Saturday, June 15, 2019

Nikon Nikkor 85mm f/1.8 and f/2 lenses ~ a closer look

It so happens that I picked up another Nikon Nikkor 85mm f/2 Ai lens.  I owned one when I moved here, sold it, and got to wondering how it compared to my other, older 85mm f/1.8 K pre-Ai.  So here is yet another look at how they compare from a resolution point of view.

Setup -
  • Sony A6000, 100ISO, 2 second delay timer, RawTherapee conversion software 
  • Big Beefy Manfrotto tripod 
  • Lenses compared
    • Nikon Nikkor 85mm f/1.8 K pre-Ai
    • Nikon Nikkor 85mm f/2 Ai
  • Lens Turbo II focal length reducer
It's become obvious to me over the years that nearly all lenses suffer from field curvature.  Some lenses, as we might expect, have more pronounced curved fields than others.  So to account for this, I have taken to shooting two images at each comparison aperture.  The first image is focused in the center of the field, and the second image focuses at the very edge of the field of view.  It is important to note that I'm not attempting to measure how much field curvature there is.  All I'm looking at is, at the edge of the field, how sharp the optic is.  If one photographs flat subject matter, nearly all lens will be more or less out of focus at the edge of the field.

Comparison Results -
[If you click on the image it'll take you to the Flickr hosting site. Once there, look at the file at full resolution. In many cases the differences between lenses is small and likely can't be seen until you take a squint at the comparison at 100 percent.]

Nikon Nikkor 85mm Comparison

Comments -

Comparing the Nikon Nikkor 85mm f/1.8 K pre-Ai to the Nikon Nikkor 85mm f/2 Ai, I see that wide open the early f/1.8 lens is slightly less sharp than the newer design f/2 Ai.  At the edges of the frame, the f/1.8 K pre-Ai lens clearly lags the f/2 Ai.  Yet both lenses sharpen up very nice from f/4, and from f/4 on down the aperture range both lenses appear to be equal in the center and edges of the frame.

In terms of field curvature, I see that the f/1.8 K pre-Ai suffers from greater field curvature than the newer f/2 Ai optic (remember, I'm not trying to measure the field curvature, only noting the curvature in relative terms).

Since I don't sharpen the comparison images in any way, I thought it might be interesting to see what a rather aggressive unsharp mask might do to images from both lenses at wide open and f/2.8 in the center and at the edges.  The USM was set to a 2 pixel radius and a 0.5 contrast step.  This is pretty steep and I normally put the USM image in a separate layer and adjust its opacity over the original image to balance the harshness of the USM to make the final image more "film-like" (I feel some of the software tools can make an image appear "artificial" and cell-phone-like).

In both cases, the USM makes the wide open and f/2.8 images appear sharper than their f/4 to f/8 non-USM equivalents on the two lenses.  The f/1.8 K pre-Ai lens still lags the f/2 Ai wide open.  But, this shows what is possible if one wants to clean up an image that was shot at the widest aperture.  The results can be pretty darned impressive.

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