amfoto1 wrote:
Both have decent image quality. The Canon STM are mostly very recent designs, often the latest-and-greatest version of the lens with other improvements besides the better AF drive system. For example, the 55-250 STM is more compact, closer focusing than the earlier, non-STM version. It also has a non-rotating front barrel (which makes it easier to use with rotating filters such as polarizers).
So long as it meets reasonable standards, image quality factors really shouldn't matter very much comparing these two lenses because they serve different purposes. You should base your decision more on what focal length range you need and/or what other lenses you already have. If you already have an 18-55mm, for example, the 55-250 might make more sense. If you want a single lens to carry most of the time, the 18-135mm might be a better choice. If you mostly shoot landscapes, portraits, etc. the 18-135mm might be more than enough (or you may want to complement it with a wider lens like the EF-S 10-18mm STM IS or EF 10-22mm USM). If you want to shoot sports or wildlife, you are more likely to want the stonger telephoto "reach" of the 55-250mm.
Both have decent image quality. The Canon STM are ... (
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