on May 08, 2024

In optics, the flatness of glass is crucial because it affects how light is transmitted and reflected, which in turn influences the quality of the images produced.Flat glass


Telephoto and the Influence of Filter Flatness

As the focal length of a telephoto increases, optical precision and flatness are required for any filters or other components attached to the telephoto 

When using a camera with a telephoto lens, especially one over 300mm, the flatness of the filter can significantly impact the image resolution. The lack of flatness in the glass, which can be proved by the existence of Newton's rings, can cause the image to go out of focus, leading to poor results.

The telephoto needs to maintain precise focus for high-magnification and high-resolution observations.  A flatness in filters or other components can prevent the image from going out of focus, degrading the resolution. The longer the focal length, the more critical this becomes.

The optical flatness is an inevitable quality and also proof of the need for high-precision technology to maintain modern photography. 


What are Newton's rings?

When using a camera filter, the presence of Newton's rings in the flatness testing can significantly degrade the actual image quality. Maintaining a high level of flatness without the appearance of Newton's rings is crucial. 

Achieving this level of precision can significantly impact the quality of the product. High flatness is expensive to achieve but is necessary to ensure the product's quality."

Newton's rings are a series of concentric, light, and dark bands that appear when light interferes between two surfaces, typically one flat and one slightly curved surface. 

Isak Newton

Picture from Isaac Newton from Britannica

This phenomenon is named after Sir Isaac Newton, who first studied these interference patterns quantitatively. When a camera filter is not perfectly flat or placed against a lens element that is not ideally flat, the slight curvature difference between the two surfaces can create an air gap. This gap leads to the formation of Newton's rings.

The image below shows a part of production lot samples of Marumi Prime Plasma Sputtering C-P.L, none of which show Newton's rings, compared to company A 's high-end model Circular Polarizer.


Newton's ring example

 Newton's rings appearance example

The presence of Newton's rings can be a significant problem because it degrades the overall image resolution, which is particularly critical when using telescope lenses.   

What can photographers do to mitigate the out-of-focus risk?

To mitigate low resolution and out-of-focus risk, especially using a telescope lens in high-magnification, photographers can take: 
  • Use high-quality filters with better flatness specifications
  • Remove the filter if the filter is not essential to take the image

Indeed, removing filters can be a solution, but we do need a filter to take a particular moment. In that case, selecting a filter with ascertained optical flatness is essential.

    In summary, the flatness of glass surfaces in optical components, including camera filters, is essential for producing high-quality images. Newton's rings are an interference phenomenon that can degrade image quality, and they are particularly problematic when there is a mismatch in the curvature of closely spaced glass surfaces.

    Thank you for your attention!

    Marumi Team