Why is 24mm, the 35mm equivalent of 4.3 on 1/2.3 sensor? at DVinfo.net

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Old May 14th, 2011, 06:35 PM   #1
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Why is 24mm, the 35mm equivalent of 4.3 on 1/2.3 sensor?

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
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Old May 14th, 2011, 10:10 PM   #2
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Re: Why is 24mm, the 35mm equivalent of 4.3 on 1/2.3 sensor?

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).
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Old May 15th, 2011, 02:51 AM   #3
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Re: Why is 24mm, the 35mm equivalent of 4.3 on 1/2.3 sensor?

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).
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