F-numbers: let me get this straight...

[gathertherosebuds]

I'm looking to buy a Canon 18-200mm. The f/3.5-5.6 is cheaper than the f/3.5-4.5.
Part of me says, this must be because lower aperture is always better and, at the maximum focal length, the f/3.5-4.5 lens will have a lower aperture of 4.5 instead of 5.6...
But another part of me wonders if it's really worth the $200-$300 difference because after all, 4.5 is included between 3.5-5.6!! And isn't the SMALLEST aperture (3.5) the most important f-number, anyway?!

Thank you

[sk66]

the smallest aperture is the largest number, and it's not listed in those figures (probably around f/22). The smallest number/largest aperture *may be* the most important. The lens with the 4.5 will be wider open at max focal length.

But this is just part of the story. With variable aperture lenses it's also about *when* the aperture shifts. Cheaper lenses shift way earlier in the zoom range than more expensive lenses do (generally). And of course, it's also about overall image quality.