Why do older camera lenses have faster F-stops?

Started Apr 14, 2013 | Discussions thread
EinsteinsGhost
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Re: f1.8 equivalents...
In reply to Joseph S Wisniewski, Apr 15, 2013

Joseph S Wisniewski wrote:

EinsteinsGhost wrote:

Joseph S Wisniewski wrote:

EinsteinsGhost wrote:

Joseph S Wisniewski wrote:

Yes, because it's micro four thirds, and the DOF and you need f0.9 to have the light gathering and DOF equivalent of the "plastic fantastic" $100 f1.8 lenses on FF. So, some old "cine" lenses get reincarnated.

DOF yes (at same FOV), exposure no.

Exposure yes.

Cut the sensor area into a quarter of what it was, and you have 1/4 of the photons in the picture, so you need to throw 4x the light at it to get the same results. Just like in the film days, when tri-x was probably my most shot film on the 4x5, while tech pan was one of my favorites on 35mm.

Physics is a bear.

Why would you need to throw 4x the number of photons in an area 1/4 the size for the same exposure?

Because, if you don't, you get very different pictures. A micro four thirds sensor at ISO 800 looks nothing like a FF sensor at that ISO, more like the FF sensor at ISO 3200. Same number of photons makes for the same image.

"Very different picture" will be in terms of DOF and FOV. A 50mm f/1.4 on MFT will produce the same FOV as 100mm f/2.8 lens on FF.

As a matter of fact, smaller sensor is simply throwing away the light that falls outside of it (which a larger sensor uses for additional FOV).

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