Try a Raynox macro adapter - bargain 30 quid, off Amazon.
It's a more correctly a 'close-up supplementary lens' that clips on the front. Very easy to use and works well on lenses over about 50mm. There are two flavours, DCR-250 which will get you very close indeed, or the DCR-150 which is better for flowers and stuff. It may be all you ever need.
Here's what they can do - a few I prepared earlier
http://www.flickr.com/groups/raynoxdcr250/
Good advice. If you are new to macro work then a Raynox close-up lens would be an economical way to see if macro photography "grabs" you. Quite possibly if it does grab you, you may want to upgrade to a dedicated macro lens. But jumping straight in with a £500 or so lens is a bit of an expensive experiment - not everyone gets on well with macros. I have been heavily into macros using close-up lenses for a couple of years and only now am thinking seriously about upgrading to a dSLR and a dedicated macro lens. YMMV of course.
I use the 250 and the 150, and also the Canon 500D close-up lens. They are all
achromatic lenses, which use two pieces of glass (or I think three for the Raynox lenses) to cut down the chromatic aberration you tend to get with close-up filter sets where each filter only uses one piece of glass. All three can produce quite nice results, even on the small-sensor bridge camera I use (given some careful PP). Your 500D (camera) should do rather better.
I imagine you would use the close-up filter on your 55-250 - you may find there is some heavy vignetting on the 18-55 (not just light fall-off at the corners, but quite possibly a "looking through a porthole" effect).
The 250 is the most powerful of these three and best if you want to capture small insects or parts of insects (e.g. flies' eyes). The 150 is my favourite for insects, as I tend to capture middle-sized insects. The 500d is my favourite for flowers. It does vary though - I sometimes use the 150 for small flowers and the 500d (or straight telephoto with no close-up filter) for large insects. I tend to use the 250 for water droplet close-ups.
With a dedicated (prime, fixed focal length) macro lens you alter the framing/magnification by changing the distance to the subject, which is pretty straightforward. You need to be aware that there is a complication with using a close-up filter - you need to get the lens a particular distance from the subject to get a sharp image, and you alter the framing/magnification by changing the amount of zoom you are using. The required distance is around 4 inches for the 250 and around 6 inches for the 150. The 500D is less sensitive to the distance to the subject, but I use it at about 14 inches. This distance sensitivity makes these lenses more tricky to use than a dedicated (prime) macro lens, with the 250 being the most difficult. Unless you are only really interested in getting as close in as possible, I think the 150 is a good general purpose starter lens - it covers a nice range of sizes (at least for the insects and flowers in our garden) and is a bit more forgiving than the 250.
FWIW I have come to the conclusion that getting good light on the subject is as important as having good optics. I tried to use just available light for a long time but found my success rate and picture IQ improved a lot when I got a dedicated flash unit and spent some time working up DIY arrangements for diffusing and reflecting the light on to the subject. The improvements were not only when taking pictures in the all too common murky, overcast conditions we often get in this country, but also to my great surprise in bright weather, where using flash can help avoid the excessive contrasts you can get in bright sunshine.
Macro opened up a new world for me. Hope it does for you too.
ell the Tamron and get some autotubes for the 50mm, theres no glass in them to affect IQ and you will get better than 1:1 with a full set on your 50mm, you can also use them on your 55-250mm.
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