Nikon Nikkor 24mm f/2.8 Ai ~ Lens Turbo II APS-C vs Full Frame

I've begun this latest round of "let's have a look at things" by considering a well-used Nikon Nikkor-UD 20mm f/3.5 pre-Ai objective.

It's now the turn of a Nikon Nikkor 24mm f/2.8 Ai.

 

This is the very optic I complained suffered from field curvature.  At least this was the case when I used it with a Lens Turbo II focal reducer on my APS-C format NEX-5T, A5000, A6000, and NEX-7 cameras.

This blog entry is to confirm or deny that the field curvature is the fault of the lens.

Setup - 

• Sony A7 with straight-thru adapter - ISO100, 2 second timer, in-camera levels used to square the whole plot up
• Sony A6000 with Lens Turbo II focal reducer - ISO100, 2 second timer
  
• Manfrotto tripod - it's capable of securing an 8x10inch view camera, so it's sturdy enough for this
  
• Nikon Nikkor 24mm f/2.8 Ai - shot at two f-stops - f/2.8 and f/8
• Rawtherapee RAW to jpg conversion - Auto-Match function (no "Capture Sharpen")
  

 

Comparison - 

Here is the scene setup.  As you can see, I moved away from using the apartment scrims (curtains) to shooting a copy of le Canard Enchaine' tapped to a flat wall.  I'm looking more carefully at field flatness, right?  Even given the all-too-often shoddy work standards around Paris, this wall is sufficiently flat for my purposes.

 

 

[As always, click on the image and look at it to 100percent file size to see whatever there is to be seen.]

 

Comments - 

Though the A6000 images show less contrast and appear "softer" than the A7 center images, the Nikon Nikkor 24mm f/2.8 Ai looks reasonably sharp in the center wide open and at f/8.  When contrast is matched with the A7, the A6000 24mm images look fairly comparable.

Checking the extreme lower left corner of the scene we can see what I observed years ago.  For me it's the Smoking Gun (as it were, I'm really a peaceful kind of guy).  Right here is why I questioned the focal reduced 24mm Nikkor.  

Even stopped down, the corners at f/8 remain out of focus.  When used with a Lens TurboII the 24mm's field is very strongly curved.  It is so strongly curved, in fact, when the center is focused at infinity the corners of the frame remain out of focus.

On a straight-thru adapter on a Sony A7 the lens is beautifully sharp across a nicely flat field.

The Nikon Nikkor 24mm f/2.8 Ai does not work well when mated to a Lens Turbo II Nikon F to Sony E focal length reducer. 

I observed something very similar with a 35-105mm f/3.5-4.5 AiS that I briefly owned.  Not realizing what was going on, I blamed the extreme field curvature on the cheap little zoom and quickly sold it.  Could it be that I jumped too soon?  Maybe that zoom's field, too, isn't curved under normal circumstances?

Which raises yet another question - does the Lens Turbo II work better with some lenses and not very well with others?

In the meantime, I freely admit that I was rather slow to realize that field curvature I'd observed in an otherwise beautiful Nikon Nikkor 24mm f/2.8 Ai was the fault of the Lens Turbo II focal length reducer and not the Nikkor lens.