Chromatic Aberration ~ software intervention Nikkor, Pentax-M, Takumar

Looking at software intervention for correcting chromatic aberration in a new Sigma 24mm f/3.5 i-Contemporary lens and learning just how important (reliant?) lens correction profiles can be to image processing, I turned my attention to three old manual focus lenses to see what might be done for them.  The challenge is, of course, that there are no lens correction profiles (.lcp) files for old lenses.

Fortunately, RawTherapee provides a surprisingly complete set of tools to work with.  Opening this software I see a tool that might be useful for correcting chromatic aberration.

Rawtherapee -> Transform -> Chromatic Aberration Correction 

There are two sliders to work with and I can look at contrasty edges of a subject and use the tool to remove CA color tints.  I can then save the settings as a recipe and recall it later as needed.

Looking around the software a bit further I stumbled upon an automated method.

 Rawtherapee -> RAW -> Chromatic Aberration Correction -> select Auto-correction

This tool automatically detects and removes CA shortly after demosaicing an image.  This tool appears to operate earlier in image processing than the Transform CA tool.  All I need to do is save a single recipe with the RAW CA correction enabled and I no longer have to store individual lens recipes.  This is the tool I used in the following comparison.

Setup ~ 

• Sony A6300 set on a tripod 
• +1EV (because of the strong white back-light) 
• 2 second self timer
• ISO100 focusing on the stick 
• Processed in RawTherapee 

I compared three old manual focus lenses ~

• Nikon Nikkor 24mm f/2.8 Ai
• Pentax-M 28mm f/2.8 
• Takumar 28mm f/3.5 SMC (second version)
  

Voila! a mundane scene of my Rescue Orchid - 

 

 

In RawTherapee and for each of the three lenses I took three processing steps and share the results in the following image.

• Demosaic and color management only as the starting point
• Add RAW -> chromatic aberration correction to the starting point
• Add Capture Sharpen to chromatic aberration, demosaic, and color managed image

 

Comments ~

 

RawTherapee's automated chromatic aberration correction tool works the treat.  It cleans up the CA tints and my impression is that an image becomes sharper in just this one simple step.  Then, with the addition of Capture Sharpen, images from old manual focus lenses can match their modern optical counterparts.

 

The old lenses I used here showed less CA than the Sigma 24mm I compared them against.  These lenses must be lighter on the tool to achieve similar results. Modern versus old?  At this level it doesn't seem to make much difference.  

 

My Nikon Nikkor 24mm f/2.8 Ai produces images that have felt "fat" to me.  I've often wondered about that.  Now I understand.  The "fatness" comes from un-corrected CA.  When the CA is cleaned up I doubt anyone could tell the difference between the old Nikkor and the new Sigma.  Images are "clinically" sharp after a little processing.

 

All hail software intervention.