Why does night photography require the use of manual controls while daytime photography can get by with Auto Mode?
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There’ll be a better answer along in a minute but…
The electronics in cameras work because they expect a lot of light.
The autofocus, for instance, needs to see a line/shape and then make it the best line/shape it can be by adjusting the lens. It needs a good chunk of light to do that.
Night photography can involve leaving the shutter open for 1,2,5 up to 60 seconds or more.
The software in the camera to is built to come to a decision in milliseconds. And by the time it times out (0.1 sec?) then it doesn’t have enough information to decide.
Cameras can have ‘night’ modes to try and counteract these issues…?
The extremely low light levels at night require the camera shutter to be open for a much longer time — sometimes several seconds to minutes. Daytime has hundreds to thousands of times more light, so the shutter is only open for tiny fractions of a second.
Because the shutter is open for so long, cameras require a tripod (or extremely stable mout) and had a manual release cable installed so the vibration caused by pressing the shutter button didn’t cause light trails on the film. The manual release greatly reduced any movement caused by your finger so the camera didn’t shake.
Modern cameras assume the area of interest is the brightspot in the field of view so the iris and shutter are set for those levels. If you are interested in a galaxy or the milkyway “skyglow”, you need to override the automatics and manually tell the camera what you want it to do.
There are different ways of doing night photography and the camera can’t tell which you intend using. Just for starters, it’s so dark that the camera can’t get a reasonable measure of exposure and auto-focus can’t work. About all it can do about that is set the ISO and aperture to capture the most light but it still needs you to manually set the focus and shutter speed. Even then you might want some depth of field and so need to reduce the aperture at the expense of a longer exposure time.
While the camera is making an exposure it isn’t getting any information from the sensor; the reading of the sensor happens at the end of the exposure. Some astrophotography takes many shorter exposures and later adds them together in software but that’s very specialised and the smarts happen outside your camera using computer software.
Your camera guesses from what is in the frame what you intend to take a picture of and in most situations it guesses correctly and exposes for that.
The problem at night is that it is not clear what the goal is.
Do you intend to take a picture of what is close to what is in darkness and what needs to use the flash? Do you intend to make a long-time exposure to the dark scene? How could the camera know the correct answer? If it guess incorrectly you need manual settings.
Even if it is a long time exposed is it the ground, stars or the moon that is what you intend to capture? all of them can require different amounts of exposure.
The result is if your goal is the same as the camera guesses you need to use manual control, there can also exist an appropriate mode for what you capture
Auto mode does not work all the time during the day either. Take a picture a person standing in a door where it is dark with bright light behind them .It the goal that the are just a silhouette you you like to see them? The camera can only guess.
You can often on phones click out what you are the intended subject. In regular cameras, it is done in a slightly different way. So auto mode does not work all the time during the day just most of the time.
If you look at more professional and advanced amateurs you will see more manual control than for the general public. This is because even during the day for example a change in aperture size has an effect on the image like the amount that is in focus. So there is multiple ways of capturing the same scene, non is right or wrong all depending on the situation.
Look for an example at https://cloudfront.slrlounge.com/wp-content/uploads/2017/05/aperture-depth-of-field-example.jpg the same image with different depths of field. I would ay aperture f/2 looks better as a portrait but what if a famous building is in the background and you like to have it in focus too? A camera can’t know that so that auto mode works during the day is a bit of a simplification.
Light meters have traditionally worked by determining “middle grey” which is halfway between pure white and pure black. If you expose a photo in the daytime based on middle grey, you’ll have some areas that are bright and some that are dark. That’s what we expect to see in the daytime.
But at night, we expect that most things will be dark. If we expose for middle grey, most things will look unnaturally bright.
So at night, you have to tell the camera that you expect less light to be present. You can do that with manual controls or with a camera’s night mode.
I’m not in auto mode often so I might be wrong. But I haven’t seen auto mode flat out not working in low light without giving you a reason on the LCD display.
But, a couple of things I can think of:
1. Most cameras has a function which the shutter wouldn’t trigger if it wasn’t able to acquire focus. During night time with everything being darker, it’s more difficult to acquire focus. So if you see it in your viewfinder that the camera tries to get focus but never takes a shot when you press the shutter, that’s the reason.
2. Night time photography is much more about trade off between various settings. You can have the aperture wide open, you can expose for longer, you can increase ISO. Each of those has their own draw backs. What auto mode does is automatically juggle between these settings to acquire a correct exposure. But because they all have their drawbacks, there are usually settings that limit how far they can go, especially shutter speed and ISO. During nighttime, the correct exposure might require shutter speed and/or ISO beyond your set limit, so the camera’s system is literally not allowed the correct exposure because of these settings. You can try to raise those limits in your settings and see if it works in nighttime again. But beware of the drawbacks they present and compensate for them in other ways.
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