I personally don't resist 3rd party lenses. In general, they are usually cheaper than Canon lenses but they're usually not as good. One of the most distinguishable aspect is that Canon lenses with ring USM motors (on most of their recent mid - high quality lenses) focuses very fast, quiet and accurate - the hit rate is dead high. So far from my experience none of the 3rd party lenses (regardless brand) can match the AF performance of the Canon optics. So if you shoot sport or anything that requires fast and accurate focusing, you will clearly see the advantage of using Canon optics.


Canon lenses are generally better in terms of image quality. You can use the [url="http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=9&Sample=0& FLI=0&API=0&LensComp=373&CameraComp=9& amp;SampleComp=0&FLIComp=1&APIComp=0]ISO12233[/url] on this site to see exactly how. However, some recent 3rd party lenses actually have flawless OPTICAL performance, such as the [url="http://www.the-digital-picture.com/Reviews/Sigma-150mm-f-2.8-EX-DG-HSM-Macro-Lens-Review.aspx]Sigma 150/2.8 EX Macro HSM[/url], [url="http://www.the-digital-picture.com/Reviews/Sigma-50mm-f-1.4-EX-DG-HSM-Lens-Review.aspx]Sigma 50/1.4 EX HSM[/url], and [url="http://www.the-digital-picture.com/Reviews/Tamron-17-50mm-f-2.8-XR-Di-II-Lens-Review.aspx]Tamron 17-50/2.8 XR Di II[/url]. My point is, if one of the best regarded 3rd lenses fit your need, buying them is a bargain since Canon does not make identical lenses or even falls behind in image quality department.


But generally speaking, Canon optics will most likely be right choices over identical 3rd party lenses. Here are a list of problems that you may encounter with 3rd party lenses:
[list][*]<span style="color: #ff0000;"]Slow AF with much less hit rate in sport shooting, does not focus well in low light[*]<span style="color: #ff0000;"]Image quality (Sharpness, contrast, color, back ground blur, color fringes, flare, vignette...) <span style="color: #ff0000;"]is generally inferior (with exceptions)[*]<span style="color: #ff0000;"]AF calibration is frequently a problem <span style="color: #000000;"](heavy front or back focus)
[*]Reliability is not as good (but always comes longer warantee than Canon lenses)[*]Almost none is weather sealed like Canon "L" lenses[*]Build quality/ergonomics is generally inferior[*]May have capability issues with new Canon DSLRs[/list]


The top three are always the reasons why I don't buy 3rd party lenses. Those are the things 3rd party lenses do not offer but are big deal for me. As said, if one of the best regarded 3rd party lenses fit you need, buy them. For anything else, if money allow Canon lenses are generally better choices. "L" lenses are always better, and much better![]


Ben