I have Canon 18-135 and 70-300 zoom lenses. People ask me how much zoom my camera supports.
What am I supposed to tell them?
"135 ÷ 18 = 7.5" and "300 ÷ 70 = 4.2"?
As you've pointed out, the question is meaningless in absolute terms. People whose exposure to photography starts and ends with point-and-shoot cameras don't really know what the term means.
They'll be thinking in terms of compact cameras, and the "zoom" on those goes from a moderate wide angle (about the 20mm mark on your lens, which is about 32mm equivalent on a full-frame sensor) as "1" to some multiple. The long end of your telephoto zoom is 300mm, or about "15x" in terms they'll understand.
If they seem disappointed that all you get is "15x" with that "huge" lens, you can always mention that there's an accessory you can get to go to 80x (that'd be the 800mm f/5.6 with a 2x teleconverter, and together they're about the price of an "entry level" automobile, but you don't need to mention that part). That'll kill the disappointment.
Its the factor between your widest focal length and tele end. For example, you have 18-135mm lens. Your widest focal length is 18mm and tele end is 135mm. So your lenses 'zoom' is 135/18=7.5X, for your 70-300 its 4.2X. This is optical zoom, some camera supports digital zooms as well. Zoom doesn't necessarily mean longer reach, its means longer range. As you see, your 4.2X zoom lens have more reach than your 7.5X zoom lens.
Compact cameras mostly have fixed lenses and zoom is used to denote the capability of the lens attached to it, but this isn't used for DSLRs as their zoom depends on the lens being used.
One thing is, your 'camera' isn't limited to any specific zoom. "how much zoom my camera supports" is not a valid question. It depends on the lens attached to it. Your camera supports unlimited zoom as long as it has got a lens to support it.
Since your question is "What am I supposed to tell them?", I'm going to go a little bit off the literal math with my answer. What you should say is:
The more "times-zoom" a lens has, the more compromise on image quality it has to have. Since my camera allows me to change lenses, I can have the flexibility of a huge zoom range without compromising on the results.
Then, I'd steer the conversation this way:
In fact, some very popular lenses for SLR cameras have no zoom at all, because they can be high quality and let in a lot of light without being expensive and gigantic. On the other hand, some lenses like my 18-135mm have a relatively high zoom ratio, allowing you to choose that convenience when you want it.
At that point, they may just smile and nod and back away. But they may also learn something. :)
You could just explain that the zoom factor (ie 3.5x, 10x, 20x) is just marketing babble and doesn't really make much sense for comparing different types of lenses, when you have the ability to change lenses.
Explain that for a given camera, the focal length is what makes the difference to how "zoomed in" you are.
Then go on to explain that you could buy a 7mm lens for this camera, or a 1000mm lens. If you own both these lenses that's a bit like having a 142x zoom.