Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
KenBalbari Regular Member • Posts: 276
Re: Equivalent focal length for MFT lenses

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.

The same object shot with a 300mm m4/3 lens (on a m4/3 camera) and a 600mm FF lens (on a FF camera) will only look exposed the same way if the F stop number is the same on both lenses because that's the only way to get the same light density (same number of photons per mm2) to reach their respective sensor.

If the F stop number is set to F/5,6 on the FF lens (in order to have the same DOF), the image will be darker. So you have to set the ISO higher on the FF camera and consequently, lose some of the IQ advantage of the bigger sensor.

Agree.  Though you might maintain an advantage in resolution, if there was any at the sensor level. Since we are comparing different lenses, lens resolution will depend also on the individual lens, but there is no reason in theory why it should be any different at the lens level given equvalent quality glass; that is, the glass does not have to be "twice as sharp" to produce the same resolution.  The sensor is where you need more pixel density to match the resolution.

The only place I see people getting confused on equivalence is when f-number is also mentioned without specifying ISO or mentioning the effect on depth of field.

But I think the nearest you can get to an overall equivalent image, assuming same image size and viewing distance, is usually to say the 135 film format camera has to use 2 times the focal length, 2 times the f-number, and 4 times the ISO (and assuming the same shutter speed of course).  But even that is imperfect (though it at least comes close on recent models such as the 6D).

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