does a faster lens mean better autofocusing in low light ?

Started Nov 30, 2012 | Discussions thread
Anders W Forum Pro • Posts: 21,466
Re: Depends what "better" means

JosephScha wrote:

Faster: certainly, since it's wide open for auto focus.

Sharper where you think focus should be: possibly not - depth of field at f/1.4 is incredibly shallow. Any focus error (or, decision by the camera to focus on some spot other than what you wanted) will show.

Fortunately, the CDAF system in MFT cameras is as a rule very precise. Furthermore, the large max aperture of an 1.4 helps keeping that precision up even in low light/contrast by reducing the noise of the signal the AF system must work with. So the only thing I'd really worry about when shooting at f/1.4 is to place focus exactly where I want it. In that regard, there won't be a whole lot of latitude due to the fairly shallow DoF.

Just because light is low doesn't mean f/1.4 is what you must choose to use. You could raise your ISO and go to a higher f stop. For still subjects you could use a tripod and use ISO 100, a higher f stop, and let the shutter speed be whatever it needs to be. Shooting at the most open aperture of a fast lens is decision, you should consider what DOF you want, not just wanting to hand hold the camera in low light at base ISO.

So yes it focuses faster, but you might not choose to use the very widest aperture even in low light.

And, as pointed out above, not all fast lenses are equally fast. My comments assume everything else is equal, but different lenses have different autofocus motors - so some will actually be faster than others in any light.

Better in low light: Is that better???

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