F4 is not F2.8...some thoughts

Started Oct 26, 2012 | Discussions thread
em_dee_aitch Veteran Member • Posts: 3,675
Re: F4 is not F2.8...some thoughts

Grig wrote:

Well, May I add one more thought from the wedding photographer here?

F4 is not 2.8 also in terms of AF accuracy and speed... It is exactly 2 times harder for the AF system of the body to focus in very low light (i.e. weddings) with F4 than F2.8...

Just my 2 cents...

No, that is completely wrong. You have not read how PDAF sensors work.

Assuming that you are doing most of your wedding photography using the optical viewfinder (PDAF) and not live view (CDAF), then there is no difference in the "brightness" that the PDAF sensor "sees" between a 2.8 lens and a 4.0 lens.

Why is that?

Because there are multiple optical paths through the camera (destination viewfinder, destination capturing sensor, destination submirror/PDAF array), and the aperture in the lens is just the first possible aperture in the path, not the last. Any additional aperture that comes after the taking lens will restrict the light path as if the taking lens itself had been stopped down. In the submirror/PDAF path the restrictive physical aperture(s) built into the PDAF array stop the light down well past f/4. Once you get into f/5 or smaller it's a bit more hazy, though Nikon's extension of the PDAF system to f/8 coverage in this latest generation means that they may have tightened it further.

I'm not an engineer, but I'm just repeating for your benefit what they engineers who participate in this forum have explained many times.

Though it does not affect image capture, the etching pattern of the focus screen in the viewfinder also acts as a type of aperture, because it effectively masks out any depth of field differences faster than f/2.5 (with respect to typical Nikon default screen, as screens vary in this threshold). This means a 50/2 and 50/1.4 will look pretty much the same in the optical VF with regard to DoF.

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David Hill
San Francisco & San Jose, CA | Austin, TX
Wedding Photographer and Apparent Gearhead

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