That *is* a hell of a lens. With that range, it's conceivable that it's all the lens you'd ever need, which would be great for traveling. It is $750, so you're definitely paying for the convenience of having a nice wide end and a great telephoto lens. B&H Photo has them, though backordered.
I don't think Canon has any lenses like that. The 28-105 and 28-135 are somewhat similar in their zoom ranges, and both of those are regarded as great lenses (at least they were back when I owned Canon gear). A bit cheaper too at $450ish with Canon's IS technology, which is the same as Nikon's VR. But there is quite a difference between the *awesome* flexibility of 18-200 vs. just *great* flexibility of 28-135....
The wide-open f-stop number is normally inversely proportional to how expensive the lens will be. The lower that number, the more money it will suck out of your wallet.
Seriously though, it stands for 'f-stop', and it's a fractional measurement of how much light the lens allows to pass through it. For bright scenes, you (or the camera's software) will 'stop down' the lens to restrict the amount of light hitting the imager. Less light means the opposite. How long the shutter remains open is the shutter speed. Longer time = more light on the imager. It is this balanced dance of light, aperture, and shutter speed that is photography. Throw off this balance, and you have under- or over-exposed photos.
Of interesting note, the human eye is about an f2.1 to f8.3 lens (according to Wikipedia). A camera's iris works the same way. Opening in the dark, closing for bright. Walk into the bathroom and flip on the lights in the middle of the night and you have just experienced overexposure. Stub your toe on the dresser on the way back to bed and you've just experienced underexposure.
That 3.5-5.6 number means that at full wide-angle, it goes down to f3.5. At full telephoto, f5.6 is as much light as it can pass. Those are pretty common numbers for a consumer-grade zoom lens. Adequate for most situations, but possibly not great enough for lower-light photography without a flash. Though its my opinion that with modern digital cameras that can do ISO 1600+ and the amazing noise reduction done both in-camera and post-processing, this is less of an issue than it used to be. My 28-70 lens is also an f3.5-5.6, and I've never really had any problems capturing low-light situations. Really, that range should be completely adequate for a beginner. If you're curious what can be accomplished with a really low f-stop lens, I'd encourage you to pick up a cheap 50mm f1.8. They're usually less than $100. Maybe *way* less than $100 if you buy used.
A really exquisite and expensive telephoto lens might be an f2.8, where as a 'common' telephoto lens might be f5.6. That 2.8 will allow you to shoot in lower light (or more commonly) at faster speeds. Remember, shutter speed and f-stops go hand-in-hand, and it's fairly important that you understand that relationship if you want to take your photos 'to the next level'.
Those giant telephoto lenses you see at sporting events are really low f-stop lenses. This allows those photographers to use higher shutter speeds to stop the action without using a flash. They also cost as much as a nice used car.
For instance, the Nikkor 70-200 f2.8 lens is $1,600. The Nikkor 70-300 4.5-5.6 VR ED is $480. A lot of that price difference is in the quality of glass and lens engineering to produce a variale zoom that maintains a stable f-stop throughout the range.
I'd highly recommend reading
this. Quote:
After reading Rockwell’s sight and other users, that Nikon 18 – 200 VR lens looks like one hell of a lens and about perfect for a traveling, one lens set up.
What would the comparable lens to this one from Canon be??
And when looking at lenses, they always have an ‘F’ number. What’s that? That question alone should confirm any suspicions you may already have about my photographic experience….
