Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
KenBalbari
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Re: Equivalent focal length for MFT lenses
In reply to JeanPierre Martel, Apr 13, 2013

JeanPierre Martel wrote:

rrr_hhh wrote:

The F number is a ratio and like all ratios also a dimensionless number, unlike the focal length which is measured in mm. It is the result of the focal length divided by the diameter of the entrance pupil of the lens. In fact F6.7 is a shortening of 1/6.7, aka the F number is the denominator of a ratio whose numerator is reduced to 1; this is why a greater F number corresponds to a smaller aperture and a small number to a greater aperture; F6.7 is an indication of the density of the light which will be allowed on the sensor ; it will remain the same for any sensor because it is a density. Here is what Wikipedia says about it : http://en.wikipedia.org/wiki/F-number

You're absolutely right. When you say the "density of light", allow me to say it otherwise: it's the number of photons that will hit the sensor per square mm.

No that is not right.  The f-number does NOT tell you anything about the density of light or number of photons.  That is going to depend as well on the scene luminance and exposure time. The f number only tells you the relationship of the focal length to the aperture.

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