The 1:3.5 - 5.5 means that is the range of the maximum lens opening at various focal lengths. So depending on focal length, your lowest fstop number (maximum lens opening) could be anywhere between those values.

A simpler way to explain it is that the first number is the maximum f stop at the wide end of the zoom and the second number is the maximum f stop at the telephoto end of the zoom.

Variable aperture lenses are not the easiest to control depth of field and isolate a subject. This is an example of trying to do something with a consumer quality lens that's easier to do with a single focal length lens, or fixed aperture zoom, ie. more expensive pro quality lenses. The other problem is the smaller sensor creates more depth of field making it even more difficult to isolate a subject.

Your best bet is to shoot wide open at the telephoto end of the zoom and come in as close as possible to the subject to make the rules of depth of field work for you.

Larry,

I regret to say so, but I cannot agree with you on this issue. The depth of field is constant at a given fstop if the subject size is held constant. In this case your suggestion to use the telephoto end of the zoom results in a larger fstop and thus a greater depth of field, again when the subject size is held constant. Keeping the subject size the same is the only valid case to look at how focal length impacts depth of field and it doesn't, only fstop does.

It's not about disagreeing or not. It's not about the numbers either. It's about real world use and the perception of how to capture narrow depth of field to accomplish something visually. Using the wide end will visually distort so it's impractical.

Larry,

Your statement "Using the wide end will visually distort so it's impractical" is way too vague and misleading as well. The issue here is to get the shallowest depth of field and that is obtained by using the smallest fstop which is at the wide end. So, the wide end should be used to get the shallowest depth of field UNLESS that results in unacceptable distorsion, in which case a COMPROMISE needs to be made between shallow depth of field and distorsion.

Frans, Photography is not just about reading the numbers. It's more of an artistic form of creation and the last lens a person reaches for when trying to limit depth of field is a wide angle, no matter what the maximum f stop.

Larry,

We were not discussing the artistic aspects of photography. We were discussing the issue of depth of field and how to minimize it with a given lens. And as such, I stand by my remarks as being accurate.

I never said that you were wrong. It's just that no one does it that way in the real word of image capture. When someone needs limited depth of field for an effect, they inevitably end up at the telephoto end of the zoom because you can see the depth of field effect happening in the viewfinder.

This is one of those arguments that makes no sense. Either you end up capturing images or read a book on lens design.

Larry,

Why wouldn't I see the depth of field effect in the viewfinder if I used a shorter focal length and moved closer to keep the object the same size? That doesn't make any sense to me. And the fact still remains, whether or not you want to acknowledge it, that for the lens we are discussing here you can use a lower fstop and get less depth of field.

The choice is not either capture images or understand the equipment; you can do both.

Wow! I hope you got your answer despite the online debate over who is right!

There are various dof tools available, such as:
http://www.dofmaster.com/dofjs.html
You will see that the variables in the tool are format (image size), focal length, distance to subject, and aperture. The first three all affect magnification. However, it isn't silly at all that photographers think in terms of focal length. They note that shorter lenses have more dof in the sense of nearer hyperfocal distances, all else equal. You could say, this is because they don't magnify as much, but in everyday use what they see is more dof. Also, larger formats have lenses that, with the equal framing and angle of view, have less dof - I assume because they need to magnify in the sense of focus the image on a larger negative or sensor. Also, it is not practical at all to think in terms of getting the same magnification from all lenses. I do not and cannot run out onto the soccer pitch with a wide angle lens to get a picture of my daughter playing. Similarly, taking a picture of many people around a table I don't try to find and probably couldn't find a far-off vantage point from which to use the telephoto I use for soccer. Nor would I want the way the space would be represented (flattened), most of the time. So in practice it is fair to say that shorter lenses have more dof, even if another variable is what drives this.

Alex,

I know I start to sound like a broken record, but shorter lenses do NOT have more dof. Dof, for a given film or sensor size and circle of confusion, is SOLELY dependent on fstop. Now, if may be that you are limited in how close to or far from your subject you can get in real life, but I don't think you or anybody else would like his composition be dictated by the focal length of a lens. In other words, most people want a certain composition and then dof is solely determined by fstop; you may have limitations as to what lens you can use to get that certain composition, but that doesn't change the basic truth about what determines dof. If you can't use the lens with the fstop that gives you the desired dof and composition, then you are making a compromise. It is good to understand what drives dof and when you are making compromises.