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:

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.

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".

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.

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.

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Sony Cyber-shot DSC-F828 Sony SLT-A55 Sony Alpha NEX-6 Sigma 18-250mm F3.5-6.3 DC OS HSM Sony 135mm F2.8 (T4.5) STF +12 more
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