Could someone please explain zoom factor

When I bought my first lens about three years ago, I told the camera store owner that I wanted as much reach as my P&S, which had a 10X zoom. He sold me a Sigma 18-200, which he said was roughly the equivilant of 11x. Now I read on the "Most Ridiculous Comments" section that when folks ask how much zoom a 70-200 L has, the most common answer is 3X. Could someone please explain how you arrive at 3X? Was the salesman mistaken, did he lie, or did I just misunderstand?

The zoom factor is simply the ratio between the shortest focal length and the longest. For instance, with the Sigma lens you mentioned, it is 200 divided by 18 which is 11.11. The Canon ratio is 200/70, which is 2.85. The salesman wasn't mistaken. I think you might be confused by thinking that the zoom ratio is the longest focal length. It isn't - it is just the amount of 'zoom' (ratio) that the lens possesses.

200/70=2.86
200/18=11.1

It's a mostly meaningless measurement. What you want to know is the focal lengths involved, not the zoom factor.

multiples of the minimum focal length to the max focal length

You say your old P&S had a "10X" zoom factor. What you need to ask yourself is "10 times what?"

The zoom factor is simply a comparison of the shortest focal length to the longest. It shows how much of a range the lens covers but does not refer to the actual focal length. On the 70-200, 200 is roughly 3 times 70 so it has a zoom factor of 3. The 24-70 and 100-100 also have zoom factors of 3. The 24-105 and 100-400 have zoom factors of 4. On your Sigma 18-200, 200 is roughly 11 times 18, so you have a zoom factor of 11.

If you want to find a lens that provides similar field of view to what you had before, you will need to find what focal length your old camera has in the built in lens and adjust for the crop factor of that camera. Or if you've been happy with the Sigma lens you've been using for the past 3 years you can just not worry about it and be happy you learned a new bit of trivia today.

liquefied wrote in post #6559576
200/70=2.86
200/18=11.1

It's a mostly meaningless measurement. What you want to know is the focal lengths involved, not the zoom factor.

Agreed, the zoom factor is just a ratio between short end and long end, it has no link to reach at all.

You would get much more reach from a 100-400 zoom, as 400mm is twice the length of the 200mm on your 18-200. However the 100-400 is only a 4x zoom factor, compared to your 11x.

Equally, the 24-105 zoom is also a 4x (actually about 4.4x for the pedantic) but is much shorter than either lens, so you get less reach.

Forget 'zoom factor', it's of no use with SLRs. It's a selling point on a p&s because you can't change the lens, so you need something that is versatile and can handle wide angle and long telephoto requirements.

With an SLR you can change to a lens with a more suitable length and having choices gives you much more range than you get with a p&s. I have lenses covering all focal lengths between 10mm and 400mm (boosted to 800 with my 2x converter), so I have an effective zoom factor of 80x which comprehensively stuffs ANY p&s.

James P wrote in post #6559512
When I bought my first lens about three years ago, I told the camera store owner that I wanted as much reach as my P&S, which had a 10X zoom. He sold me a Sigma 18-200, which he said was roughly the equivilant of 11x. Now I read on the "Most Ridiculous Comments" section that when folks ask how much zoom a 70-200 L has, the most common answer is 3X. Could someone please explain how you arrive at 3X? Was the salesman mistaken, did he lie, or did I just misunderstand?

The salesman was right and he understood your question correctly.

A 70-200 is roughly a 3X zoom, because the ratio of the longest length (200mm) divided by the shortest (70mm) is close to 3. But this is no help to a P&S user such as yourself.

What you wanted to know is, what focal length do you need to buy on a 1.6X body SLR to match the 10X (longest position) on the P&S you own. Your salesman knew that your P&S probably has a short end focal length of about 36mm (FF equivalent) and a 360mm (FF equivalent) long end.

36mm FF equivalent is 22mm on your dSLR and 360mm FF equivalent is 225mm on your dSLR.

So a 22-225 zoom would pretty much exactly give you the same angle of view range. His suggestion of an 18-200 is thus very reasonable.

Well, James is certainly getting some advice. Everybody has said the same thing!

Thanks to one and all for explaining this so well. I have long since retired my Olympus P&S and joined the wonderful world of SLRs, but that "zoom ratio" thing is just something I never figured out. Now when someone asks me "what's the zoom on that lens?" I can give the correct answer. I sincerely appreciate your explanations and for not clobbering me for asking such a noob question.