Re: Focal length and angle of view.

alertjean@xxxxxxxxxxxxxx wrote:
If two lens have same angle of view, does it necessarily imply that
both of them has the same angle of view ?

For the same sized image sensor yes.

Two caveats:

1) When a lens is declared to be "100mm" it is often really 1 or 2mm off either way. The rounding is a typical marketing adjustment.

2) For 35mm format, the angle of view assumes a 36x24 mm image area. So if the sensor is a 1.5x crop, the the angle of view is reduced. This is where the "crop" factor and "equivalent focal length" mentions are rooted.

So how is a 100mm macro lens(say canon 100mm f/2.8) getting more
magnification compared to a zoom lens (say a tamron 18 200) set to
100mm ?

It's not. It's just that a macro lens is designed to achieve focus at a reproduction ratio of at least 1:2 and preferably (for 35mm anyway) to 1:1. The zoom magnification has little do do with it.

For example if you're taking a photo of an ant that is 10mm in length at a ratio of 1:1, then the image formed on the sensor (or film) will be 10mm long.

If you enlarge 10X, then on the print, the ant will be 100mm long.

Zoom macro lenses usually do not achieve 1:1 focus.

Is it that the only difference between these two lens is that canon
macro has a much closer minimum focussing distance compared to
tamron ?

That's the basic point.

What about MP-E 65mm f/2.8 1-5x Macro ?

Same, except it goes beyong 1:2 and 1:1 and achieves a range of 1:1 to 5:1. There are not many magnifying lenses like this.

Cheers,
Alan.

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