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Magnifying glass. This can be acquired from a number of sources – think garage sales, hardware stores, the bottom of the drawer. It just need to be free of scratches and approximately the size of your lens barrel—check to make sure that the size is right before purchase! If there are scratches, don't try and use “liquid scratch filler” or the like—it has a different index of refraction than the magnifying glass, and will only exacerbate the problem. Also, test out the lens by just holding it in front of your camera and seeing how the macros turn out before purchase.
Old Filter. You need something to connect this apparatus securely to the barrel of the camera, and the only real convenient way to do that is with another filter. Used UV or clear filters are cheap or even free—they don't even need to be clean, as you can fix that. Ask around amongst your friends, or look through your local Craigslist.
Cutting utensil. You need something to cut through the handle of the magnifying glass. Depending on the material, a box cutter or a small saw would probably do the trick.
Sand paper. This is to make the whole apparatus a little bit smoother, more of a polished look, so it's not necessary unless you do an especially sloppy job cutting.
Tape. Black electric tape looks a bit classier, but anything will do. Some sort of tape to attach your camera lens to the filter.
Lens cloth. You'll need to be cleaning the lens during the construction process, so having one of these around is an A+. Lens cleaner might also be a good idea.
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Your first task is to prep your future lens—the magnifying glass. Your first task is to clean the magnifying glass of any smudges and dust. After, carefully cut off the handle of the magnifying glass. The closer you cut it to the main frame of the Sand down any stray bits you didn't quite get with the knife to make the edge smoother. You've probably got more dust and fingerprints on the lens at this point, so clean it off again for good measure. Once you've attached it to the filter, it'll be a pain to clean any smudges off.
Now, your used filter. If there are any marks on it that might affect image quality, then you can take the filter itself out using that same cutting utensil and sand the inside edges clean with the sandpaper.
Now, just tape the two together, the magnifying glass lens on the outside, filter on the inside. The closer the magnifying glass lens and the filter are in size, the better this will work. Now test it out, and check out your cool macros!
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The most obvious variation with this lens apparatus is to try with multiple magnifying glasses for even more macro power. You can also try out different magnifying lenses, such as those on binoculars, to see if it'll create an interesting effect. Before you try anything out, however, try just holding your combination up to your camera and try taking a few macros with it to see how (if) it'll work.
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How To Use A Macro Lens
Your automatic focus mechanism is likely to get confused by the lenses between it and the subject, so it's a good idea to use manual focus. You'll have better control over the image anyway, which is important for macro photography.
Also, be prepared for a slight fisheye effect on the fringes, or slight vignetting. Some distortion will happen on even the best of store-bought, professionally-made macro lenses, but depending on the quality of the magnifying glass you use, this may be a bit more distinct. These are all things you can check out in your individual lens before purchase, however, so keep an eye out for good quality!
Little else has really changed as far as macro photography technique is concerned: just keep experimenting and playing with the lens!