vendredi 20 mai 2011

Nikon Primes, the AF 85 mm f/1.8 test

The 85mm f/1.8 AF Nikkor is a short telelens, well suited for reportage or portrait in available light. It's an AF, first manner, with a "screw" and needs a camera with a motor. While focusing, the rear lens moves back and forth and the front lens doesn't rotate. But the focus ring does.

Short and rather wide, it fits well in the hand. In front of a small camera body (Nikon 3100), the weight (400g) is well distributed. The outer barrel is all plastic, the rest is glass and metal. After 20 years of light use, focus got a bit noisier but otherwise it is in perfect condition.

Image quality

In a first series of tests, at f/1.8, small printed letters looked as if they were small blots of ink and hardly readable. Compared to other Nikkor primes at full aperture, the 85mm seemed a bit less accurate. I suspected a bad focus. But after more trials, I draw the conclusion that these results were consistent.

Optical defects

Click on the images to see them full size.
Nikon Nikkor 85 mm f/1.8 test
Nikon Nikkor 85 mm f/1.8 test

The 50 mm f/1.8, the 60mm f/2.8 and the 35 mm f/2 all give better defined images in a large centre spot, fully opened. Throughout the field of the 85, edges of bigger letters (black on a white background) are not neatly cut out but slighly fuzzy, like seen through a luminous haze. This is probably a consequence of a residual spheric aberration, a flaw inherent to lenses without parabolic surface and more obvious as diameter grows. Therefore, fast lenses like this 85 mm exhibit optical artifacts like the coma (light sources look like a comet tail) or coloured fringes.

The two images below show a visible coma and purple colour fringes at f/1.8. At f/2.8, these artifacts are not really annoying.

Nikon Nikkor 85 mm f/1.8 coma
Nikon Nikkor 85 mm f/1.8 coma

This said, the f/1.8 stop is usable thank to a good general contrast. At this aperture, the light falloff is not really important and isn't noticeable at f/2.8.

When stopped down, to f/2.8, the precision of fine details gets a lot better in the whole field.

Nikon Nikkor 85 mm f/1.8 test

At f/4, then f/5.6, the image gives an impression of brute sharpness. It looks very close to an other image of the same subject at the same magnification ratio, from a 50 mm or 60 mm Nikkor, compared side by side.
Nikon Nikkor 85 mm f/1.8 test
Nikon Nikkor 85 mm f/1.8 test

On a DX format, this very good result is even throughout the field.

Shall we conclude ?

In a nutshell, this 85 mm is a good workhorse. Reasonably fast and functional at full aperture, good at f/2.8 and outstanding beyond this stop. It looks modest compared to the awe inspiring, all-metal 85 mm f/1.4 of the same manufacturer. But for a mere 300 €, second hand, you've got a very appealing and yes more modest, alternative.

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