Thursday, September 13, 2018

Nikon Nikkor 24mm f/2.8 Ai ~ a closer look

[UPDATE 29 January 2022 -  I was definitely out of my depth here with this post.  I have since learned that the field curvature I observed is not the fault of the Nikon Nikkor 24mm f/2.8 Ai lens.  The curvature is introduced with the use of the Lens Turbo II focal reducer.  It does not change the fields of all lenses to this degree.  The Nikon lens in question is beautiful and I'm glad I kept it.  You can read my mea culpa in a post I made four years after this one.]
I've wanted to like the Nikon 24mm f/2.8 manual focus lens.  Really, I have.  But every time I compare it against the cheap, small, light, modern Sigma 19mm f/2.8 EX DN E, it loses out.  The corners are soft every single time I try to conduct a comparison.  I had a f/2 version of the 24mm Nikkor that behaved just as poorly.

Having owned these lenses for years I sold the f/2 version out of frustration.  The entire experience has been nearly maddening.  I couldn't imagine how Nikon could've screwed up two versions of the same focal length.

Early one morning I was cogitating on the edges of a dream-like state and something occurred to me.  I suddenly felt I should check for field curvature.  Figure 1 illustrates the effect I'm talking about.

What I thought about was how Nikon might have designed these lenses to have a plane of focus that was equidistant from the lens (IOW, a curved field).  So I quickly set up a test.  The scene is simple.  To place the two bottles near the edge of the frame, I scribed an arc from the lens where all three bottles sat on that arc.

Then I compared three lenses.
  • Nikon Nikkor 24mm f/2.8 Ai at f/4
  • Nikon Nikkor 28mm f/3.5 Ai at f/4
  • Sigma 19mm f/2.8 EX DN E at f/2.8
I thought the 24mm Nikkor would show an arc that described the point of focus for that lens.  After having used the 28mm f/3.5 I thought it would show a point of focus as a straight line.  I knew the Sigma would be a flat field lens.  I've used this one for years and it is my reference lens for resolution and field flatness in this focal length range.

Here is the scene setup.  As you can see, the bottles near the edge of the frame are well forward of the railing.  The railing represents the straight line.  The three bottles were placed on an arc equidistant from the lens, where the center bottle is resting against the railing/straight line.

Scene Setup ~ Nikon 24mm f/2.8 Ai

Here are the results (click on the image and select the full resolution image to look at the details).

Field Curvature Comparison
Indeed, the 24mm f/2.8 Ai Nikkor appears to be designed with a point of focus that is equidistant from the lens across the field.  This, to me, means it is deliberately not a "flat field" lens.
With this lens, however, the portion of the image that is in focus does not "pull" or exhibit sagittal distortions.  The subject is rendered "naturally".  And this may be the very reason why the lens was designed this way in the first place.
Considered in this new understanding, the Nikon Nikkor 24mm f/2.8 is actually a very fine optic.  I just have to keep in mind it's characteristics when using it.
As for the 28mm f/3.5 lens, the edge performance at f/4 is not sufficiently good for me to draw any conclusions about it's design nor field curvature.  I could've stopped the lens down further, but the depth of field might have been too great for me to detect it's point of focus at the edges of the frame.
The control lens, however, is quite outstanding and, though the effect is subtle, the railing is more in focus than the bottles.  This re-confirms for me that it's design is more "flat field" than the Nikkor 24mm.  Not "better" than the Nikkor 24mm, just "different" and more in line with what I expected (before conducting this little test).

Illustration ~ Fig 1
Figure 1

Friday, September 07, 2018

Hyperfocal Distances ~ Nikon Nikkor 24mm f/2.8 Ai

Recently a friend explained to me that he was having trouble focusing a Zeiss 25mm manual focus lens on his Nikon DSLRs.  Since very wide angle lenses give deep depths of field I suggested to him that he set the lens at it's hyperfocal distance, stop the lens down to f/11 and "call it good to go."

He wasn't familiar with the term "hyperfocal."  So I whipped out my favorite depth of field calculator and suggested he put the focus at 6 feet 6 inches, set the aperture to f/11 and that everything from 2 feet to infinity would be in focus.  I asked him to let me know what he thought after he tried it out.

Well, needless to say, he was thrilled and he sent me a couple sample images.

Which gave rise to the question of how this might look in practice.  So I took out a Nikon Nikkor 24mm f/2.8 Ai, setup up a tripod, and photographed a scene with different subjects at different distances to show how it works.

Here is the base scene -

Scene Setup Hyper Focal

And here is a look at the results for the lens focused at the hyperfocal distance, where it is focused at infinity, where it is focused on the foreground, and a look at what happens when a smart sharpen is applied to the hyperfocal image.

Comparison ~ Hyper Focal

What I see is that setting a lens to it's hyperfocal distance does indeed work.  Everything from the orange clothespin in the foreground to the windows at infinity are "acceptably" in focus.  It's a matter of how much of an "airy disk" we can accept before saying something is "out of focus."  Frankly, the detail at infinity isn't all that bad.

Of course, if I took the time to focus on infinity, subjects at that distance were slightly more in focus and the foreground dropped resolution.  When I focused on the foreground the clothespins were slightly better focused and the background dropped resolution.  From the above image you can see for yourselves by how much resolution changes in the various scenarios.

Taking the hyperfocal image and applying a light smart sharpen was rather interesting.  Of course sharpening an image does not add resolution.  It adds contrast to the dark/light transition zones of an image.  That is to say, it adds "apparent" resolution because the human eye sees increased contrast as increased resolution (up to a point).

Walking out into the "real world", here are a couple examples how how setting the Nikkor 24mm at f/11 and the focus at the hyperfocal distance looks in practice.

Passages ~ Paris, France
Passages ~ Paris, France