I started writing and then split my post into three levels of detail for people who have varying lengths of time they can be bothered reading!
The most basic answer is probably that at night, the camera needs to let in more light to get a decent image formed on your sensor and the easiest way to do that is to hold the shutter open/switch the sensor on for a longer period of time. This is why if you have a point & shoot or a phone cam set to night-mode, you'll find that without the flash you'll get a much blurrier picture. That probably explains your focus and colour bleed, and the graininess is probably because of another trick digital cameras use - amplifying the signal to get a brighter image. Every pixel still gathers as much light as nature intends, but the camera boosts that and the downside is that any random noise gets boosted too.
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There are a couple of probable reasons. If you've got it on automatic, does it tell you which settings it's using as you take a picture? The three to look out for are ISO (how much electrical amplification is applied to each pixel to artificially boost your sensor's sensitivity), shutter speed (written as whatever fraction of a second the shutter opens/sensor is switched on) and aperture (the size of yer hole).
If you're in the dark, setting your ISO to a large number means you'll get brighter photos but you'll sacrifice in that you'll get some grain. Setting your shutter speed to something quite slow will let in more light, but you'll need to sit still. Ideally use a tripod or a table if you're using longer than 1/30th of a second. If you're zoomed in, you'll need to make it even faster because the longer lens means wobbles will be amplified. And finally set your aperture as big as it'll go (smaller numbers = bigger holes). I've included a description below to explain what the numbers on each setting really mean, and how you can change one of them (for instance, use a larger hole) and work out what to do with the others (for instance, compensate by knowing how much faster to make the exposure).
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ISO - usually 100, 200, 400, 800, 1600, possibly 3200. As the numbers suggest, each setting is twice as sensitive as the previous one. 100 is 'not' sensitive while 3200 is 'very'. Low end is useful in daylight, high end in darkness, but since you're amplifying electrical signals you're also amplifying any noise which is why the night-time pictures are grainier. (Geek note: in film, sensitivity was generally speaking increased by putting bigger crystals of light-sensitive chemical onto the film. Light rays had a better chance of hitting them, and when one eventually did, it made a relatively large chunk of film 'change colour'. It's this loss of resolution that makes photos taken on high-ISO film look grainy - and it's not necessarily an aesthetically bad thing!)
Shutter speed - if you have an old manual camera then you'll get increments on a scale which double from one setting to the next, in line with the difference in ISO settings. Digital SLRs usually further divide these into thirds, so you get mid-way points too.
The 'classical' scale is 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500 and so on. (I know, a couple of them don't
exactly double/halve from one to the next, but the numbers are less fiddly this way. Know anything about binary?
)
Aperture - this scale seems complicated but it's really really really not. It's not necessary to do this, but if you're interested, set your camera to a loooong shutter speed (several seconds), press the shutter release then quickly turn the camera round and look down the lens. You'll probably see the blades of the aperture creating a hole, and as the exposure finishes they'll retract back to the default position at the edges. Imagine you measured the diameter of that hole they made. Now, if you were to measure the length of your lens and divide that by the diameter of the aperture, you would end up with a number somewhere on the scale that your camera provides for setting the aperture. The number is written as f/'x' where f is the focal length and x is the diameter of the hole. Thus, a picture taken at f/2 means the diameter of the aperture was half of the 'zoom' length (which is a very big hole) and f/16 means the aperture was a sixteenth of the lens length.
Again, the 'classical' scale is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and so on.
And again, the difference between one position and the next is double/half the amount of light, and digital SLRs will break these down into thirds so you can fine-tune your photo.
Now,
the really useful part about knowing all that is this: remember each position on any scale equals a doubling or halving of the light getting in. This means if you move one position along a scale, to keep your brightness even you'll have to move one position
in the opposite direction along another scale.
Imagine you took a photo of a sprinter for 1/30th of a second at f/5.6 - it might be well lit, but the sprinter is blurred. You want to use a faster shutter speed to freeze them, but you don't know how it should affect your aperture. Well, let's say you move to 1/125th of a second. That's two positions faster, but that also means it'll be two positions darker (geek note: remember each position equals a doubling, so that means you've got a quarter the light coming in). Flip over to your aperture scale and work out how to compensate - you started at f/5.6, so you need to make the hole two positions bigger - f/2.8! You could also have gone two positions on your ISO scale, but then you're messing with graininess and you've only got 5 or 6 possibilities there.
- Code: Select all
More light || 1 sec 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 || Less light
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Big hole || f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 || Wee hole
(I hope that aligned on your screens too)
(Note: the two scales will align at different positions depending on how much light you have. In a dark room, 1/30 may align with f/1.4 to get a decently lit exposure.)
Well done if you got that far! Phew!