Why do older camera lenses have faster F-stops?

Started Apr 14, 2013 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: f1.8 equivalents...

HumanTarget wrote:

EinsteinsGhost wrote:

In other words, the exposure is matched.


ISO does not change your exposure (aside from changing your aperture or shutter speed in an auto/priority mode), but relates exposure to image brightness.

Exposure value depends on aperture value and time value. So, yes, ISO doesn't play a role there. However, neither does "equivalent aperture".

No, but we're talking about equivalent apertures here, aren't we?

Only for DOF, not for exposure value.

When you change to the equivalent f-stop, you're matching the DOF, and thus the exposure.  Of course, the FF requires a larger exposure, so your image would be 4 stops underexposed; set the FF at ISO 1000, they'd be pretty evenly matched.

In other words, aperture equivalence is not affecting exposure, only DOF.

I'm not sure whether you're agreeing or disagreeing here.  Using equivalent apertures with the same shutter speeds will result in the same exposure AND DOF (which is why it's called "equivalence").  Unless you change your shutter speed, too, you cannot change DOF without changing exposure.

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.

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