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.
- 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.
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.
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