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Hi Miqspix,
The Canon 28-135 has a maximum aperture of 3.5-5.6. This means that the widest aperture the lens can achieve is f/3.5 at the 28mm end and f/5.6 at full zoom of 135mm.
The Tamron has a maximum aperture of f/2.8 across the entire zoom range, so whether you have it set to 28mm or 75mm, the maximum aperture will always be f/2.8. It can of course be stopped down to a smaller aperture (not sure what the smallest is but probably f/32 or something in that region)
The focal length is determined by the lens. What you are referring to is the depth of field, which is controlled by the aperture. The wider it is, the narrower/shallower the depth of field is, which creates the nice out of focus backgrounds. The smaller the aperture is the greater the depth of field and the more in focus the background becomes.
I hope that helps.
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