Why prefer the 18-55mm and 55-250mm lenses vs 18-200mm?
There seems to be a preference for to have the
- 18 mm - 55 mm
- 55 mm - 250 mm
lenses, but no mention of why they're better than a single 18-200mm. Could someone please explain why the two separate are better than the one combined, and if anyone has quantified this difference?
(I'm curious about Canon in particular.)
Image quality. The wider the range of focal lengths on one lens, the more design compromises are made and the more correction must be applied deal with things like geometric distortion, chromatic aberration, and light fall off in the corners.
Aperture. Even though the EF-S 18-200mm f/3.5-5.6 has the same maximum aperture of f/5.6 as the EF-S 55-250mm f/4-5.6 IS II at each lenses respective maximum focal length, the 18-200 is slower for most of the range they share in common. The 18-200 is at f/4 by 28mm, f/4.5 at 45mm, f/5 at 55mm, and f/5.6 from 80mm on up. The 55-250, on the other hand, doesn't reach f/4.5 until 74mm, f/5 until 96mm, and f/5.6 until 154mm.
Price. Depending on where you buy, the kit with the EF-S 18-55mm f/3.5-5.6 IS II rarely costs much more than the body only for the cameras it is offered with. I have seen occasions, usually during camera+lens rebate promotions where the kit was actually cheaper than the body only! The current difference at amazon.com for a T4i body is $16 less than the kit. So the 18-55 costs very little. The EF-S 55-250mm f/4-5.6 IS II goes for about $300 in U.S. stores, but you can get it from amazon.com for $174. The EF-S 18-200mm f/3.5-5.6 IS sells for around $700 in the stores and $569 on amazon.com. The 18-55 & 55-250 will run you around $190 more than a T4i body. That's $379 cheaper than the 18-200. The Sigma Sigma 18-250mm f/3.5-6.3 DC OS HSM IF is even slower than the Canon 18-200, costs $349 and performs about the same or worse than the Canon 18-200.
DxO Mark has all the quantification you could want of theses three lenses.
Here's a screen grab comparing the Canon 18-200, Canon 55-250, and Sigma 18-250. The charts show sharpness at maximum focal length and aperture of each lens. Green is sharper, yellow is in between, and red is less sharp.
So you really do pay substantially more for a lesser product if you go with the 18-200mm than the two lenses 18-55mm and 55-250mm
A smaller zoom range means fewer compromises in the optical design and usually better quality. It's better to have a boat and a car and use them where appropriate than to have some sort of boatcar that doesn't do either job as well.
Although the two answers are correct, I would like to chime in with a slightly different opinion. Note that I am not a professional by any means.
I used to have the two kit lenses for my Sony Alpha 55 (18-55 and 55-200), but switched to a new configuration where I have a 18-250 and a couple of nice, sharp primes (a 35/1.8 and a 105/3.2 macro).
I have probably lost a bit of sharpness by switching from the two zooms to the new one, but I really am much more comfortable with this set-up. I now have a lens that covers a wide focal range to get the work done for small prints, the web, and family snapshots. It lets me switch from a moderate wide angle to a nice zoom — got that squirrel stealing a nut to my daughter while I was photographing the view of a lake.
When I see something more worthwhile, that I would probably want to enlarge and print, I use the primes, which perform much better than the two kit zooms ever could.
If you'd like a bit of humor on this theme, read John's Shermann take on "18-300 is bad".
Go with the kit. As what others have mentioned, you compromise image quality and aperture openings for a superzoom.
However, I use one (an 18-250) for travel purposes. It can get the job done, but don't expect good image quality no matter what the price of the lens.
Plus I usually find these superzooms to be more expensive than 2 kit lenses (say a $350 superzoom, a 55-250 cost as low as $160 and an 18-55 costs $140, plus its way cheaper if bought as kit)
Lastly, kit lenses (individually) aren't as heavy than the superzooms, so you won't feel like your camera's weight is centered at the front.