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Both images show the same artifact. enter image description here

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  • Without a little more context in each of the images it is a bit hard to tell exactly what we are looking at. – Michael C Jun 17 '18 at 17:07
  • It's the rear lens element. I got it in a kit that I purchased used. I missed them when examining the lens sadly. I can upload an uncropped image – Chai Jun 17 '18 at 17:32
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If bubbles are something you're really interested in there's Standards for them:

Hoya Group Optics Division - Internal Quality of Products

ISO 19742:2018(en) - Optics and photonics — Optical materials and components — Test method for bubbles and inclusions in infrared optical materials

Schott Glass - TIE—28 Bubbles and Inclusions in Optical Glass, Technical Information Advanced Optics, Schott, 2016 (.PDF)

TIE-28 - Bubbles and Inclusions in Optical Glass

  1. Generation of Bubbles in Optical Glass

The compositions of glasses are frequently given in form of oxides. In reality however raw materials used for melting may be carbonates or hydrogen-carbonates and others. Melting of such raw materials produces reaction gases forming bubbles in the melt.

These bubbles will be removed with the refining process, which is part of all glass melting. During the refining process the glass temperature is increased. The solubility of the gas components decreases. Bubbles will be formed and grow much more rapidly. The elevated temperature decreases the viscosity of the glass supporting the bubbles to move up and vanish from the melt.

Residual fine bubbles, which do not succeed to leave the melt, will be removed by chemical reactions caused by refining agents. These are elements, which change their valency during cooling down. When such an element searches for a chemical binding partner, it will take the gas of the tiny bubbles and dissolve it back into the glass. The gas atoms still will be present in the glass but not disturbing anymore.

Residual bubbles in optical glass may come from different sources. In most cases they come from non-perfect refining.

Nevertheless a certain amount of gaseous bubbles within the glass melt during the melting process is essential to achieve a good homogenization of the melt. Ascending bubbles lead to an additional convection movement of the glass melt and therefore improve the homogenization process.

To inspect for bubbles this is the recommended setup: place the glass on a black surface and shine the light from the side, unfortunately that involves removing the lens. You can probably get acceptable results leaving the lens in place and using a laser pointer at an angle to see if there's a reflective spot.

Lens inspection

TIE-28 - Bubbles and Inclusions in Optical Glass

  1. Inspection of Bubbles and Inclusions

The inclusion quality will be assessed by visual inspection. To visualize the bubbles and inclusions the following measurement setup is used in general (figure 5). The glass is placed on a black background and illuminated from the side. The glass is viewed from above by looking through it toward the black background. The bubbles and inclusions become visible as bright spots. This arrangement is very sensitive for the quantification of bubbles and inclusions. To determine the sizes either comparison standards or microscopes are used. The evaluation includes all bubbles and inclusions with dimensions ≥ 0.03 mm. Figure 6 shows a typical view using this measurement setup.

It matters where on the lens and which lens of the group whether or not there will be a big problem. Any inclusion anywhere on a lens reduces the contrast in that area, if it is imaged then it might be detectable.

Where it matters

TIE-28 - Bubbles and Inclusions in Optical Glass

  1. The Influence of Bubbles and Inclusions on Optical Application

In theory every part of a lens has the same imaging properties (if one may neglect image aberrations). For example covering a lens completely with only letting the light pass through a tiny spot in the upper corner one can still see the complete image but with less intensity. In figure 4 a) very simplified part of an optical system is displayed. This optical system consists of two lenses. The first lens is positioned at an intermediate image plane with the image of an arrow. The second lens images the arrow to the final image plane. Additionally four light rays are constructed. It can be seen clearly that independent from another the two upper rays and the two lower rays both image the tip of the arrow correctly. The position of the first lens in this optical system is an image plane. The position of the second lens is a pupil.

In figure 4 b) the influence of bubbles and inclusions that are located in lenses near the pupils of an optical system can be seen. They do not disturb the shape of the final image but they reduce the image contrast and intensity because part of the light is scattered. The size of this effect depends on the amount of bubbles and inclusions per area of the glass.

Bubbles and inclusions located in optical elements near the image plane can be harmful, because they might be visible in the final image (depending e.g. on inclusion size and image detector pixel size and sensitivity). This is displayed in figure 4 c). The inclusion within the first lens can be seen in the final image.

In reality lens systems consist of many different lenses and are much more complicated than the displayed example. Therefore the designer has to take into account individually the possible bubbles and inclusions in the optical system for any lens.

It could be a bubble, or it could be a fleck of platinum from the melting tank.

  • I have updated with uncropped images. – Chai Jun 18 '18 at 11:18
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It’s difficult to determine from your posted images. A bubble in the optical glass of a modern lens is uncommon, but not unheard of. Generally such bubbles do not degrade image quality. If the bubble is close to the surface, damage due to a blow increases. Mostly a bubble on the lens is a cosmetic defect. One would not expect a bubble on an expensive lens. If the camera was recently purchased from a reputable source, they will likely exchange it. Otherwise, take some test shots. If they pass muster, let it go, and next time inspect your purchase.

  • Alan, I got it from ebay, unfortunately, even after checking the optics when I got it, I missed this, which is unfortunate. – Chai Jun 18 '18 at 15:15

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