# Equivalent focal length for MFT lenses

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

JeanPierre Martel wrote:

KenBalbari wrote:
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."

That's what she says: it's a ratio.

Yes, that's what I said in the first place, but the f in that ratio is for "focal length".  All f/6.7 means, is that the aperture is equal to the focal length divided by 6.7.

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.

It's more complicated than that.

It's not more complicated than that. All f/6.7 means, is that the aperture is equal to the focal length divided by 6.7.

F/2,8 means the focal length of the lens (let's say 50mm) divided by 2,8. It indicates the relative diameter of the opening of the iris. For a given lens, the more that diameter is big, the more the F stop is small. So it's an reverse relation for a given lens.

However, when we compare two different lenses, that diameter will be smaller in a 12mm prime lens compared to a 60mm prime lens when both are at F/2,8. Why ? Because the F stop number is a ratio and nothing but a ratio.

Exactly what I said in the first place.  If you change the focal length, you change the f-number.  Unless you fix the f-number instead, which means you are changing the aperture (which also changes the depth of field in the image).

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Olympus PEN E-PL2
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