# I finally 'get' equivalence

Started May 22, 2012 | Discussions thread
Re: The Hokey Pokey.

Great Bustard wrote:

Fygaren wrote:

Yes, the FF would gather 4x the light because the FOV is 4x bigger, whitch is a totally different photo. But when we talk about equivalence it should always be with the same FOV??

Yes -- it only makes sense to compare systems at the same AOV. Let's talk about 12mm f/2 on mFT vs 24mm f/2 on FF. The diameter of the virtual aperture (entrance pupil) at 12mm f/2 is 12mm / 2 = 6mm. For 24mm f/2 on FF the aperture diameter is 24mm 2 = 12mm.

Since the aperture diameter is twice as large, the area is 4x as large. That means 4x as much (2 stops more) light will pass through the aperture and fall on the sensor in any given time interval.

As the AOV is the same on both lenses the same amount of Lumen(lm) will enter the lens. As the m4/3 sensor is 1/4 of FF this will mean that the lux(lm/m2) on the m4/3 sensor will be 4x higher given the same aperture area. Now the aperture area on the m4/3 is 1/4 of the area of the FF with the same f-number, making the lux the same on both sensors(giving the same shutter speed). Oh, as lux is measured in lm/m2 and the area of the FF sensor is 4 times bigger it gets hit by 4 times the lumen...s*it! :S I stand corrected, thanx

Tom

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