've been quite happy with the two element Canon 500D Close Up lens I use for macro work with my Canon DSLRs and telephoto zoom lenses.