Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
rrr_hhh Veteran Member • Posts: 6,022
Re: Equivalent focal length for MFT lenses

KenBalbari wrote:

JeanPierre Martel wrote:

KenBalbari wrote:

Yes, that's what I said in the first place

Bravo. We are all saying the same thing. So sorry if I misunderstood your message.

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).

Say that otherwise. I have no clue about what that means.

If you mean that 300mm m4/3 lens at F/2,8 = 600mm FF lens at F/5,6, that's true for the depth-of-field and for the angle-of-view only. But that's not the case for the brightness of the picture (the ISO being the same of course).

I mean that f-number equals f/D.  So if you change f, you change the f-number.  Unless you choose to change D (the aperture diameter) instead.  But as I explained it above in my first post, if you keep D the same and change the f-number, then you have also changed the exposure, and have to increase ISO as well to compensate.

This is where you are in error : you can change the focal length of the lens without changing the F number which is a standardized ratio. Things are simple : set the ISO, set the shutterspeed and the F number you need to expose your scene correctly : whether you are using a 28mm wide angle, or a 90mm tele you will have to use the same aperture on both lenses, although you have changed the focal length. Now take two cameras with a different sensor : use the same ISO and same shutterspeed, you will need a 50mm on FF or a 25mm on MFT in order to keep the same angle of view, but in order to get the same brightness, you will have to use the same aperture, aka the same F number on both (Well there will be some difference in reality, because each camera manufacturer design their jpegs engine differently, but that won't be linked fdirectly to the size of the sensor).

In fact, you don't even need a lens to measure the light and the aperture you need : you can find it using a separated lightmeter; it will be valid for all lenses on all formats.

You have three parameters to consider in the equivalence :

  •  the exposure producing a certain level of brightness : F5.6 will remain F5.6 whatever the format is;
  • the DOF : F5.6 on FF correspond to F4 on APSC and to F2.8 on MFT
  • the different kinds of noise produced during the process of capturing the light; the MFT sensor being four times smaller will produce more noise than a similar sensor on full frame; in theory this could correspond to two stops difference in matters of ISO, but in practice it is much more complicated (the smaller sensors are usually packing more pixels than the larger sensor and their efficiency are usually different too). This is better determined by experience and by reading results of tests like those of DXO. 
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