i'm a little confused here...
I'm going to take a P&S Canon powershot as an example to illustrate my confusion.

IXUS 220 HS
Sensor: 1/2.3" CMOS
Lens: EFL: 4.3-21.5mm (35mm equivalent: 24-120mm)

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Now, if a 50mm lens equals 80mm on APS-C 1.6 sensor.

then why a 4.3mm lens equals 24mm on a 1/2.3 sensor,
and not 9.89, if 4.3 x 2.3 = 9.89 ?.


There's something i don't quite understand yet.
Could someone explain this for me?.

Thank you

Because in your equation you've mixed up sensor size and crop factor. You've inserted the sensor's diagonal measurement rather than the crop factor for a sensor of that particular size. The math you've used would only be accurate if a 35mm sensor was equal in size to a 1" sensor, which it is clearly not.

As sensor size decreases crop factor increases. I'm not sure how to figure out the crop factor for a sensor of any particular size compared to 35mm, though using the stats provided I would say that 1/2.3" sensor has a crop factor of about 5.58 (24/4.3=5.58 to the nearest 100th).

Lets not forget that a 24mm lens is still a 24mm lens regardless of which sensor size your camera uses. The camera is just applying a crop factor to the lens which in turn gives it the same angle of coverage as say a 36mm lens (using 1.5X ratio).