I agree with tpe - avoid the MEP 65mm macro for now - its a fantastic macro lens but its one of the hardest lenses to learn how to use (distance from lens to subject can be only about 4cm when at max magnification!). Its also a macro only lens - it will not focus on anything but macro subjects, which can be a bit limited when out in the field - it also needs proper flash lighting to work well - idealy the twin positional macro flash head since it has a lights on feature which activates the flash for a period of time - this is important as the lens can be very dark to see through so the added light is important - together both items make for an expensive package
As for "normal" macro lenses there is a great selection of choice:
Canon EFS 60mm macro - the only downside to this lens is that its EFS which means it will fit crop sensor cameras only, a full frame camera (like a 5D) will not work with this lens - this is only a limitation if you are thinking of moving to full frame cameras and want to keep your lenses -the 50D will work well with this lens. Its build and sharpness are great.
Canon 100mm macro - this lens is another strong choice, a good focal length and great optics make it a sharp lens for macro work. It's a popular choice for many; its only downside is that lens hood and tripod collar are not included in the price or package - they have to be got extra as additions. A lens hood is essential and really should not be left off so its a must addition, the tripod collar is only important if your going to mount the lens on a tripod, for this use the collar is very important as it really helps with weight distribution. The camera mount can suffice, but its going to put a lot of load on the front of the tripod which might cause dipping (only visable at macro level) and be a pain for framing a shot
Tamron 90mm macro - a good budget macro lens - again sharp and well built. It does not have innner focusing so the lens will extend as it focuses
Sigma 70mm macro - reportedly their sharpest lens, this is another strong choice and a good one for budget purchasers. Note that this lens is not listed as compatable with sigma teleconverters, but I have read that users of this lens have found that it will fit a sigma 1.4 teleconverter. It does not have innner focusing so the lens will extend as it focuses
Sigma 105mm macro - very much like the canon in use and optical quality. It does not have innner focusing so the lens will extend as it focuses
Sigma 150mm macro - this lens, and the sigma 180mm macro are a class above the other 2 sigma options; HSM focusing motors, inner focusing (lens remains the same length no matter how you focus - movements happen internally) and also compatability with sigmas line of teleconverters - both the 1.4 and 2*. It also includes both hood and collar in the set - and its price is similar to the canon 100mm if bought with both hood and collar. This lens is also the longest (focal length) lens that can be used handheld without trouble. Again this is a well built and sharp macro lens
Sigma 180mm macro - as above this is the top line macro lens from sigma, its offers a long focal length and also is able to give the same optical quality as the Canon 180mm macro, whilst being a lot cheaper. The only downside is that users of this lens tend to regard it as being a bit too heavy for prolonged handheld use for macro work - for tripod work though its one of the best choices.
Notes: Focal length - the longer the focal length of a macro lens the longer the working distance - distance from camera to subject. When working with insects you idealy want the longest length you can get so as to put as much distance from you and your subject; this lessens the chances of spooking the insect. Generally lenses greater than 90mm are recomended for newer macro insect photographers.
Also note that the longer the focal length the greater the degree of background blur in a shot - often a desirable feature in macro work.
Auto Focusing - macro lenses on the whole are poor auto focusers because macro work is done almost totally in manual focusing - the canon 100mm, sigma 150mm and 180mm with their better motors tend to be the better when using their auto focusing - however all are prone to hunting at times.
Inner focusing - note that macro work tends to be done with the lens at a fixed focus point and the camera and lens are then moved to achive focus on the subject, so whilst lenses that focus normally (ie they extend/contract as focused) will be longer at the maximum magnification it will be a fixed distance (mostly, sometimes adjustments are needed to get the desired framing of the subject).
Note that I have left out the 50mm options as they tend to be poorer lenses, often aimed at the budget level and thus tend to be of lesser quality. Also note that some 50mm lenses require filteres to be added before they are able to work as proper macro lenses - instead they offer a lesser magnicication than standard without the filters.
Teleconverters - note that adding a teleconverter will not only increase focal length (and thus working distance) but also will increase magnification of a shot and will also reduce your plain of focus. A 1.4 Teleconverter is a good investment as it will let you get that little bit closer to smaller bugs without compramising image quality nor making macro work much harder. A 2*TC is much harder to use well as the plain of focus is very very thin (which makes it very hard to focus the lens) and also will compramise image quality noticably. Some 3rd party teleconverters will work with lenses like the canon 100mm, but I do not know which makes and models do
In the end its very hard to make a bad choice and any of the above will work very well at macro work - the biggest factors tend to be focal length and price
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