# Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
Re: Equivalent focal length for MFT lenses

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.

At F/6,7, the same number of photons per mm2 will hit the sensor whatever the mount (m4/3 lens or full-frame), the focal length (12mm, 60mm, 300mm, 600mm). So the brightness of the image will be the same.

Since the sensor is 4 times bigger on a FF camera, of course the 600mm FF lens is a lot bigger than the 300mm m4/3 lens. But the end result is the same except for the Depth of field, which is a different matter.

At F/6.7, a 300mm m4/3 lens will have the same DOF as a 600mm FF lens at F/13.

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