Originally posted by Pablom afaik focusing distance is measured from the sensor. 25cm sounds pretty reliable to me.
Working / focusing distance is measured from the lens objective (front), not from the sensor. And minimum focusing distance for ANY non-reversed lens is its focal length. If I put a 50mm lens on 250mm extension, the focusing distance is NOT 250-300mm, but a minimum of 50mm.
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The OP's question was, how to shoot macro with the 18-55. Several ways exist.
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Cheap/fast/dirty: Use +diopter lenses that screw into the lens' front threads. These typically come in sets of +1 and +2, and +3 or +4. A set of 52mm-diameter diopters to fit the 18-55 can usually be found on eBay for well under US$20. These are easy to use and don't consume light; but their magnification is limited, and they distort, especially towards the edges. These are the cheap way to get a taste of close work.
NOTE: 'Macro' is usually defined by magnification, where 1:1 means the image on the sensor is the same size as the actual subject. 1:2 is half-size, 2:1 is double-size, etc. Usually, anything less magnified than 1:2 is considered close, not macro.
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Costlier, cleaner, fast: Use an adapter lens that is optically corrected, such as the Raynox DCR-150 (+6.66 diopters) or DSC-250 (+4 diopters). These adapters cost ~US$50 and give much less distorted results than simple +diopter lenses, and their universal mount will fit onto many lenses including the 18-55. See the RAYNOX CLUB here for fine examples.
NOTE: For either the simple or corrected adapters, working distance varies with both diopter and focal length; so, experiment.
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Better, closer: Get the cheapest manual 24mm lens you can, with 52mm threads and an aperture ring; and get a 52mm thread-reversal ring, ~US$5. Screw the reversal ring into the front threads of the 18-55 (the primary), and screw the 24mm lens (the secondary) onto that, reversed. This is called "stacking" and can achieve great magnification (M). Stacked like that, if the 18-55 is at 24mm, M= 1:1; if the 18-55 is at 48mm, M= 48/24 = 2:1 (double-size). Great magnification reduces the light that raches the sensor. No free lunch here, eh?
NOTE: If any Pentax-compatible lens is the secondary, your working distance is about 45mm.
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Cleanest: I forget who makes them, but you can buy macro extension tubes that send signals between camera and lens, so the lens still works automatically. Such tubes aren't cheap, but they're cheaper than an AF macro lens, and they give the cleanest magnification, since there's no extra glass between subject and sensor. If you have 50mm of extension and the 18-55 is set to 25mm, you reach 2:1; if the 18-55 is set to 50mm, you get 1:1 (full-size) magnification.
NOTE: Cheap non-auto macro tubes ARE NOT suitable for the 18-55 kit lens.
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Warning: Don't try to use the reduction ray from HONEY, I SHRUNK THE KIDS. It doesn't work.
Of course there are many other ways to shoot macro, but what I've mentioned are suitable for the 18-55 kit lens. Have fun!