# Equivalent focal length for MFT lenses

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

rrr_hhh wrote:

KenBalbari wrote:

No, you will have very different depth of field in your example, plus the FF sensor will have less noise, better dynamic range, and likely better color depth. In addition, you don't really need to say "twice as soft" for the lens, as a 300mm lens will naturally have twice the power of a 600mm lens of equal quality.  If the glass is equal, the 600mm will be "twice as soft" so to speak, if you are comparing on a lp/mm basis.

But the best way to do equivalence is to realize that the apperture changes if you fix the f-stop and change the focal length.  This is because what the "f" in f/6.7 stands for is "focal length".  Those who deny this simply don't understand what an f-stop is. So if you change the focal length from 300mm to 600mm, in order to keep the aperture (and depth of field) the same, you need to also change f/6.7 to f/13.4.

F doesn't stand for focal length, although it has something to do with the focal length. 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.

rrr_hhh

Your Wikipedia source agrees with me on this.  First sentence in the summary at top:

"In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil."

And then again, first sentence in the first section:

"The f-number N is given by N=f/D where f is the focal length, and D is the diameter of the entrance pupil (often called the aperture). It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the entrance pupil diameter in terms of f (a symbol denoting the focal length), and the f-number. "

It is quite clear that the "f" is for "focal length".  And f/N means focal length divided by f-number.  So f/2.8 means focal length divided by 2.8. And that is a measure of the aperture.  If you measure f in mm, then the aperture is also in mm.

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