Sunday, April 07, 2019

Nikon Nikkor 85mm, Pentax 85mm Soft ~ point light source comparison

I heard that spending time along the Cote d'Azur could be good for one's art.  While I make no claims to be an artist, spending time in the sun along the Mediterranean Sea inspired me to more deeply consider optics as applied to the craft of photography.

For many years I looked at just one dimension of commercially available optics - resolution.  Now I wanted to start looking at other aspects of optical performance.  For this series I look at under, neutral, and over corrected spherical aberration in out of focus rendition on subject matter behind the point of focus.

Setup -
  • Sony NEX5T, ISO 100, 2 second timer, -1 EV
  • Big Beefy Manfrotto tripod
  • Lenses using with a Lens Turbo II focal reducer -
    • NikonNikkor 85mm f/1.8 K pre-Ai
    • Pentax 85mm f/2.2 Soft in Nikon F mount (rare? I've never seen this lens in a Nikon mount before)
  • NOTE1: Lenses were shot at their widest apertures only
  • NOTE2: Out of focus samples are from points _behind_ the point of focus to compare background out of focus rendition
  • RawTherapee to convert RAW files into black and white and to set the black levels
  Comparison -

If you click on the following image you can inspect it at 100 percent.

Out of Focus Rendition ~ Comparisons

Comments -

NOTE: I feel the Lens Turbo II focal reducer adds a bit of under-corrected spherical aberration.

The Nikon Nikkor 85mm f/1.8 pre-Ai shows under corrected spherical aberrations.  In normal photography I would expect, based on these comparisons, that there will be a smooth out of focus field rendition.

The Pentax 85mm f/2.2 Soft shows very strongly under spherical aberration corrections.  In normal photography I would expect, based on these comparisons, that there will be an extremely smooth and very soft rendition across the field.

There is something interesting about the Pentax soft that should be carefully noted.  The point light source remains strongly represented in the center of the out of focus disk well beyond what might be considered the normal point of focus. This is physical evidence for how a severely under-corrected spherical aberration optic can exhibit extreme depths of field, even when the lens is shot wide open.

Resources -

For further information on how the topic of out of focus rendition, optical properties, and Nikon lens design history, please refer to the following -

A PhD thesis on the impact of "soft focus" lenses on the history of photography -

An excellent starting point for understanding out of focus rendition (I might not completely agree with his interpretations/observations, but his foundation of understanding is quite good) -

Nikon lens design histories -

Point light source discussions -

Zeiss comments on optical design -

Metabones Focal Reducer whitepaper -

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