How can you compare magnification on camera lenses (i.e. 135mm = ?X?

The magnification is directly related to the "mm" number of the lens. So a 400mm lens has a stronger "telephoto" effect than a 100mm lens.

As the "mm" number gets smaller, the lens effect changes from "telephoto" to "wide-angle".

The "X" typically used in camera specification measures the range of the zoom lens.

So a lens with a range of 35mm (wide angle) to 350mm (telephoto) is said to have a 10x zoom. (350 divided by 35)

But a camera with a range of 25mm to 250mm also has a 10x zoom. (250 divided by 25) But the telephoto of this camera is much lower than the first camera.

So, if you want a more powerful telephoto, get a camera with a higher telephoto "mm" number.

I think your question stems from digital camera manufacturer's habit of marketing their varioius telehoto lens equivalents. Unfortunately, the "X" factor (3X, 5X. 10X) has no standard. The X simply designates the difference in magnification from the image taken without zoom.

For instance, I had a Fuji camera that claimed some X times magnification. Unfortunately, I noticed that at the highest magnification, the image in the viewfinder was about equal to the naked eye image. Without the zoom, the camera was equivialnt to using a wide angel lens.

Each maker has its own standard. So, if you are shopping, keep in mind that for best magnification, look at the camera's optical zoom capability. Obviously the higher the better. Digital zoom doesn't mean much if you intend to print images.