A lens like 17-70 would be sufficient for most travel photos, plus another 50-200 or 55-300, you are set to take excellent travel photos
Of course 18-250 is an excellent choice too. But from my personal experiences, I seldom use focal length longer than 70mm when I'm out traveling, no point carrying the extra weight and length of a 70-250mm all the time.
If you don't want to change lens, just use whatever mounted on your camera for whatever you see on the road. It's not like you missed anything, 'cause you are traveling, the most important thing is to see a different part of the world with your eyes, and enjoy the trip
Depends on your shooting style. Sure, you don't want to miss any shots when vacationing, but sometimes, something's got to give way.
An 18-2xx is good if you don't want to bother changing lenses, especially when you're taking a wide architecture shot and follow up with a close-up of the details. It's also good for street/candid shooting in good light.
But since I find ultrazooms too long and unwieldy, I travel with a UWA (Sigma 10-20) and some primes. With small primes, I simply store them in my jacket pockets for fast access during changing.
Recently went to China especially Mt Huangshan last Oct and brought 18-250mm. Yes, I agree with what Fengwei said best about 17-70mm. I seldom use long 100-250mm only if you are in mountain and wanted to shoot photos at pavilion or rocks in another mountains, 250mm is good advantage. Depend on landscape or street etc what you want to shoot.
I love Mt Huangshan in China very much and 18-250mm was worth using.
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This is what i recommend actually...just bring a 70-300 or 55-300 and a fast prime for night shots. for short range use a good point and shoot.
main advantage, if taking photos of locals or maybe historical/sacred places...a PNS is more acceptable in such places most of the time.
It all depends what is your objective of bringing a dslr over in the first place. In my case, it was pretty rushed so didnt have time to consider if dslr would be better and in the end simply machine gun my PNS because convenient and somehow did the job. I only used my dslr when die die need to shoot in raw. bring just about everything but in the end used a PNS 90% of the time. hahaha//
i don't really like not having the benefit of changing lenses when travelling, i suppose it depends on how serious you are. you need to maintain a balance between what you need, as opposed to carrying your entire lens collection on your back like a pack mule.
i could give you many reasons for why you should want to change lens:
1) option of uwa, no all-in-one lens is going to give you anything in the region wider than 18mm, which is definitely useful for many locations.
2) limited fastness of lens, at best f/2.8 for zoom lenses i think.. there are situations where you might need f/2.0, or even f/1.4. rare, no doubt, but like i said, depends on how serious you with regards to photography on the trip.
3) image quality.. as much as megazoom lenses have improved since their conception.. still lagging behind.
4) size, street situations, 18-250 is still going to give you more attention than a simple flat lens. don't even need to be pancake, like a 28mm manual lens is so small
if you must have an all-in-one, i think the best compromise is 17-50 f/2.8 , tamron, or 16-50 f/2.8 DA*.. most versatile i guess.
18-250, 18-270, possibly doable, but for that price, i rather bring two, 18-55 kit lens + a telephoto zoom.
here's what i usually bring, i don't think it's particularly painful, since i have done 4 hour walks with it without rest or water.. sigma 10-20 + 28mm + 50mm + 70-300mm. on trips where i don't even think i might walk that much i will bring 2 bodies (one modified for IR), the previous + 18-55 kit lens.
some people have suggested an all prime set up..
it is not that incovenient to change lens using a shoulder bag or a hybrid bag.. all you need is practice, and the desire to get photographs that you can be proud of.
but like i said, if you are looking more to have a great time with your family, and capture memories, then 17-50 f/2.8 will be a good choice. and of course, it's your choice in the end.
When I sent overseas, I brought along my ex-Sigma 18-200 and ex-sigma 30 1.4.
Although the 30mm's IQ was much better, I hardly used it much. its too troublesome to change lens while carrying stuff and watching the kids, so the 18-200 stayed on 99%
of the time. If you simply want to capture the holiday moments, the 18-200 is very versatile (i have since upgraded to a Pentax 18-250). While it is not fast, at 200mm, it can still achieve the bokeh you need. I like that I am stationary and only the lens moves and I use the entire range. I do not fuss over vignetting or barrel distortion, etc!
But I do concur with the others the IQ on the 18-250 is probably the worse in the entire pentax range but on holiday, flexibility and simplicity is more important. If you fumble with lens changes, the kids are already out of your camera frame!
However, if you are holidaying with a friend or alone, it makes sense to bring a short zoom and primes because you will have more time to compose and shoot.