A while ago, I posted a detailed article about a very defined pattern of red dots / artifacts that I saw on the Fuji X-series cameras when shooting against the sun. This was the first time I encountered such a problem, so without fully researching the issue and understanding the real cause, I wrongfully blamed the Fuji X-trans system for creating those patterns (my sincere apologies to all the Fuji fans!). A couple of our readers pointed me to some other links on the Internet that show a similar issue on different camera systems from Sony, Panasonic, Olympus and a number of others. The pattern indeed seemed to be quite similar between those and what I saw on Fuji cameras. I then decided to take my Olympus OM-D E-M5 camera for a side-by-side comparison and see if I could reproduce the issue on it as well. Now that I have done enough research to understand the root cause of this problem, I will not only explain the red dot phenomenon in detail, but also show image samples from two different mirrorless systems to illustrate the point.
The red dot patterns can be quite frustrating to see in images. Although this particular phenomenon only happens when the light source is very intense and the lens aperture is small, one would still probably wonder what causes it to happen and how one could minimize or even eliminate it. Before I talk about those things, let me first demonstrate that the red dot flare issue is not related to a particular camera or a lens. When shot in the same conditions, pretty much every modern mirrorless camera will show this and even our DSLRs are potentially prone to the same problems, as discussed below. Take a look at the following image taken by the Fuji X-Pro1 camera and the Fujinon XF 14mm f/2.8 lens at f/22:
The two images are taken 2 minutes apart, with about the same light intensity from the sun. As you can clearly see, the red dot pattern is visible in both shots, at slightly different intensity levels. But it is certainly there. And if I took every other APS-C mirrorless camera, whether it was Canon, Nikon, Sony, Panasonic, Pentax or Samsung, all of them would produce similar results. The effect can look worse at different angles and sun intensity levels, as demonstrated in my original article.
Before I explain why the red dot flare issue happens, let me first explain the conditions when you will be seeing this problem. The red dot issue only occurs when all of the below conditions are met:
- The camera is pointed at a very bright source of light (i.e. the Sun)
- The source of light is very intense
- The lens is stopped down to a small aperture (typically f/11 and smaller, but can be visible at f/8 on smaller systems)
This is not seen at large apertures, as clearly seen in the below image (Fuji X-Pro1 + 16-50mm @ f/3.8):
While the effect can be seen on pretty much any digital camera (yes, including DSLRs), it is highly amplified on mirrorless cameras with short flange distance – see more on this below.
So what causes the red dot flare issue? Basically, this has largely to do with the reflective nature of the sensor surface. As light rays enter the lens, they get squeezed into a very small aperture. At this point, each internal reflection in the form of flare is already part of the image. The light rays reach the sensor and immediately bounce back to the rear element of the lens. In essence, each pixel on the sensor that gets hit with the bright source of light reflects some of the light back to the rear element – that’s what creates the grid pattern. The reason why we see the red dots so large, is because of the flange distance. The small pixels from the sensor become larger when they first reach the rear lens element, then when the light reflects back from the rear element to the sensor, they are even bigger in size!
The reason why the effect is amplified on small mirrorless cameras has to do with the shorter flange distance. All of that back and forth reflection madness is happening because the intensity of reflections is higher at such short distances. If the flange distance is doubled like on DSLRs, those pixel reflections get too big to cause much trouble. This does not, however, mean that DSLRs are immune to this particular issue. If the lens is stopped down enough, DSLRs can produce different patterns too.