Sunday, April 14, 2019

Nikkor Soft filters 1 and 2 ~ point light source comparison

Here's a question: What effect might a soft focus filter have on out of focus rendition?  I decided to have a look.

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.

More specifically, I wanted to see how Nikkor soft filters might influence the out of focus regions of under-corrected spherical aberration lenses.

Setup -
  • Sony NEX5T, ISO 100, 2 second timer, -1 EV
  • Big Beefy Manfrotto tripod
  • Lenses using with a Lens Turbo II focal reducer -
    • Nikon Nikkor 50mm f/1.8 AiS
    • Nikon Micro-Nikkor 55mm f/2.8 Ai
    • Nikon Nikkor 85 f/1.8 K pre-Ai
  • NOTE1: Lenses were 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
 Comparison -

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

Nikkor 50mm, 85mm, Soft Filter ~ point light source comparision

Comments -

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

The Nikon Nikkor soft filters appear to mute out of focus disk intensity.  The stronger the number filter, the stronger the effect.  In general, the basic underlying effect of spherical aberration appears to remain, such as the bright center spot that is common to under-corrected lenses.

So, what have I shown?  Not much that is readily/easily visible from the point light source comparison.  Based on experience, however, the effects of using a soft filter are more easily judged across a full image.  Even though I doubt I will significantly deepen my understanding of soft filters, I will take a look at this effect in a future post.

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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