G'day DJ

Nice to hear from you & Welcome to the Forum

Firstly - there are no such animal as a "close up filter" ... a filter is an optical device that changes the colour or tone of an image - it has no effect upon focus. A lens, any lens is an optical device that is able to create an image and focus that image onto a receiving device, be it your eye or film or whatever

Close Up Lenses are optical devices similar to a spectacle lens. Mostly they are simple in construction, being one piece of glass (ie: one element), although in the US you can purchase 3-element close up lenses if you desire

Nearly all Close Up Lenses are designed to screw into the "attachment thread" on the front of a camera lens - and as we photographers use coloured & other filters, screwed into this "attachment thread" we loosely call it the "Filter Thread"

A Close Up Lens, being a lens, has a focal length, and the "strength" of the lens is referred to in "Dioptres". They are sold in sizes from about 30mm diameter to about 72mm diameter - check the Hoya catalogue if you like. The price is "about $1 per mm of diameter"

A "+1 Dioptre" lens has a focal length of 1-metre
A "+2 Dioptre" lens has a focal length of 1/2 metre
A "+3 Dioptre" lens has a focal length of 1/3 metre

For example ...
If you attach a +2 close up lens to a camera lens and that camera lens is set to infinity, then the close up lens will alter the camera's point of focus to 1/2 metre

If you screw the camera's focusing ring closer than infinity, then the new point of focus will become closer than 1/2 metre as well.

Being a simple, one-element optical device, it will never provide the empirical sharpness of a dedicated Macro-Lens for your SLR, however, these lenses are expensive as well.

Where the Close Up Lens becomes extremely useful is to lower the minimum focus point of a long focal length lens. Say you've got a 300 or 400mm lens which stops focus at 1.5metres. If you place a +2 close up lens onto that tele lens, then the lens immediately focuses at 1/2 metre ... great for insects or spiders who easily become frightened when a true Macro lens is used and it needs to come to 10-15cm from the subject to show it in detail

My camera lens is a 35mm to 420mm zoom - elsewhere on this forum (I think wildlife) you can see images of a Golden Orb Spider in its web, shot using a +2 close up lens from the 1/2 metre distance. If you look very critically at those images, you will see some colour changes towards the corners (looks a bit like a prism breaking up the colours) ~ this result you would not (should not) get with a true Macro lens, but there aren't too many 400mm macro lenses ...

Hope this has been of help to you - come back with more queries as they arise

Regards, Phil