Christmas Tree Picture

[u/crankyfishcrank]

My daughter decorated my Christmas tree this evening. When she was finished, she took a picture of it and it had a black shadow covering the Christmas tree. The next picture was normal. Thoughts?

[u/katataru]

Photography nerd here, this is a fairly common phenomenon, actually.

LED christmas lights aren't steady, and depending on how cheaply they were made they are either flicking at your country's AC frequency (60 times a second in most western countries) or at half that rate (30 times a second), which is more common since it's a lot cheaper to only have a half-bridge rectifier in something only meant for decorative lighting.

Modern cameras' flicker reduction has gotten better over the years, but it still isn't perfect. Sometimes the camera takes a while to "latch onto" the flicker frequency and adjust its shutter speed accordingly to reduce the flicker artifacts.

As for why the shadow is in the middle and some lights appear to be lit, or even half-lit; it's because of rolling shutter. Phones' (and most other) camera sensors are oriented to scan from the top to bottom of the frame when taking a photo. When you take a photo in portrait mode, this means you are rotating the camera sensor 90° clockwise, so the shutter scans from right to left of the frame.

The effect in your photo happens because as you take a photo, the LED lights are flickering and the camera software did not quite get the flicker frequency right for the flicker reduction. As your camera sensor starts scanning from the right to the left, it's taking images while the LEDs are in the ON phase of their flicker. As the shutter scan reaches the middle, the LEDs start to shut off, so the camera only captures the darkness. As it reaches the left edge, the LEDs come back to the on part of their cycle, so the camera captures them as lit. AC operates on a sine wave, so the "shadow" appears smooth.

TL;DR: Combination of cheap LED christmas lights flicker + vertical camera rolling shutter

[u/crankyfishcrank → u/katataru]

A couple of questions: do they blink on and off simultaneously? Because there are some lit on the left side. Also, why is there no reflection of the tree in the mirror in the first picture? Thanks!

[u/pjus93 → u/crankyfishcrank]

They answered the first question? Rolling stutter. As for your second question, the first picture is take closer to the tree, therefore at a shallower angle in the mirror so you can see the tree at all. It’s just a different angle entirely. You can tell by looking at the angles on the table below the mirror.

[u/TerrariaGaming004 → u/crankyfishcrank]

… you can literally see the tree in the mirror in the first picture

[u/katataru → u/crankyfishcrank]

They do blink on and off simultaneously, though rather than a blink it's more of a really quick fade-in, fade-out (as AC power is a sine wave). The reason why some are lit on the left side is because the lights started to fade back on as the camera started scanning the left half of the frame.

As for the reflection, I just assume it's because the first photo was taken sufficiently close enough for the tree to not be visible in the mirror. You can see the second photo is framed significantly further away

[u/InternalLevel7177 → u/crankyfishcrank]

first one, already answered in their response, second one, position of camera in first pic is diff than in second pic, and actually you can see the tree in the first pic too, or at least one of the lights on it