Well, DavidL: "close-up" and "close-up" can mean different things.
If you shoot from a distance
(say 10m) and you want the subject at this distance "closer" to you, you need a longer focal length than your current lens. For this purpose, you can buy
(a) a camera-lens with longer focal length (tele-lens=best solution)
(b) Teleconcerter to put between your body and the existing camera-lens (teleconverters range from 1.4x to 2.0x magnification)
(c) There are also "teleconverter" that can be screwed onto your camera-lens, esp. for cameras, where you cannot change the lens. They also provide something like 1.7x magnification.
With all these solutions above you still can focus from infinity to the closest distance of the camera-lens.
If you're working really "close-up"
, that means at a distance from your sugject of less than 1m, you are normally limited by the shortest focus distance of your lens. In this case you take a close-up lens ("lens" in this case just being a single glass) that can be mounted in front of your camera-lens (phew - quite confusing: "lenses" and "lenses"). With the close-up lens mounted, the focal length of your camera-lens is shortened marginally but you can now focus much closer, so again magnification of the subject goes up. But other than with teleconverters, a close-up lens is not designated by a single magnification-factor (say 2x) as the real amount of magnification that you can get depends on the camera-lens that you're using: with wide camera-lenses, the benefit/magnification of a close-up lens is minimal and with camera-lenses from 50mm upwards the effect/magnification of a given close-up lens is increasing. Beware though that the combination of camera-lens plus close-up lens can no longer focus to infinity: The farthest distance you can focus now is the the focal length of the close-up lens, which normally is the designation of the lens, e.g. 500 = 500mm=50cm. Or the designation of the close-up lens is diopters (e.g. 2, 4, 5). then you get the focal length of this lens by dividing 1m/diopter, i.e. a "2"-diopter lens has 50cm focal length.
As to your question: There is no such thing as a "2xclose-up lens" or "2x close-up filter": either it's a "2x teleconverter" or it's a "2-diopter close-up lens". As you know from the above now both these gears work totally different
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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