Why do older camera lenses have faster F-stops?

Started Apr 14, 2013 | Discussions thread
HumanTarget Senior Member • Posts: 1,646
Re: f1.8 equivalents...

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?

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.

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