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:

rrr_hhh wrote:

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.

No, this is where you are in error.  Yes, it's a ratio, but the focal length is the numerator of that ratio.  So if you change the focal length, you also change the ratio, unless you also change somthing else.

Ooops, yes, I goofed confusing numerator and denominator, but the resulting ratio remains the same. And what I said still hold. I used the term aperture the way most photographers use it : aka to indicate what is on the lens aperture ring, or what appears on the LCD when you adjust aperture. By the way : how many of us know the actual aperture of the lenses they are using  (aka the diameter of the entrance pupil of the lenses in mm ) ?

As for the rest, I hate these equivalence debates and won't go further. You have been caught of guard stating that the F stands for focal length while it isn't and you are now trying to rationalize this, using the term aperture in a way most photographers won't use it, unless they are into optics and lens design. Just admit you have goofed with that minor thing,  like I admitted myself above.

All I need to know is that

  • for brightness F5.6 will aways be F5.6
  • for DOF F5.6 will be like F11 on FF
  • and for noise : the total amount of light reaching the sensor will be 1/4th of what reach a FF sensor which has some consequence with noise. Anyone knows that smaller sensor have a disadvantage there. Theoretically, all other things equal this could lead you to two EV loss in matters of ISO with respect to FF (aka ISO 800 instead of ISO 200) but this is practically of few usability since all sensors are different and it is better to look a tests comparing the noise of the cameras you are considering rather than draw arbitrary conclusions based on the unprobable hypothese that everything else is equal on the sensor. 

I don't think it as simplistic as you want to make it, it is just very operational and all what people on these forums will ever need to know.

With this I'm done with this argument which I find extremely boring.

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