Why do older camera lenses have faster F-stops?

Started Apr 14, 2013 | Discussions thread
The Davinator
The Davinator Forum Pro • Posts: 21,225
Re: f1.8 equivalents...

HumanTarget wrote:

EinsteinsGhost wrote:

Nope.Here's an image taken with a small sensor camera (4x crop), at 200mm FF equivalent:

Sony F828, ISO 64, 51mm (200mm equivalent), f/2.8 (f/11 equivalent), 1/800s

Are you going to tell me that a FF camera will get this same exposure with a 4-stop faster shutter speed (1/12,800s) at same ISO and f/11?

No, but at the same shutter speed and f/11.  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.  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.
  Remember, exposure refers to how much light you collect.  When two systems use the same shutter speed and equivalent f-stop, they will collect the same amount of light.  Where FF has an advantage over smaller sensors is that it has a larger aperture for any given f-number (since the larger sensor requires a longer focal length, and aperture is f/n).  Use the same sized aperture (equivalent f-stop) and same shutter speed, and FF loses its advantage over smaller sensors.

I think I understand ya.  I thought you, and Joseph were saying same shutter, iso and f stop.  I did not consider equivalent DOF

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