A Closer Look at Nikon’s New Phase Fresnel (PF) Lens Technology

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Weighing just 1.66 pounds, Nikon’s newly announced 300mm f/4 is the world’s lightest 300mm full frame prime autofocus lens. It’s also the first Nikon DSLR lens to feature the company’s new Phase Fresnel (PF) technology, which is the main reason the lens can be so small and lightweight.

The lens is 1.5 pounds lighter and 30% shorter than its predecessor.

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The PF in the lens name indicates that the lens contains a new Phase Fresnel lens element as one of the lens elements. Here’s a diagram of the new 300mm (the green element is the Phase Fresnel lens):

diagrm

Fresnel lenses were originally developed by French physicist Augustin-Jean Fresnel for lighthouses:

The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet.

In Nikon’s new lenses, this special lens element is used to reduce chromatic aberration, something traditional lens designs use multiple lens elements to do. Since less elements are needed when a PF element is used, the resulting lens can be lighter (and smaller) than traditional lenses.

Chromatic aberration occurs when a lens fails to focus all colors to the same convergence point. You’ll notice it as fringes of color in places of your photographs where dark and light areas meet. Here’s an example of a photo showing noticeable chromatic aberration:

aberration

Here’s a diagram showing how a Phase Fresnel lens element can be combined with a normal glass lens element to reduce chromatic aberration through “mutual cancellation,” causing the different colors to converge at the same point:

diagram

One of the main downsides of including a Phase Fresnel lens element is that it can cause colorful ring-shaped flares when strong light sources are inside or just outside a frame:

before

To address this issue, Nikon is including a new feature inside its Capture NX-D RAW processing software called “PF Flare Control.” The feature can minimize the appearance of this PF flaring:

after

It will be interesting to see whether photographers can work around this flare issue enough to embrace this new lens technology.

(via Nikon via Nikon Rumors)

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