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 spherical aberration (looking for under, neutral, or over correction) in out of focus rendition on subject matter behind the point of focus.
Setup -
- Sony A6000, ISO 100, 2 second timer
- Big Beefy Manfrotto tripod
- Lens using with a Lens Turbo II focal reducer -
- Nikon Nikkor 100-300mm f/5.6 Ai zoom
- NOTE1: Lens was shot at the widest aperture 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 100-300mm f/5.6 Ai zoom at 100mm shows a flat out of focus disk. The further out of focus a dot becomes, the more a slight under-corrected spherical aberration in the out of focus regions is revealed. Based on this comparison, in normal photographic use this lens at this focal length should produce a smooth out of focus rendition.
At 200mm, this Ai zoom continues to show a flat neutral disk. The further out of focus a dot becomes, the more a slight under-corrected spherical aberration in the out of focus regions is revealed. Under normal photographic conditions I would expect that this lens at 200mm will continue to show a smooth out of focus rendition.
At 300mm, this Ai zoom shows very neutral out of focus rendition. Under normal photographic conditions I would expect this lens at this focal length to produce extremely smooth out of focus rendition.
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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