On Thu, 02 Jun 2011 22:31:49 -0400,

(E-Mail Removed)
wrote:

> I've read that the magnification of a lens is calculated by

>dividing it's larger zoom number by it's smaller zoom number.

>

> If true, an 18 - 200mm lens has a magnification (not

>considering crop factor) of 11.1...

>

> But a 70 - 200mm lens would only have a maximum magnification

>of less than 3 times?

>

> Did I miss something?

>

> If that's true, why would anyone buy a 70 - 200 in preference

>to something that can better pull in distant objects?

>

> Obviously, I haven't yet taken a photography course or this

>would have been one of the first questions I'd have asked.

>

> Any help is appreciated.
You're confusing zoom range with magnification. Zoom range is easy.

It's just the larger focal length / smaller focal length.

Magnification is a little more complex. For a telescope or binoculars,

it is the focal length / effective exit diameter, which is the

eyepiece focal length. That's all well and good because you are

looking right through the lenses at an object. With a camera, it's a

little different because it's projecting onto a sensor. You can take

the exact same lens and it will have different *effective*

magnification for cameras that have different sensor sizes. This is

true even though the actual magnification of the lens hasn't changed,

and is the focal length / effective exit diameter.

It gets a little more complicated for many zoom lenses with internal

focus that have a small min focus distance. Take the 18-200 as an

example. When you focus close, say less than 10', with the lens zoomed

out to 200mm, it isn't really a 200mm focal length. For close focus,

the lens compensates for the fact that it's length is restricted due

to internal focus by reducing the focal length. That lens can't get

much more than 140mm when the subject is close. At infinity focus, it

does go out to 200mm. So don't believe your zoom ring markings unless

the subject is far away.

I have a quick and dirty calculus for lens magnification which relates

to the "fact" that a 50mm lens on a 35mm camera is considered to be

about 1x magnification. You just take your focal length and divide by

50. Thus, a 200mm focal length would be a 4 power lens if you wanted

to relate it to something like binoculars magnification.

If you're shooting with an aps sensor that has a crop factor of 1.5,

just take whatever you get above and multiply it by 1.5. So on a Nikon

APS sensor, a 300mm lens would be a 1.5 * 300/50 = 9x lens and would

have the same magnification as 9x binocs.

There are more detailed analysis you can get into if you're really

interested. Things like the lens reproduction ratio, which is

dependent on subject distance and therefor ability to focus at

distance X. Just google reproduction ratio and lens focusing formulas

if you really want to get into it.

Steve