The 18-135mm is an f3.5-5.6 variable aperture zoom lens.
What this means is that at the shortest focal length (18mm), it can be set as wide as f3.5. But at it's longest focal length (135mm), the largest available aperture is f5.6. At focal lengths in between the max aperture becomes progressively smaller as you zoom the lens from 18mm to 135mm.
So, it depends upon the focal length you have set, what the max aperture will be.
Try it at different focal lengths. You should be able to set f3.5 if set to 18mm, and will see that change to f4, f4.5, f5.0 and f5.6 at longer focal lengths. If this is what occurs, the lens is working fine and there is no issue. If the lens truly can't be opened beyond f4.5 or f5.0 at the shortest focal lengths, there might be a problem with the lens and you should get in touch with Canon service.
A variable aperture zoom is less expensive to design and build, plus it might give better image quality while being more compact and lightweight. Compare, for example, with EF 24-70/2.8L II or EF 24-105/4L IS, both of which are constant aperture zooms but give less of a range of focal lengths, are larger and heavier, and are considerably more expensive.
As to depth of field, you can render a softer background even with the apertures that are available to you, depending upon how close you are to your subject, as well as the subject's distance from the background. Shorter focal lengths naturally render deeper depth of field and less background blur, while longer focal lengths will make for shallower DOF and stronger blur effects.
For example, the image below is shot with a similar variable aperture zoom at 37mm focal length, f7.1 aperture with the subject fairly distant from the photographer and relatively close to the background. As a result of this combination, DOF renders lmost everything in this image in sharp focus...
Now here's another example, aobut the same distance from photographer to subject and the same f7.1 aperture, but using a longer 117mm focal length and with the background behind the subject much farther away. Notice how much more background blur is rendered....
Another example... This time the focal length is actually a little less, 100mm, and the aperture is the same f7.1. What has changed is the photographer is closer to the subject and the subject is even farther from the background, thankfully rendering even stronger blur (since the background includes rather ugly horse trailers and heavy equipment)...
My point is just that you should experiment with distances and focal lengths and the apertures your lens provides, to see what you can do with depth of field effects. Or, plan to spend some serious money to upgrade to a large, constant aperture zoom.
Or maybe a prime lens.... for really strong background blurs, zooms can't beat prime lenses that can have much larger apertures without being too large and heavy, and without completely breaking the bank.
This is shot with EF 135/2L, wide open at f2.0 and very close to the lens' minimum focusing distance. The background is only a few feet behind the subject, yet is almost obliterated by the strong blur...
Canon EF 85/1.8 and 50/1.4 are also very capable of shallow DOF effects. If money is no option, there are also the EF 85/1.2L and 50/1.2L!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
