Here is a guide about EXPOSURE for 360 camera beginners, novices, or even some intermediate shooters. Learning about exposure is very helpful for your photos and videos, and can let you shoot high quality photos or videos in low light (without an expensive camera), and make creative effects, among other things.
If you’re a beginner, just focus on the big questions with big headings. For novices, more detailed questions are in sub-headings smaller font.
1. What is exposure?
360 cameras, like other cameras, record images by recording light. Exposure means the amount of light recorded in the sensor. Sometimes, the photo or video is too dark (underexposed). When it’s too bright, it’s overexposed. If it has the correct exposure, it is “normally exposed“.
2. Why is exposure important? Can’t you just brighten or darken an image using software?
Yes and no. Yes, many software can let you edit a photo or video to make it darker or brighter. But sensors have a limit. Digital sensors are like little buckets that collect light. Usually sensors measure the light as a number from 0 to 255. When a bucket is full, it cannot collect more light, so the sensor just writes 255 when it is full, regardless of whether it just barely got full or was actually 10,000 times brighter.
The important thing to remember is that if you hit 255, you’ve ‘blown‘ or ‘clipped‘ the highlight. It will be shown simply as pure white with ZERO detail. When there is no detail, darkening that part of the image doesn’t increase the level of detail. At best, it will just change from pure white to pure gray, but still no detail. So when you clip the highlight, the detail is irrecoverable.
What about shadows? Is it possible that the image is so dark that you can’t recover it? Yes in practical terms. What actually happens is that there’s so little light recorded that the sensor cannot distinguish between that little amount of light and just plain electrical noise. When that happens, it is also effectively irrecoverable. Often, manufacturers will prefer to just show this area as totally black, instead of black with technicolor spots from noise. Since the area looks totally blacked, these effectively irrecoverable shadows are called “blocked” shadows.
As technology improves, the electrical noise in sensors decrease, so sensors get better and better at recording ever smaller amounts of light. But blown highlights are still blown highlights.
2.1 What’s channel clipping?
Clipping can also occur on some colors before others. A digital camera sensor has red, green and blue sensors. It’s possible that in some areas the red channel is clipped while the green and blue still have some capacity left to record. For example if you have a bright red object and shine a bright red light on it, there’s so much red that the channel will probably clip while the blue and green sensors are just yawning. When a channel is clipped the color will become inaccurate.
2.2 What is a histogram?
Many non-360 cameras have a histogram, which is a column graph of the number of pixels in each brightness value from 0 to 255. It is especially useful if it is shown in realtime while you are still framing the image. For 360 cameras, a histogram is still a very rare feature as of 2018. Insta360 Pro (reviewed here) is one of the 360 cameras that has a live histogram.
3. When is a photo or video ‘too dark’ or ‘too bright’? What is the correct exposure?
Objectively, the correct exposure is the one that shows a real world object at the same ‘brightness” as in real life. However, you can do intentional overexposure or underexposure for creative reasons, or for example to avoid clipping.
3.1 How does a camera measure exposure?
Believe it or not, a camera cannot actually sense the correct exposure. If you get a totally blank sheet of paper and you get a normal camera (such as your smartphone), and the white sheet fills the entire frame, then the camera will get confused. It doesn’t know if it’s looking at a white sheet of paper in dim light, a gray sheet of paper, or even a dark gray sheet of paper in bright light.
In this regard, a fully spherical 360 camera is less susceptible to getting the wrong exposure because it can evaluate the whole scene around the camera. Even if you put a white sheet of paper in front of one lens, the other lens can probably see something other than white paper, and by looking at the range of highlights and shadows, it can probably figure out that the white sheet of paper is “really” white.
3.2 What do you mean by expose for the highlights, or expose for the shadows, or expose to the right?
Expose for the highlights means to choose an exposure that makes sure the highlight details are not clipped, and then let the exposure for midtones and shadows fall where they may. This is usually the right approach for digital sensors or for shooting with slide film.
Expose to the right is similar to exposing for the highlights but different. Expose to the right (ETTR) means to look at the histogram and adjust the exposure so that it is just shy of clipping. If a scene has low dynamic range, ETTR means you would intentionally overexpose it until it just barely avoids clipping. This lets you record as much light as possible (for higher quality) without blowing the highlights. Then you normalize the exposure in postprocessing.
Expose for the shadows means to choose an exposure that makes sure the shadows are not blocked. This is a technique often used when shooting with negative film, because negative film has the opposite problem from digital sensors. You can easily recover highlights (it is hard to blow the highlights with negative film), but the shadows can look too noisy. So film shooters used to intentionally overexpose.
5. What are the elements of exposure?
There are three things that together determine the exposure: aperture, shutter speed, and ISO. We’ll examine these terms one by one.
