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I have the Canon 28-135 IS USM lens and I am pondering replacing it with the newer 18-135.

I know I'll be giving up the USM and FTM focus and gaining 10mm on the wide end (replacing my old kit lens) and an extra stop or two of IS by going with the 18-135.

Does the 18-135 represent a step down in image quality? I'm most concerned about sharpness, as distortion & CA (chromatic aberration) can be fixed in post-processing.

Does anyone who has shot with both lenses have a preference?

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Canon has three primary grades of lens quality: silver, gold, and red (L-series). The EF 28-135 IS USM is a gold band lens, which generally means it has better build quality, better focus, and usually a bit better optics. The EF-S 18-135 is a silver band lens, which generally means bottom-rung build quality, bottom-rung focus, and basic optics.

In this specific case, the 28-135 has quite a bit better build quality, as its part plastic and part metal,where as the 18-135 is all plastic except the mount. The focus is considerably better on the 28-135 as it has USM (ultrasonic motor) focus, which is smoother and allows FTM (full time manual focus). The 18-135 has a simple gear motor, and does not feature FTM.

Optically, I think these two might be pretty close. The 18-135 has a UD glass element, which is pretty nice for a bottom-rung Canon lens. The 28-135 is decent optically, but its never produced top-notch quality, and has some distortion problems. The UD glass element should help with dispersion (CA), which might produce better corners than the 28-135. Distortion is probably about the same on the two lenses, at least at the wide end. I know the 28-135 is known for a fair bit of corner distortion and vignetting. The EF-S build of the 18-135 might resolve the vignetting problems.

Finally, the 28-135 is an EF mount lens, where as the 18-135 is an EF-S mount lens. The latter will ONLY work on cropped sensor bodies (APS-C sensors). If you ever wanted to upgrade to a full-frame camera like any one of the Canon 5D's or even one of the 1D bodies, you would not be able to use the 18-135 on them.

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    \$\begingroup\$ There is no difference between the gold and silver rings in terms of quality. Silver just means that it is an EF-S lens! The EF-S 17-55/2.8 IS is silver, yet very good quality. And you forgot the green ring, for DO lenses :) Again, green is not an indication of quality but of technology. The red ring of an L lens usually indicates top of the line quality but there have been some underwhelming lenses in that lineup as well... the 17-35/2.8L, 14/2.8L, TS-E 24/3.5L for example were not exactly stellar performers optically. Neither was the 50/1.0L for that matter. But they were built like tanks. \$\endgroup\$
    – Staale S
    Jan 28, 2012 at 1:53
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    \$\begingroup\$ @StaaleS: Thats not correct. The EF 75-300 is silver band, and the EF-S 10-22mm, EF-S 17-55, and EF-S 60mm Macro are all gold band. It is indeed an indicator of build quality, regardless of the mount. I'm not really sure how to classify the green ring other than its always on DO lenses, but of the same build quality as red-ring L-series lenses. When I refer to build quality, I mean construction (which is always better on gold than silver lenses), and features (the USM on silver lenses is REALLY crappy and gritty, while on gold lenses with maybe the exception of the 50/1.4 is pretty smooth.) \$\endgroup\$
    – jrista
    Jan 28, 2012 at 2:03
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    \$\begingroup\$ Hmm... you are right, silver is not an EF-S indicator; my mistake. On the other hand, the 50/1.4 has a gold ring and has a truly crappy micro-USM motor that breaks if you look hard at it. It is an old lens though, newer micro-USM lenses seem to go with silver. I suspect that gold used to mean USM motor, no matter what kind, but no longer does. The 50/1.4 is ancient... Maybe they have redefined it as "ring USM only" on new lenses? (continued...) \$\endgroup\$
    – Staale S
    Jan 28, 2012 at 3:07
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    \$\begingroup\$ Rock-solid lenses like the TS-E 45 and 90 have no ring at all, nor does the MP-E 65. Of course all these are manual focus. Green is DO, no more. The 70-300 has normal good build quality, not the bulletproof L-level. The 400 DO is another story, it is an L in disguise. Green was used on the first fluorite lens back in the late sixties, early seventies, I suspect that it is used for what you might term "exotic" lenses. \$\endgroup\$
    – Staale S
    Jan 28, 2012 at 3:08
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    \$\begingroup\$ A-ha! Straight from the horse's mouth, and just to add to any confusion: tinyurl.com/8xkje8f (Canon Professional Network). Summary: Red: L-series. Green: Diffractive optics. Silver: "Current non-professional EF lenses". Gold: "You will see a dashed gold ring on some older lenses...with an ultrasonic motor". And "older consumer lenses do not have an identifying ring". So they seem to have changed their minds over the years. Alternatively, they are as confused as the rest of us. \$\endgroup\$
    – Staale S
    Jan 28, 2012 at 3:57

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