The Red Dot Flare Issue

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:

Fuji X-Pro1 Red Dots

And now take a look at an image captured by the Olympus EM5 with the Olympus 12mm f/2.0 lens at f/22:

Olympus OM-D E-M1 Red Dots

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.

Occurrence

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:

  1. The camera is pointed at a very bright source of light (i.e. the Sun)
  2. The source of light is very intense
  3. 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):

No Red Dot

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.

Cause

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.

Comments

  1. 1
    ) Christobella
    November 14, 2013 at 7:00 am

    Interesting stuff. Can we assume that sensor manufacturers are already trying to develop sensors with less reflective surfaces? Is there an inherent problem with trying to do this?

    • November 14, 2013 at 4:54 pm

      I don’t think there is much that can be done to “coat” sensors. Ideally, you want the rear element of the lens to be coated (as Nikon does on some lenses) – that will probably significantly reduce the effect seen above.

  2. 2
    ) craig
    November 14, 2013 at 7:55 am

    Interesting, so how does this work on a sony a77? I’m guessing the space required for the translucent mirror would minimize this, but does the fact that the mirror never flips up mean the light is not reflected from the sensor to the back of the lens? I have not seen this red dot before but don’t recall seeing it on my nex-5n either.

    • November 14, 2013 at 4:56 pm

      Craig, the Sony A77 has a long flange distance, so you should not see it as bad as on mirrorless cameras with short flange distance. I wonder how the new A7 and A7R will look in comparison – hopefully the larger physical aperture size will not cause these red dots to appear in images, but that remains to be seen.

  3. 3
    ) Luc Poirier
    November 14, 2013 at 9:04 am

    Hi Nasim
    Interesting article. Can you tell me when looking directly at the sun through the camera viewfinder when its safe and when its not ? Last year I was working on my Nikon SB900 flash and the flash fired at an angle from my eyes and i could see for over ten minutes the flash tube as red impregnated on my retina even when I went outdoor or closed my eyes. I was quite scared that my retina was damaged but thank god it was not.

    regards
    Luc

    • November 14, 2013 at 4:57 pm

      Luc, it is never safe to look at the sun when using an optical viewfinder – you can severely damage your eyes, especially when using long focal length lenses! It is really no different than using a telescope – don’t be another Galileo!

      From that standpoint, mirrorless cameras have another advantage – they do not burn your eyes :)

  4. 4
    ) HomoSapiensWannaBe
    November 14, 2013 at 10:51 am

    Nasim,
    At Imaging EXPO in Atlanta this past January, I spent some time with a Nikon rep looking at lenses. He told me that Nikon’s Nano coating is usually only on the REAR element, specifically to prevent image degrading reflections between the sensor and rear element. I don’t know if this is true, but in consideration of this article, it makes sense that this critical part of the optical path deserves the best coatings available.

    • November 14, 2013 at 5:11 pm

      That’s very interesting – I had no idea that Nano coating was used only on the rear element. Would be great to find some documentation / more info on this. If the rear element is coated, it would surely reduce the red dot effect!

  5. 9
    ) furulevi
    April 4, 2014 at 10:32 am

    Example video:

  6. 10
    ) _sem_
    July 17, 2014 at 3:12 pm

    Check this video, SGS2: https://www.youtube.com/watch?v=x1pdjLrXAXo – smartphone lenses make huge blobs rather than dots.
    One more thing to mention is that the distinct dots appear because the sensor acts as a diffraction grating.
    I think the dots on DSLRs tend to be very small, but sometimes a + shape of the outline of the diffraction pattern is noticeable (distinct from the sun-star where the number and directions of the rays are related to the aperture edges).

  7. 11
    ) Chris.I.
    July 18, 2014 at 7:37 am

    I have a Canon Mark11 camera when I take photos and my lens is on autofocus I get red dots on the photo but when I switch to manual focus no red dots .
    Coul you please advice how do I get rid of the red dots?
    The same thing happens with all different lenses.
    regards
    Chris

  8. 12
    ) dgatwood
    July 24, 2014 at 3:25 pm

    There’s a menu option in your Canon camera’s menu that lets you show which focus point was in focus when you took the picture. Turn that off. Note that the dots aren’t actually part of the picture. They’re just being displayed by the camera. Also note that this has nothing to do with the subject of this article.

  9. 13
    ) Chris.I.
    July 25, 2014 at 7:33 am

    I turned the af off and more dots thanks

    • 14
      ) Chris.I.
      July 25, 2014 at 7:34 am

      no more dots that’s what I wanted to say

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