The first way to control the amount of light that reaches the sensor is by the size of the hole in the lens through which light enters. That hole is called the aperture, which is measured by an f-number or f-stop, such as f/2.0 or f/2.8. The smaller the f-number, the larger the hole. For consumer 360 cameras, the aperture is generally fixed just like in action cameras and most smartphones. However, for conventional cameras, especially DSLRs, the aperture is usually adjustable. If a DSLR lens has an aperture of f/3.5 that means that at its widest, it can have an aperture of f/3.5, which can adjusted down in several f-stops to f/22. If a lens is a zoom lens, it will often have a different maximum apertures at the wide and and at the telephoto end.
The second way to control exposure is by controlling how long the sensor is exposed to the light. This is called shutter speed because in DSLRs, there is a shutter that opens and then closes. (Technically it should be called shutter duration but no one calls it that.) The higher the shutter speed, the less light you receive and the darker the exposure. Shutter speeds are measured in seconds and generally follow a scale like this: 1 sec., 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. Each doubling of the shutter speed doubles the amount of light that reaches the sensor.
The third way to control exposure is by changing the sensitivity of the sensor, measured by ISO. In the film days, you did this by using different types of film, such as 400 film . For digital sensors, they can change the ISO on-the-fly. ISO is measured usually along a scale like this: 100, 200, 400, 800, 1600, 3200, 6400. The higher the ISO, the greater the sensitivity (but it also has more noise). Each doubling of the ISO doubles the amount of light that reaches the sensor.
5.1 What are ‘stops’ of light?
Each stop is a doubling of the amount of light. So if I increase exposure by 2 stops, it means allowing 4x more light to reach the sensor.
5.2 What are the f-stops and what do they mean exactly? Why does f-stop go up as the aperture decreases?
The size of the aperture can be expressed as a size of a hole, such as 10mm. But the problem is that’s not very useful. You won’t know if 10mm is large or small unless you know how long the focal length of the lens is. If the focal length of the lens is 20mm, a 10mmm aperture is quite big. For a 40mm, a 10mm aperture is not as big.
So instead of expressing the size of the aperture as its actual diameter, it’s more useful to know what fraction of the focal length it is. So the f-number is the denominator of that fraction. For example, if the aperture is 1/2 of the focal length, then the f-number is f/2. If the aperture is 1/4th of the focal length, then its f-number is f/4. So, if you see a lens with aperture of f/3.5, then its aperture is 1/3.5 of the focal length.
Since the f-number is a denominator, then the larger it is, the smaller the fraction, and the smaller the aperture is.
5.3 Why do many lenses have similar aperture numbers? How do photographers remember them all?
If you look at the apertures of lenses, you’ll see that certain apertures occur again and again. For example, it’s much more common to see f/4 than f/4.1 or f/3.9. That’s because there’s a standard f-stop scale, and it’s based on the square root of 2 (which is 1.4 — see 5.3.1 to understand why). So the standard f-stops are f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32, f/45, f/64. Each f-stop is double the amount of light of the next f-stop. So f/1.4 has twice as much light as f/2.0.
You’ll notice that every other number, it gets doubled. The second number from f/1.0 is f/2.0. The second number from f/1.4 is f/2.8 and so on. That can help you remember the f-scale. Another way to remember is that each f-stop is 1.4X higher than the previous f-stop. For example, f/2.8 is 1.4 times the previous f-number which is f/2.
But what about f-stops like f/1.8 or f/2.2 or f/3.5? They’re also part of the standard f-stop scale, because the f-stop scale is usually divided into thirds (or sometimes halves) of a stop. So the first part of the f-stop scale in 1/3 stops is: f/1.0, f/1.1, f/1.2, f/1.4, f/1.6, f/1.8, f/2.0, f/2.2, f/2.4, f/2.8, f/3.2, f/3.5, f/4.0 . So f/3.5 is 1/3 stop brighter than f/4.0.
5.3.1. Why do we use square root of 2 as the base for the f-numbers?
We want a scale where the light will double at each f-stop. If the sequence is f/1, f/2, f/4, f/8, i.e., doubling each number, then actually, each f-stop would be quadruple the amount of light, because it would double both the x-axis and y-axis of the hole. Since we only want a doubling, then each step in the f-scale uses the square root of 2 (instead of 2).
6. If the overall exposure is correct, then does it matter what the aperture, shutter speed, or ISO are? Why not just worry about the total exposure?
An exposure of f/2, 1/60, ISO 100 is the same as f/2, 1/125, ISO 200. In the second exposure, the shutter speed is 1 stop faster (1 stop less light), but the ISO is 1 stop higher (1 stop more sensitive), so it’s a wash. So there are many ways to reach the same exposure in terms of brightness. This is called exposure reciprocity.
Why choose between different equivalent exposures? Because using a different aperture, shutter speed or ISO will have a different effect on your shot. This is sometimes called “creative exposure.”
