What is this polygonal artifact with my SEL50f18 and Sony a6000 after extreme shadow boosting in Lightroom?

Question

I recently got a SEL50F18 OSS APS-C Lens for my Sony a6000. As it is my highest aperture lens (f/1.8), I did some experimentation with raw files and tried to push the limits in Lightroom with highlights and shadows. I do know that my Sony a6000 hasn't the biggest sensor, but it's currently my highest dynamic range camera (vs. Nikon D5000). The problem also only occurs with this lens.

I chose this picture, taken with full aperture, as it has the most problem with increasing the shadows. Depending on the dynamic range of the image, the problem isn't as visible.

While I can without problems lower the highlights to -100 in Lightroom to recover awesome details, I can't increase shadows over +10 without revealing this polygonal artifact.

raw unedited exported to jpeg:

raw highlights -100; shadows +100: exported to jpeg

Here is a version with heavily increased brightness to expose more of the shape:

I don't understand this behavior at all. I'd be very happy if someone could explain why this happens when boosting the shadows and how I can prevent it and still have the ability to recover depth data.

Note that this does not occur when focus point is on a near object with indirect lighting.

Answer 1

Pretty much every photograph has lens flare and other reflections from the internals of the camera in it. Most of the time the primary light from the scene is strong enough to mask the flare and other reflections. Sometimes, though, we take a photo that has large areas that are dark enough for the flare and reflections to be visible. This is especially the case when we start pushing the shadows in which those very faint artifacts are hiding.

Here's an image of the Moon and Jupiter.

Here's an image taken the same session with the same lens and camera but exposed roughly 12 stops brighter. It is also cropped less and rotated 90° to show the effect of exposure on flare and reflections.

For a more complete discussion of these two images and how they were taken, please see this answer to Can you photograph the milky way with a full moon out?

Here's another example. The same model lens as the one shown on the left had a flat UV filter on the front of it when the image on the right was taken. The image on the right shows reflections off the ridges in the lens that are then reflected into the lens by the back side of the flat UV filter. (Just one more reason to dump the UV filter and use a lens hood for better protection unless one is truly in an environment where sand, salt water spray, or other small projectiles can strike the front of the lens).

In the case of the examples in the question, it appears that you are seeing reflections of scattered light off baffles in either your camera or your lens. Such baffles do a fairly good job of absorbing scattered internal light. But when you really push the exposure of areas that receive no direct light, eventually you'll see them.

I'd be very interested to know what the texture of the front side of the baffle panel near the rear of the SEL50F18 OSS APS-C lens looks like.