Light reflecting off a camera sensor?

D13H4RD · April 19, 2019

I was out shooting with the Fujifilm X-T3 when I noticed that under certain circumstances, especially when a direct strong light source is shining into the camera through the lens (and there is no surrounding light to even out the exposure), rainbow reflections appear around the light source in a very tidy grid pattern, in exactly the same way as you'd see a phone's display reflect when you shine a strong light source on it.

Wanting to know more, I searched for a possible answer, which came out as to potentially be the light simply reflecting from the color filter off the sensor to the lens' rear element, then back to the sensor itself.

Spoiler

The grid of rainbow flare is caused by strong light reflecting off your camera's sensor pixels, forward towards a surface (such as the rear element of your lens, or perhaps the IR filter over your camera's sensor if it is not bonded to the sensor's color filter array and/or microlenses). The light is then bounced back towards your sensor again, but greatly attenuated.

It's the 2D grid nature of your camera's sensor (like all digital cameras) that is causing the regular gridlike pattern.

Note that every surface inside a camera and its lens reflects light. Optical elements are very smooth, and therefore reflect specular (distinct) light. Some elements are coated, specifically to reduce reflections and therefore glare. The coatings don't entirely eliminate reflections, but they to greatly reduce, or attenuate, the reflections. Thus, the other lights in your image actually do reflect off the sensor and back, just like the brightest lights. But reflections from the dimmer lights are so much dimmer than the light they came from, that they aren't visible. It is possible they could be teased out if you cranked the exposure in post processing. But it might also be possible that their reflections were too faint to pick up at the ISO and shutter speed you used.

You can see a similar effect by shining a flashlight directly at a dark LCD monitor. It's a bit difficult to photograph, but very easy to see with your eyes. You will see an interference pattern caused by multiple near-parallel light rays bouncing off the individual pixel elements of your LCD. Some of the rays will constructively interfere, while others will destructively interfere, canceling each other out. Depending on the layout and orientation of the color pixels in your monitor, you probably won't see the same perfect grid pattern.

Source

Given that there is a lot of green dots/squares closest to the light source, in a similar manner to the color filter on an X-Trans sensor, light reflecting off the sensor seems to be a plausible theory. I tried it with a smartphone and a Canon point-and-shoot and they produced similar results, albeit in a different look owing to their different color filters. Strangely however, my Sony a6000 didn't seem to do the same, or at least do so in a similarly obvious manner.

Just wondering if anyone else sees the same.

historicalpoultry · April 20, 2019

There has been many incidents of this ranging from all kind of cameras. I remember there being "issues" on the original A7 from Sony and older x trans sensors as well. I would say that it is in fact most likey the filter stack in front of the sensor. What lens were you using? Maybe using adapted lenses that aren't optimised for the angle of incident of mirrorless cameras can produce a stronger effect. Just guessing here though.

Besides all this: I doubt you will have any issues when shooting under normal conditions. I'm assuming here that you're not really shooting such bright lights at the exact right angle all that often

I bet I could achieve similar results with my x-h1 if I tried. But do I worry about it ever ruining a picture? Not really

Cheers

D13H4RD · April 20, 2019

21 hours ago, historicalpoultry said:

What lens were you using? Maybe using adapted lenses that aren't optimised for the angle of incident of mirrorless cameras can produce a stronger effect. Just guessing here though.

I'm using the Fujinon XF 18-55mm f/2.8-4 R LM OIS. Basically the default kit lens on the X-T3 (and the "premium" option on the lower end models). I might have exhibited a bit of OCD when it came to maintaining it (I think I worry way too much about keeping the front element shiny when I never actually needed to xD) but I feel that the SEBC coating on it should be very robust (first used in broadcast glass), so it shouldn't really be much of an issue.

I actually did check the Fujifilm-X website for photos submitted by photographers with this lens and in similar types of shots (where there is a strong light source shining in the composition window), these artifacts do indeed appear. Also seems to be a thing with most other cameras, like that Canon EOS DSLR in the link.

21 hours ago, historicalpoultry said:

Besides all this: I doubt you will have any issues when shooting under normal conditions. I'm assuming here that you're not really shooting such bright lights at the exact right angle all that often

Haha, you bet.

99% of my shots tend to be street, cityscapes, and a bit of food plus portraits. Very rarely do I shoot with a very strong light source shining through and even then, I would adjust my composition and exposure to minimize flaring and other artifacts. Also worth pointing out that proper focus and exposure makes this artifact wayyyy less obvious. You will still see them if you overexpose a ton in post, but we're not here to find artifacts that most people likely won't even know exists if it's exposed properly, eh?

21 hours ago, historicalpoultry said:

I bet I could achieve similar results with my x-h1 if I tried. But do I worry about it ever ruining a picture? Not really

Pretty much this.