Using a wider aperture will decrease the depth of field (the range of the scene in focus, often abbreviated DOF). So if you see a portrait with a blurry background, some of them are from using a lens with a wide aperture (although DOF also depends on other factors). For 360 cameras, this is generally irrelevant because the aperture is generally fixed, and the depth of field on 360 cameras is very deep.
Using a faster shutter speed will freeze fast action. A slower shutter speed will have blurred movement, but it can also result in blur from camera shake. The stabilization in stabilized 360 cameras will not reduce blur from camera shake.
7. How do you control exposure? What is auto exposure and what is manual exposure? What are the different exposure modes?
Exposure is based on aperture, shutter speed, and ISO (see #5 above). Manual exposure (M mode) is an exposure mode where you specify all of these variables yourself (except aperture for 360 cameras). Manual exposure is uncommon for consumer 360 cameras.
Auto exposure means the camera chooses the aperture, shutter speed or ISO. All consumer 360 cameras have some type of auto exposure. Although the camera chooses the exposure, even auto exposure can be adjusted to become brighter or darker, by using a setting called exposure compensation, which is measured by stops such as +1.3 or -0.7 (sometimes it will say EV which means exposure value). If you want the image to be brighter, you increase the exposure compensation, and to make it darker, you decrease the exposure compensation.
Program exposure (P mode) is similar to auto exposure but the camera will try to guess the conditions and then choose an exposure algorithm that is optimal for that situation. For example, if it is dark, it will use “night photography mode,” which usually means it will use slow shutter speeds, and might increase the ISO if necessary.
Shutter priority (S mode) means that you specify the shutter speed, then the camera chooses the aperture and ISO to get an equivalent exposure. As with auto, you can also make it brighter or darker with exposure compensation. You use this mode to achieve effects such as light trails (by using a slow shutter), or to freeze fast action (by using a high shutter speed).
ISO priority means you specify the ISO, and the camera chooses the aperture and shutter speed. As with auto, you can also make it brighter or darker with exposure compensation. You use this mode if you know for example that you want to shoot at the base ISO, and you let the camera figure out the shutter speed needed to achieve a normal exposure for the base ISO. ISO priority was popularized by Pentax but is a bit less common for DSLRs although several 360 cameras have it.
Aperture priority (A mode) means you specify the aperture, and the camera chooses the shutter speed and ISO. Once more, you can also make it brighter or darker with exposure compensation. It is a popular mode for amateur DSLR shooters but for 360 cameras with fixed apertures, it is never offered.
PASM means Program mode, Aperture priority, Shutter priority and Manual.
7.1 I used +3 exposure compensation but my image is still too dark. Why?
Exposure compensation is not magic. If your aperture is maxed out, you’re using the highest ISO, and the slowest shutter speed available, and it’s still too dark, exposure compensation will not magically make your photo or video brighter. You’ve simply hit the limit of what your equipment can do.
7.2 Why are lenses with wider apertures called ‘faster’ lenses?
Because the wider the aperture, the higher the shutter speed that you can use while still maintaining the same exposure.
8. Is there a difference in exposure technique between photos and videos?
For videos, the general concepts are the same, with a few differences.
– For most 360 cameras, you have limited exposure control (usually just exposure compensation). An exception is Insta360 One, which has manual exposure for video.
– Unlike a photo, you can’t drag the shutter forever. The slowest shutter speed is limited by the frame rate. For videos with a 30fps frame rate, the slowest shutter speed is 1/30 (in fact, most cameras will only go as slow as 1/60 at 30fps).
– Videographers can use slower shutter speeds to show blur in movement.
8.1 What is shutter angle?
To determine the amount of blur, videographers compare the frame rate with the shutter speed using a measurement called shutter angle. Shutter angle is a measure of how long the shutter speed is, compared to the frame rate, expressed as a fraction of a circle in degrees (as opposed to a percentage or fraction). If the shutter speed is half of the frame rate, then the circle is half full and therefore we would describe that ratio as 180 degrees. This article from RED explains shutter angle more detail, with a comparison of motion blur at different shutter angles. Kandao Obsidian R (hands-on here) is one of the 360 cameras with a setting for shutter angle (although you can change the ‘shutter angle’ on any camera where you can adjust the shutter speed in video mode).
How to Shoot in Low Light
Now that we know about exposure, let’s think about how to shoot in low light. When you shoot in low light, the usual problem is that the image is too noisy or it might be blurry or both. Questions:
Why is the image too noisy? It is probably using high ISO to compensate for the low light.
Why might the image be too blurry? It is probably using a slow shutter speed, again to compensate for low light.
Solution: to avoid noise, use a low ISO (use ISO priority or manual exposure). Since a 360 camera can’t compensate by using a wider aperture, won’t this result in needing an even slower shutter speed? Yes, and that is why you should use a tripod. In addition, you should tell the subject to avoid moving, to keep them sharp.
That is how I shot the low light sample photo here with the Xiaomi Mijia Mi Sphere (which does have manual exposure, ISO priority, and shutter priority as well).