Nikon Nikkor 24mm Ai ~ a reconsideration

Four years ago I wrote a little entry where-in I bemoaned field curvature that I found in a pretty little Nikon Nikkor 24m f/2.8 Ai.  This was "confirmed" in images shot at or near infinity that were distinctively soft in the extreme corners of the frame.

Since then I've thought a lot about that blog entry and the pretty optic in question.  I couldn't believe that Nikon would design a lens with that much field curvature.  Certainly, there had to be some "reasonable" explanation, or so I posited.

Some time between then and now I acquired a very low mileage (ie: <700 click) Sony A7 full frame light grabbing mirrorless monster.  While this is only the Mark I version of a long line of great Sony 24 mpixel cameras it really performs.  The sensor is very quiet (ie: no noise) and is far sharper than any Canon DSLR I ever used (and I used a lot of them).

If I weren't already wedded to the super small very light NEX/A5000/A6000 APS-C series of Sony cameras (which also outperform any Canon DSLR I ever used), I think I'd be sorely tempted to trade it all in on more Sony Full Frame Insanity.

Which leads me back to the Nikkor 24mm.  I finally found time to mount it up on the Sony A7 using a straight-thru adapter and point the lens at a fine-detailed subject.  The fine-detailed subject just happened to be our daylight back-lit apartment gaze curtains.

Setup - 

• Sony A7 - ISO50, 2 second timer, in-camera levels used to square the whole plot up
• 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 from f/2.8 through f/11
• Rawtherapee RAW to jpg conversion - Auto-Match function, but nothing further (ie: NO Capture Sharpening - that will be in a future blog entry)

Comparison - 

Here is the scene setup.  It's just a pair of closed gaze scrims in our apartment.  The details are interestingly small, so therefore useful for this kind of "wee look-see."  The center section, upper left corner, and lower right corners were used in the comparison.

 

 

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

 

 

Comments - 

Considering just the Center images, f/2.8 looks a bit soft.  Is it the lack of contrast?  Or perhaps a bit of spherical aberration?  It's not bad to my eye, but it's certainly softer and less contrasty than at f/4 or anything further south than that.  I'll have more to say about this, too, in a future post.

Looking at the question of field curvature, I feel I learned an important lesson. From f/2.8 I can see that the extreme corners of the frame are sharper than they ever were with the APS-C systems used with a Lens Turbo II focal reducer.  The corners get sharper from there on down through f/11.

So, what did I learn?  I learned that I can't just slap a lens on a focal reducer and expect it behave the same way a 35mm manual focus lens would on Full Frame.  There may be important changes to the way an image is rendered.  It's worth the time to test and evaluate something without drawing any early, hard, fast conclusions.

I need to update that old post to point out that "field curvature" is not necessarily a question of the lens.  In this case, the Lens Turbo II focal reducer induced field curvature when used with the Nikon Nikkor 24mm f/2.8 Ai lens.

Looking Forward to 2022

Even as Omicron is surging wildly out of control around the world I can't help but dream, hope, and wish the following events actually get to see the light of day.

These are all motorsport related photo-opportunities -

Retromobile - 16-20 March

Foire Photo - Chelles - 20 March *ADDED 27 January*

la traversee de Paris - 27 March *ADDED 27 January*

Tour Auto - 25-30 April

Vintage Revival Montlhery - 7-8 May

Rallye des Princesses - 14-19 May

Paris - Rambouillet avec les Teuf-Teuf - 28, 29 May *ADDED 3 March*

Cafe Racer Montlhery - 18-19 June

le Mans Classic - 30 June, 3 July

 

With luck, I will see you there!

 

*UPDATED 27 January* It appears that Europe will be opening up in two weeks.  So all these events and MORE will be on schedule.  It'll be a very busy year after all.

Someone who performs outstanding optical testing...

I've known of the Lensrentals blog for some time, now.  Yet it's only very recently that I came to more fully understand and appreciate what Dr. Roger Cicala has to say. 

I enjoyed reading Dr.Cicala's comments on prime lenses and think his conclusions are interesting.  Of course I'm now considering adding a Sigma 30mm f/1.4 DC Contemporary lenses to my always too long list of optics that I might _have_ to have.

Dr. Cicala's conclusions were based in this article on traditional MTF measurements.  That is, contrast measurements were made along a single 2D line.  No attempts were made to account for optical field curvature in a 3D space.

Then I came across a current article and it really caught my attention.  Roger writes about integrating MTF information in a way that goes beyond simple flat field measurements.  

This is wonderful in the way the information helps one "see" how lenses behave in three dimensions. I think what Dr. Cicala & Co are now doing is ground breaking. I love it!

Additionally, I appreciate is that he clearly states that he is _not_ testing for out of focus rendition.  His tests might at best remotely hint at how out of focus regions can be rendered, but that is an area he is currently leaving to others.

Which leads me to something I read some years ago.  This was something that David Duncan Douglas wrote about when he first encountered Nippon Kogaku lenses.  His story, in general, is very well known.

M.Douglas' colleague made a nicely sharp image and was asked what lenses he was using.  M. Douglas' glass was designed and manufactured by Leitz and Zeiss and he was finding his lenses were rather soft and unremarkable compared with the photo his colleague had made.

On his first visit to Nippon Kogaku (now Nikon) they used a display method where one could clearly see and evaluate the performance of a lens.  A lens projector was the display method.  The way it worked was to have a grid image projected onto a screen through a lens.  As a lenses focus ring was turned a person could judge for themselves not only resolution, but _how an optic transitioned from out of focus into focus and back out of focus_.

There is a standard, albeit subjective, method for considering out of focus rendition of optics as well as "sharpness". Lens projectors are still used today for evaluating lenses used in cinema.  In fact, at least one testing lab has added out of focus rendition information to their web pages. 

Coming back to Dr. Cicala's lens tesing, I wish I had access to the kinds of equipment he uses.  Lens Rentals looks at the latest generation lenses.  I'd like to see how the old manual focus Nikon Nikkors behave.  There would be a lot for me to learn.

Both methods (MTF in 2D/3D and grid image projection) do not take into account the image recording materials.  These methods consider only the lenses.  I will leave the pros and cons of this for perhaps another time.  

Until then, and only if you're really interested in the subject and haven't already encountered Lens Rentals blog, consider what Dr. Cicala has written.