f/1.8 on m4/3, ASP-C, FF; are they the same?

Started Nov 1, 2010 | Discussions thread
nugat Contributing Member • Posts: 699
Re: Please stop confusing the normal people....

This was simpler back when light (exposure) meters were separate handheld devices. You set the film ISO on the meter and measured the light and got a set of pairs of numbers: shutter speeds and corresponding apertures. Eg. f16/125, f8/250, f5.6/500 etc. Each of the pairs gave the right exposure for a given scene and ISO. It was never dependent on lens type or camera body, f-number from the meter was directly comparable to the one from the lens. And still is . The f-number is dimensionless and simply tells how intenive light falls on area unit of the focal plane. Two lenses with different focal length but the same f-number will deliver the same light intensity (illuminance) per area unit of the focal plane. To get the right exposure one only needs to know now ISO and shutter speed.

DOF is a subjective phenomenon and depends on more factors than aperture and focal length. It has nothing to do with light intensity.

To sum it up: just like in the old days an f-number of any lens is directly comparable for exposure purposes.

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