Why do older camera lenses have faster F-stops?

Started Apr 14, 2013 | Discussions thread
HumanTarget Senior Member • Posts: 1,548
Re: f1.8 equivalents...

EinsteinsGhost wrote:

Only for DOF, not for exposure value.

We've been talking about exposure, not exposure value.

We're disagreeing on a very basic premise: Role of "equivalent" aperture on exposure.

Brightness Value of a Scene = Aperture Value + Time Value - Speed Value (ISO)

Aperture Value is NOT based on the idea of equivalent aperture. For that matter, if you took a light meter for a scene and observed ISO 100, 1/1000s at f/2.8 for a scene, what adjustments would you make for a camera with a smaller sensor? Use an m4/3 sensor to keep it simple.

The f-number system is not based on equivalent aperture, no, and that's what makes it so easy and convenient to use.  But we have not been talking about that, we've been talking about the actual amount of light hitting the sensor, which is necessary when discussing equivalence.  If you want to capture the same image (in terms of DOF and shutter speed) on two differently-sized sensors, the larger sensor must have a proportionally higher ISO.  But the two will receive the same amount of light (the same exposure).

Take two swimming pools; pool A is twice as large as pool B.  Fill them both with the same hose/same pressure for one minute, and which has more water?  It should be obvious that they'd both hold the same amount of water.  Pool A would be less "full," but both pools would hold an equal amount of water.

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