An interesting discovery about polka-dot flares

If you know me personally, you know how much I enjoy shooting with my Leica CL and Leica Elmarit 18mm f/2.8 TL ASPH. It remains one of the cameras I instinctively reach for, especially when I want something small, direct, and uncomplicated. The image that led to this accidental discovery was made with that same camera when I was testing out an unreleased lens.

I have shot directly into the sun many times over the years, especially with this combo and also across different cameras and lenses. It is a situation I am quite familiar with, and one that rarely surprises me anymore. Which is why I was particularly shocked when one of the image I shot from the set turns out to inherit this polka-dot pattern on its flare.

Both photographs below were taken with the same camera and the same lens. The scene was unchanged. The only difference between them was the aperture — one image was shot at f/2.2, the other at f/8. What appeared in the stopped-down image, however, was something I had never encountered before: a series of organised, red circular polka-dot flares scattered across the right side of frame.

Image shot at f/2.2

Image shot at f/8

At first, the effect was confusing. The dots were clearly defined and evenly spaced, and they did not resemble the usual flare patterns I was accustomed to seeing. That led me to my initial suspicion that it could be caused by the poor lens coating which resulted in multiple internal reflections, as the dots does seems to appear at the end of each star point. And it was rather puzzling was the fact that they appeared only in one image. The wider-aperture photograph looked clean, while the f/8 frame carried this unexpected visual signature. Both images were straight out of camera.

After running several more test in the studio, the behaviour began to make sense. When stopping down, the depth of field increases, forming a crispier image. While internal reflections can occur within the lens under strong light source, in this case, I suspect it was the reflections between the rear lens elements and the sensor surface cover glass. Each reflection is actually the cover glass of each individual pixel.

To better understand whether this was specific to the Leica CL, I ran the same test across several other cameras that I have at current. Similar traces of the effect could be observed on both the Canon R5 and the Fujifilm GFX50S, though in both cases it was barely noticeably and way less pronounced. And interestingly, images from the Canon 5DSR came back completely clean. If nothing else, it was a quiet reminder of why the 5DSR still earns its place in my kit.

Despite further testing and reading, I could not find a definitive explanation. Eventually, I reached out to a friend who has better understanding of flare to compare notes. His said was that the culprit was most likely related to on-sensor phase detection, which can introduce this kind of repeating flare pattern. Which technically, this should not have occurred on the Leica CL, as it relies on contrast-based autofocus rather than phase detection.

However, upon researching further the Leica CL uses a Sony sensor derived from the same family found in the A6300 and A6500. While Leica implemented its own processing and autofocus behaviour, parts of the underlying sensor architecture were retained. It is likely that this shared design played a role in allowing the flare pattern to appear, even without active phase-detection autofocus in use. This probably explains why the Canon 5DSR does not have it as it has a separate phase detect autofocus sensor.

What stood out most was that this was not a defect, nor an error in exposure or technique. It was simply the lens and sensor combination responding to a very specific lighting condition. In an era where modern systems are designed to aggressively suppress optical artefacts, this behaviour honestly felt quite annoying but I am thankful that I manage to find out before a serious job. Otherwise, it would have been problematic. Also thankful that Canon cameras handled it pretty well.