Lens Out of Focus Rendition ~ a more personal history

Out of focus rendition behind the point of focus is where the "character" of a lens is.

For years I thought optical resolution was where "magic" could be found in a lens.  It took me a couple decades to learn otherwise.  Sharp lenses aren't hard to make.  Everyone is capable of manufacturing lenses that are "sharp."

I've learned that, for me, it's other optical properties that make a lens interesting and adds "character" to an image.  This is what I'm talking about when I write about the out of focus rendition behind the point of focus.  There are three kinds of out of focus rendition and they are as follows.

• Under-Corrected Spherical Aberration - the out of focus disk of highlights are lighter (show more energy) in the center of the disk than at the edge
• Neutrally-Corrected Spherical Aberration - the out of focus disk of highlights are smooth and evenly illuminated across the disk
• Over-Corrected Spherical Aberration - the out of focus disk of highlights show bright edges and are "hollow" in the center of the disk
  

NOTE: These effects are most often and most easily seen when a lens is shot wide open.

Nikon knows that under-corrected spherical aberration behind the point of focus can produce a "subtle", "delicate", and "beautiful" effect.  From what I see they've been designing lenses to build this into their lenses since at least the end of WWII. For their old manual focus lenses Nikon has a clear understanding of the effect.

Zeiss lenses tend to be designed for neutral spherical aberration corrections.  I have a gorgeous Sony Zeiss 16-70mm f/4 ZA OSS that appears to be been designed this way.  And I have a couple Nikon zoom lenses that behave this way, too.  One is the cheap and under-appreciated E-series 75-150mm f/3.5.  The other is the 100-300mm f/5.6 AiS which is also cheap and under-appreciated.  I find out of focus rendering to be wonderfully smooth.

Old manual focus over-corrected lenses tend to appear "sharper" at the point of focus than under-corrected lenses (where spherical aberration tends to veil an image).  I'm convinced this is why certain manufacturers chose this approach.  I'm thinking of the Zeiss 50mm f/3.5 and f/2.8 Tessar lenses and many of the Canon FL, FD, and FDn designs.  This effect is what people tend to call "soap bubble bokeh."  I don't like it, but I know of photographers who do.

Back when I shot large format film (4x5inch up through 12x20inches) I felt with no real evidence other than "Tribal Widsom" that German made lenses were the "best."  I owned a nice collection of Schneider, Voightlander, and Zeiss lenses.

It was only recently that I read about Nikon's lens design philosophy and how they applied their under-corrected spherical aberration approach to their medium and large format lenses, as well.

I saw "something" in the way a pretty little Nikkor-M 200m f/8 performed, but at the time I couldn't "put my finger" on what it was.  Well, looking at a few of my old negatives I now see it was this out of focus rendition that makes Nikkor optics so special.  It was a real missed opportunity for me to explore what the Nikkor-W series of lenses were capable of. 

How to know how a lens was designed for behind the point of focus rendition?

This is easy.  Very easy, in fact.  Using a digital camera with focus magnification -

  1) Mount a lens on a camera

  2) Find and focus on bright highlights

  3) Magnify a highlight to 15x

  4) Start to slowly turn the focus ring from farther away to closer

  5) Watch the highlights as they go out of focus, note the highlights in one of three following ways:

        a) Bright point in the center of the expanding luminous out of focus disk - this indicates under-corrected spherical aberration behind the point of focus.  Often with old lenses you will see what appears to be a brighter ring around the edges of the out of focus disks.  This is normal.  What's important is to see is that the center is brighter than the surrounding disk area (with the possible exception of the very edges of the disk).

        b) Luminous out of focus disk remains smooth across the field - this indicates a neutrally corrected optic (these tended, until recently, to be rather rare in my experience)

        c) Bright disk edges with hollow center - the out of focus disk looks like a doughnut - this indicates an over-correction leading to "soap bubble bokeh"

I do this when considering a lens I'm not already familiar with and this simple technique works a charm.

Modern mirrorless AF lenses from Sony, Nikon, and Olympus are designed to eliminate as many optical defects as possible.  With the aid of computer ray tracing software and improved manufacturing techniques many new lenses are darned near "perfect."  You can use the technique of verifying the out of focus rendition on current optics, too.