Photo of sprites in a clear dark sky, is this possible?

Question

On Earth Science StackExchange (ES-SE) we recently discussed this question: Photo of sprites in a clear dark sky, how is this possible? A photograph of Red Sprites was captured by David Finlay (see here).

The second image is the same as picture 1, just zoomed in.

While the question on ES-SE focuses on the physics, I'm wondering about the possibility of capturing the phenomenon in a photo. Let me do some explanation up front.

Red Sprites are an optical phenomenon that can occur above thunderstorms (~50 km above ground). There is not a lot known about this phenomenon, however, some sources say that red sprites last up to tens of milliseconds (see e.g. here). So a conservative approach would be to say they last for about 0.1 seconds. Although it is much likely that they have a shorter duration.

I'm not into photography, but from what I understand the exposure time needs to be increased the less light there is. This makes me wonder how or even IF such a photo is possible due to the fact that I expect the exposure time to be much longer than 0.1 seconds.

I have the following questions:

1. What are typical exposure times for light settings as shown in the image? I assume it depends a lot on the equipment used - I'd be happy for estimates including high-end and low-end gear.

2. Given these exposure times, is it possible to capture the Red Sprites in a photo?

3. If long exposure times (with long I mean at least 10x 0.1 seconds) are needed, would the sprite be visible in the image?

For short exposure times (on the same time-scale as the red sprites appearance) the photographer must have been extremely lucky to capture the phenomenon.

Answer 1

Shots I've done of the Magellanic Clouds required an 8 second shutter speed at f/3.5, ISO 3200. It is conceivable that he could have captured these events since he was trying to photograph Lyrid meteor shower which means he used longer shutter speeds (in the range of seconds). Yes, the photographer was extremely lucky since sprites are rarely seen from the ground. The "sunset" is actually a shot of an aurora according to the article cited.

Answer 2

Here you can find example settings for take photo of lighting.

Whether I'm shooting with a cable release or remote control, I usually start by setting the shutter to BULB, the aperture to f/5.6, and ISO to 400. Focus is manual.

So the exposure is few or several seconds. And it's possible to take the photo because the light which come from lighting is much more than light from the sky.

The same is situation with flashlight which have duration of flash around 1/10000 of second. But because of huge energy the scene is illuminated and you see the things illuminated from flashlight, not the ambient light (almost).

Answer 3

Typical settings for a sunset might be something like f/8, ISO 100, 1/250... that is an exposure time of .004s. The shortest typical SS capability is around 1/8000 (.000125s). And the longest is typically 30seconds (or bulb/indefinite).

But the source brightness is just as important as the exposure time. E.g. in that image you can see that the area of the setting sun is approaching overexposure saturation (complete white) due to its' brightness, whereas the left front corner is nearing completely black (underexposure clipping). One could say that source brightness is set by the aperture (% of light allowed to pass) and ISO (light "sensitivity") settings.

So the question is less "how long do sprites last," and more "how bright are sprites?" Basically, if the sprites are visible against the sky around them, then they can probably be photographed.

Answer 4

In short, it depends on how bright they are.

A given pixel on a camera sensor measures the total light it sees over the duration of the exposure — put another way, since "brightness" is a rate, it's measuring the integral of brightness over that time.

For a continuously illuminated subject (the stars, sky, landscape, etc. in your photo) the recorded value will be proportional to the exposure time, while for a briefly-illuminated subject (the sprites in your photo) the recorded value will be proportional to the time of that illumination, irrespective of the exposure time (as long as it's longer than the illumination). Many photographers are familiar with this in the form of "balancing" flash with ambient light: varying the shutter speed increases or decreases the contribution of ambient light, while the contribution of the flash (which is a pulse of 5ms or less) remains the same.

So you're right to think that a long exposure that brings out the stars like this might be long enough to completely overwhelm the sprites — but I think that Steven Kersting basically has the right of it in the last sentence. If the sprites are bright enough to see against the sky with the naked eye, then a camera adjusted to give a decent exposure for the same sky will probably see them too.

Answer 5

It matters not how long the light source lasts, assuming the shutter is open for the entire emission from a very short duration light source.

What only matters is how much total light is emitted from a light source during the total exposure time.

Capturing sprites isn't that much different from capturing lightning. In most cases the camera's shutter is open for a few seconds, during which time any particular bolt of lightning may last anywhere from a few to several hundreds of milliseconds. The total exposure time only affects the brightness of light sources that emit light constantly from the beginning of the exposure to the end, as well as the things those constant light sources illuminate.

Imagine we have two identical cameras side by side set to the same aperture and ISO, but one is set to expose for four seconds and the other is set to expose for one second. If both shutters are actuated at the same time and a lighting strike occurs during that first second, both cameras will show the lightning and the things illuminated by the lightning as equally bright. But the camera that is allowed to expose for four seconds will show things illuminated by constant light sources four times brighter than the other camera. Thus the ratio of things illuminated by the lightning to things illuminated by ambient light will be different.

Now imagine that a second bolt of lighting struck during the third second. The first camera didn't record it because its shutter was already closed. In the image from the second camera we have no way of knowing which lighting bolt struck during the first second and which struck during the third (without comparing it to the other image that captured only the first second).

Having done a bit of astro work, I can tell you that the sky was much darker that it appears to be in that photo. For that many stars to be visible the background illumination of the sky would have needed to have been jet black and totally non-perceptible to human eyes. Yet the background brightness of the sky makes it look like the photo was taken at twilight. What appears to be a sunset in the center is, in reality, the accumulation of light from who knows how many bolts of lightning over the course of the exposure.

Think of the very short duration of the sprites this way. Harold 'Doc' Edgerton at MIT practically invented electronic strobe photography in order to take images of bullets going through things such as apples or cards.

The images were taken in a darkened room so that the camera's shutter could be left open for far longer than the instant 'Doc' wished to record. The way that he captured such a short slice of time far beyond the capabilities of the camera's shutter mechanism was to precisely control an extremely short pulse of light set to "pop" a specific number of milliseconds after a microphone attached to the flash detected the sound of the gun being fired.

It doesn't matter how long the shutter is open before and after the flash, the camera records the same amount of light from the flash because all of that light energy happened while the shutter was open.

Digital and film cameras are alike in this respect. Still images record all of the light that falls on the film or sensor when the shutter is open. The resulting image is the grand total of all of the light captured at any point during the exposure.

It's not unlike a water bucket catching rain. If we only measure the amount of water before emptying the bucket once per day, then we can't really tell if that water fell into the bucket slowly during a drizzle over the previous twenty-four hours or if all of the water fell into the bucket during a five-minute cloudburst. We also can't tell if that cloudburst was five-minutes ago, three hours ago, or sixteen hours ago. All we know is that there is an inch and one-half of water in the bucket when we measured it for the first time in twenty-four hours.

Likewise, a still image of these sprites can't tell us how long they lasted. It can't even tell us if all of them happened simultaneously or if they happened at different times over the duration of the full exposure time. It can only tell us how much light each of them emitted in the camera's direction. What the exposure time can tell us is how bright light sources were that were at constant illumination for the entire exposure (assuming they don't fully saturate the sensor, in which case it's analogous to our water bucket overflowing at some point before we measure and empty it so we then have no way of knowing how much more rain fell than what it took to fully fill our bucket).