For many years I looked at just one dimension of commercially available optics - resolution. With this blog entry I continue to look at other aspects of optical performance. For the series of postings 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
- Big Beefy Manfrotto tripod
- Lenses using with a Lens Turbo II focal reducer -
- Nikon Nikkor 85mm f/2 Ai
- Nikon Nikkor 85mm f/1.8 K pre-Ai
- 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
If you click on the following image you can inspect it at 100 percent.
Comments -
NOTE: I feel the Lens Turbo II focal reducer adds a bit of under-corrected spherical aberration.
The Nikon Nikkor 85mm f/2 Ai lens shows under corrected spherical aberration as well as a bright edge ring that suggests over correction around the outside of the out of focus disk. In normal use, I expect the out of focus rendition to be "harsher" than, say, the next lens from Nikon.
The Nikon Nikkor 85mm f/1.8 K pre-Ai lens shows classic under corrected spherical aberration with a bright dot in the center of a smooth disk. In normal use, I expect the out of focus rendition to be "delicate" (using Nikon's own description of the effect) and smooth across the field.
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 - http://research-repository.st-andrews.ac.uk/handle/10023/505
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) - http://jtra.cz/stuff/essays/bokeh/
Nikon lens design histories - https://imaging.nikon.com/history/story/
Point light source discussions - https://www.dpreview.com/forums/thread/4031515
Zeiss comments on optical design - https://lenspire.zeiss.com/photo/en/article/how-does-zeiss-define-bokeh-an-interview-with-dr-stefan-ballmann
Metabones Focal Reducer whitepaper - https://www.metabones.com/assets/a/stories/Speed%20Booster%20White%20Paper.pdf
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