Lens coatings / diffraction

[Tony Parsons]

This may well be a silly question - if so, apologies. I have noticed a few images taken with 'vintage' lenses described as 'uncoated'. I have also read of the diffraction effects of stopping lenses down to near their minimum aperture, and I wondered whether uncoated lenses display the same diffraction effects as more modern, coated, lenses ? 

[Dustin McAmera]

I can't see a reason why they wouldn't. Coating only affects what happens at the air/glass surface; coating reduces the fraction of the light that is reflected back from the front surface, and increases what goes through into the lens. Coating on the rear surface does the same job; increases what comes cleanly out of the glass into the air, and reduces what gets reflected back. This reduces flare arising from that reflected light, if it eventually makes its way out to the film.
Diffraction is purely an effect of the sharp edge of the iris: light adjacent to that edge gets bent as it passes it, disrupting the image. Diffraction happens at any aperture setting, but at small apertures, the edge makes up a bigger fraction of the total area of the aperture so it shows more.
As far as I can see, those two effects shouldn't have a bearing on each other; but I bet someone is typing right now to tell me I'm wrong!

However, I guess it may be that if you have an old lens with lots of aberrations and no coating, it may be that you don't spot the diffraction among those other effects.

 

[Tony Parsons]

Thanks, @Dustin McAmera - I anticipated that this would be the answer, but I was just curious.

 

 

[paddler4]

Diffraction is a function of aperture and pixel size. Coatings have nothing to do with it. See this: 

https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

However, a lack of coatings can result in more flare, which also reduces sharpness.

[Tony Parsons]

Thanks for the link. Very interesting information.

[John Seaman]

To some extent, comparing modern coated lenses with old uncoated lenses is problematic. Uncoated lenses were generally of simple construction, typically triplets or Tessar types with only six glass to air surfaces, so that coating made less difference to their results. The 5 element Planar design only really came into its own when coating became available and it was possible to have more and more elements without introducing too much flare and without losing too much light at each glass to air interface. This is just my feeling about the issue, nothing to do with diffraction of course,

[rodeo_joe1]

16 hours ago, John Seaman said:

The 5 element Planar design only really came into its own when coating became available and it was possible to have more and more elements without introducing too much flare and without losing too much light at each glass to air interface.

Not forgetting the Dagor, or Doppel-anastigmat or Orthostigmat, as it was known when not made by Goerz. This design of two back-to-back symmetrical cemented triplets overcame the issue of multiple air/glass surfaces by reducing them to only 4 through cementing.

IMO the uncoated Dagor could compete with a coated Planar in terms of sharpness, flatness of field and coverage, with little loss of contrast to flare. 

The design freedom of having independent surface curvatures and element spacing, allowing better spherical and colour correction, did indeed only come into its own with the introduction of AR coating. 

[Tony Parsons]

My grateful thanks to all for the helpful, informative and indeed entertaining responses. I am always interested in the technical aspects of the construction of equipment.

Tony