Tuesday, October 01, 2019

Spherical Aberration and Depth of Field ~ a comparison

Looking at the effects of spherical aberration on out of focus rendition has led me to better understand depth of field and why calculating it is not as straightforward as one might believe.

A neutrally corrected lens (such as a Nikon Micro-Nikkor 55mm f/3.5 or f/2.8, or a Nikon Nikkor 85mm f/2) will show a balanced out of focus transition.  That is, the rate of change from sharp to out of focus will be similar both in front of and behind the point of focus.

A lens (such as a Nikon Nikkor 85mm f/1.8 K or 105mm f/2.5 P) that is under-corrected for spherical aberration behind the point of focus will show greater depth of field behind the point of focus than in front of that point.

Conversely, a lens (such as a Nikon Nikkor 50mm f/2 H) that is over-corrected behind the point of focus will show a shallower depth of field behind the point of focus than in front of that point.

Studying the links found under Resources (below) helped me understand how this is possible and how optical physics and photographic imaging work in real life.

I happen to have three lenses that span the range of neutral correction to severely under-corrected spherical aberration.  I didn't feel any need to compare an over-corrected lens as the effect should be obvious from looking at the Comparison image below.

Setup -
  • Sony A6000 handheld
  • Lenses using with a Lens Turbo II focal reducer -
    • Nikon Nikkor 85mm f/2 Ai 
    • Nikon Nikkor-K 85mm f/1.8 K pre-Ai
    • Pentax 85mm f/2.2 Soft (in Nikon F mount - the only one I've ever seen)
  • NOTE1: Lenses were shot at their widest apertures only
  • RawTherapee to convert RAW files into jpg using "Film low ISO" profiles

 Comparison -

The image on the left shows the effect of spherical aberration on depth of field.  The image on the right is simply the scene as it came out of the camera for each lens compared.

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

Out of Focus Comparison ~ 85mm lenses

Comments -

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

NOTE 2: Recall that if a lens is under-corrected behind the point of focus that the very same lens will be over-corrected in front of that focus point.

The neutrally corrected Nikon Nikkor 85mm f/2 Ai lens was focused on the red 80cm indication on the tape.  The 79cm (behind the point of focus) and the 81cm (in front of the point of focus) are to my eyes at the limit of what I would call "in focus" - 1cm in front and 1cm behind the point of focus means that in this situation there is a total 2cm of "depth of field".

The under-corrected Nikon Nikkor 85mm f/1.8 K pre-Ai lens was focused on the 79.5cm mark (I missed the 80cm mark).  Using the f/2 lens as a reference I would say that the 80cm mark in front of the point of focus and the 77.5cm mark behind the point of focus are at the limits of what I would call "in focus." 

This is approximately 0.5cm in front and 2cm behind the point of focus - with a total "depth of field" of 2.5cm.  Clearly, "depth of field" has contracted in front of the point of focus and expanded behind the point of focus.  This, directly, is the effect of behind the point of focus under-corrected spherical aberration on "depth of field".

The severely under-corrected Pentax 85mm f/2.2 Soft lens is an interesting study in "depth of field."  It is so severely under-corrected and so filled with spherical aberration that I missed the intended 80cm focus point.  Everything from 84cm through to 75cm appears to my eyes to be well defined sharp to be called "sharp."  In this setup the Pentax Soft is giving 9cm of "depth of 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