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Nikkor 85mm f/1.4 AF-S

September 5, 2015

 

The bulk of this review will concentrate on the lens resolution.  The Nikkor 85mm f/1.4 AF-S is considered by many to be the gold standard for Nikon lenses.  It not only has outstanding resolution, but it also produces among the best out-of-focus characteristics (bokeh) of any existing lens.  This combination, in addition to its perspective at 85mm, makes it the ideal portrait lens.

 

 

One of the few complaints (other than cost) about this lens is that it doesn’t have vibration reduction (image stabilization).  As of this writing, none of the major manufacturers have a fast prime 85mm lens with stabilization.  It’s probably non-trivial  to maintain the premium optical characteristics and add vibration reduction.

 

Due to its nano-coating, this lens is very resistant to flare.  Even still, a lens hood is always recommended.

 

Nikon, as just about everybody knows, also makes an 85mm f/1.8.  I don’t own this lens, but it is said to have at least comparable resolution to the f/1.4.  People who buy the f/1.4 primarily get it for the ability to obtain a very thin depth of focus wide open, with world-class bokeh in the background.

 

Autofocus isn’t perhaps blazingly fast, but it’s pretty good; keep in mind that f/1.4 takes extreme precision and therefore the focus is slightly slower to achieve this precision.

 

My personal major complaint about this lens (and Nikon isn’t alone) is the spherical aberration.  What this means is that focus fine-tune wide open doesn’t work for the lens when the aperture is stopped down.  My camera lens combination, for instance, needs +1 at f/1.4, 0 at f/2.0, and -4 when stopped down beyond that.  I write this calibration information on the inside of the lens cap, since I’m terrible about remembering these rules.  You can always shift to Live View with contrast-detect autofocus to avoid the focus fine-tune problem.  What is needed here is a “smart lens”, such as what Sigma offers with its USB dock, that allows in-lens firmware to compensate for this focus shift.  Even Sigma only compensates for distance/focal length combinations and not focus shift due to an aperture change.  This spherical aberration phenomenon only seems to be an issue on high-speed lenses.  If a camera could keep the aperture at the stopped-down position while focusing, then the problem would go away.

 

An aspect of spherical aberration is “spherochromatism”, which this lens shows at wide apertures (I think all really fast lenses exhibit this to some degree).  Out-of-focus objects just in front of the plane of focus are magenta and objects just behind are greenish.  It’s pretty much academic in real-world use; the effect is really small and is hard to notice unless the subject is white.

 

I briefly had another copy of the Nikkor 85mm f/1.4 AF-S.  Note this is in the past tense.  Its resolution measurements were generally about 11% worse (some were 26% worse).  Not all lenses are made the same.  Consult here if you really want to learn about lens-to-lens variation.  I read a review of this same model lens here. It sounds like they might have gotten my old lens.

If you want to go through the pain of evaluating your own stuff, then you need to get the MTF Mapper program and print out the resolution target files at that site.  The software (at this writing) is free, and the author Frans van den Bergh is to be commended.  The download site is here .

 

I discuss using his program in another article. Since Frans tends to write for an audience at the mensa level, my article is a ‘Cliffs Notes’ version.  You really should give his stuff a read, however.

 

MTF50 Measurements

 

I measure lens resolution at MTF50.  Most published manufacturer MTF charts are at MTF10 (contrast) and MTF30 (resolution).  Except for maybe Zeiss and Leica, those MTF charts are “theoretical”, meaning they’re blowing smoke you know where.

 

The "MTF" refers to Modulation Transfer Function, which refers to how light/dark transitions happen.  "MTF50" refers to the highest line frequency (line pairs per millimeter) you can have before 50% of the contrast is lost.  Values above about 30 lp/mm are considered pretty good, and anything above 50 lp/mm is outstanding.

 

I made the tests with a Nikon D7000 (APS-C sensor).  If you have a quality lens like this one, then you'll get up to 26% higher resolution by switching to the 24MP sensor D7100 (I tested it).  You lucky dogs with your full frame sensors will get slightly different results on the edges.

 

What the resolution chart looks like

 

The picture above shows what gets photographed (unsharpened, RAW) and evaluated.  The program author, Frans, has a couple of chart designs, but the main idea is to align the little squares to get their edges in sagittal (spoke) or meridional (tangent) directions. The squares need a little ‘slant’ to them (5 degrees is optimal) to get measured optimally, similar to what Imatest does.  Measurement algorithm problems arise if the little square orientations approach vertical or 26.565 degrees.  If you try testing yourself, don’t get too sloppy about orienting the chart, and bear in mind that the squares must always be bigger than 25 pixels on an edge.

 

The chart squares emanating from the center along 45 degrees (“X”) have MTF readings that can be 2 or 3 percent higher than they deserve.  This is the tradeoff between the desire to get sagittal/meridional measurements and approaching the critical ‘bad’ slant angles.  The Imatest guys have punted on this and don’t align their target squares in sagittal/meridional directions; they have 5-degree slants on all of the squares.  You’ll see an “X” pattern on some of the 2D resolution plots below, due to this effect.

 

 

MTF50 85mm f/1.4 wide open

 

This is really good performance for being wide open, in my opinion.  It’s just about even resolution across the APS-C sensor, too.  I get to avoid any nasty dips that might lurk on those full-frame camera edges.

 

The center of the chart at f/1.4

 

The MTF Mapper program provides annotations on every measured edge for every little complete square it locates in the photograph.  The annotations are in units of “cycles per pixel”.  These units are converted into “MTF50 line pairs per millimeter” in the plots, if the program options are configured to request that format.

 

Nikon Official 85mm AF-S MTF ‘theoretical’ chart from their website

 

 

f/1.4 MTF50 lp/mm over the whole frame

 

This 2D view makes it easy to see how the sagittal direction brings down the averages as you get away from the lens center.  The MTF Mapper program also has a 3D view option to turn those reds and greens into mountain peaks.

 

85mm f/1.4 worst corner for APS-C sensor

 

The worst this lens could come up with is the top-left corner at f/1.4 reading of 28.3 lp/mm in the sagittal direction (0.13 cycles/pixel).  The worst is something like 0.15 c/p or 31 lp/mm in the meridional direction.

 

85mm f/2.0 MTF50

 

 85mm f/2.0 MTF50

 

 

85mm f/2.8 MTF50

 

85mm f/2.8 MTF50

 

Again, please ignore the little chart bubble defect at the top of the plot.

 

 

85mm f/4.0 MTF50

 

 

85mm f/4.0 MTF50

 

 

85mm f/5.6 MTF50

 

You’re getting into a rarefied atmosphere here.  Due to the resolution chart individual square edge orientations along meridional/sagittal directions, it’s probably fair to knock off about 2 or 3% from the values shown for the squares that align along the main diagonals of the test chart.  Even still, these readings are outrageously good.  This is peak resolution performance for this lens.


 

 

85mm f/5.6 MTF50

 

85mm f/8.0 MTF50

Diffraction is starting to set in, so the resolution is starting to drop a bit.

 

85mm f/8.0 MTF50

 

Sample Images

 

Bear, Sequoia at dusk.  85mm 1/500 f/2.2 ISO 640 at 30 feet.

Could have used anti-shake here for my knees.  Glad the termites were more appealing than me.

 

Forest 85mm f/1.4 1/200 ISO 200

 

The background just melts away.  This is why you buy this lens.

 

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