What is shutter speed? Here’s everything you wanted to know (and maybe more).
The short, and correct, answer is that shutter speed is exposure time. Normally your sensor (or film) sits around in the dark waiting for a bit of action. It is the shutter – like the blinds or curtains which shut light out of your room – which keep the sensor in the dark. When the shutter opens, it lets in light and the sensor gets to work.
Exposure time, then, is the time interval or duration during which your camera’s sensor (or film) is collecting light to capture your image.
Somewhere in the mists of time (anyone help me out here – the Speed Graphic shutter?) the usage started that equated a short exposure time or short duration exposure with speed.
See: short equals quick equals speedy. A long exposure time was not over quickly, so it equals slow. In the days when all cameras had mechanical shutters that flicked open, then closed again, it was reasonable to think a short exposure time meant the shutters had to move more quickly. In actual fact, all shutters in use now always move at the same speed: they just open for more or less time.
And these days, millions of cameras – e.g. those in cells phones, PDAs – do not have shutters. (How are you going to squeeze them in?) So newbies to digital photography don’t need to feel silly if they are confused by talk about shutter speed. Often there’s no shutter, so how can there be shutter speed, let alone slow and fast speed? Hey, but we really have exposure time.
How do shutters work?
There are three types. Two are essentially blinds which physically block out light until we call for the exposure.
The focal plane type is like two curtains over a window. Each is wide enough to cover the whole window. We start off with one fully drawn = darkness. We pull it across to start the exposure, a bit of window is exposed. Then the second curtain starts to move – keeping pace with the first. The gap between them is narrow for a short exposure, wider with longer exposure. Finally, the first curtain is fully drawn but the second curtain now covers the window: back to darkness. This type is used mainly in dSLR type cameras.
The inter-lens type works like the iris of your eye, except that most of the time the iris is closed shut. For exposure the iris opens, then shuts again. And how long it’s open is the exposure time. It’s that simple.
The sensor type works in a completely different way. It is collecting light all the time – that’s how you can see the image on the screen so you focus and frame up. When you make the exposure, the sensor circuits are told to start collecting light, then the end of the exposure a pulse in the circuits shunts the collected light – now an electrical charge – to under a light-proof cover on the sensor. From there, the charge can be read to create the image.
(It’s tough reading all this, we know. If you ask nicely, we might create some flash movies to show how it all works.)
What do exposure times do?
You, or the camera, selects a combination of exposure time and lens aperture to make an image that is correctly exposed. But exposure time does more than that. It is your way to control motion blur.
It works this way: when the shutter is open, if there is any movement in your subject, that movement will register as a smear of the image – a streak – on the image. If there’s a lot of movement, the streak will be longer, perhaps long enough to see. If there’s movement but the exposure time is really short, the streak will be there, but it’ll be so short, we won’t think it’s a blur; it will appear sharp (assuming of course the image is in focus).
Exposure time controls blur, in addition to contributing to correct exposure.
Here’s a pictorial interlude.
- Look first at the 40 shutter setting: the exposure time is 1/40sec – about quarter the length of a blink – yet it is too long to capture the fast-moving water, so it is milky blurred.
- 125 or 1/125sec is a good average short exposure, but it’s not enough to stop the water. We need an exposure much much shorter to ‘freeze’ the movement.
- A 2500 setting or 1/2500sec exposure does the trick: we can see individual streams of water caught sharply.
Why the sequence of settings?
Shutter settings are typically numbers in this kind of sequence: 2000, 1000, 500, 250, 125 – you can see the pattern here: each is half of the previous. Then it goes 60, 30, 15, 8 – kind of half, or nearly half each time. 4, 2, 1 … then 2 again?
What is going on is that for the actual exposure time, you put a 1 over the number. So, 2000 means 1/2000sec: a very short exposure time – while 1 is 1/1 is 1sec, a long exposure time.
The sequence is because one step to the next gives a doubling or halving of exposure (assuming aperture and sensitivity are held). They aren’t quite doubling or halving because some of the numbers would be awkward to mark on cameras (1/7.5? – given them a break). In fact modern cameras give true halving and doubling steps. Which photographers confusingly call ‘stops’: a doubling/halving step is one stop.
What is shutter priority?
In modern auto-exposure (AE) systems, you can let the camera select both the shutter and aperture settings – this is programmed mode. In aperture priority AE you can set the lens aperture by hand, and let the camera figure out the right shutter setting. Or, yup, you worked it by yourself: in shutter priority AE you set the shutter setting, and the camera figures out the right aperture setting for the right exposure.
When to use shutter priority?
As we know, shutter settings control motion blur. So you use shutter priority AE when your priority is control over motion blur.
If you want no blur – maximum sharpness – you set very short exposure times and let the camera figure out a large aperture ( you may need to raise high sensitivity if light’s not too good). I do this when photographing out of moving cars, trains.
If you want blur, set longer exposure times (still on shutter priority) and use a tripod or rely on image stabilization to steady the shot.
- 1/60 – 1/30sec is good for fast-moving action e.g. in a street festival.
- 1/30 – 1/8 sec is cool for people walking in crowds, for blurring water in fountains etc. and is just about hand-holdable.
- 1/4 – 4 sec is a good start for short streaks of car lights at night.
Actually it’s a bit more complicated than I’ve suggested. As you know, if something is coming straight at you, it may not appear to be moving that fast (sadly, a cause of many a road accident). But if it’s going across in front of you, relatively slow movement can look very rapid. And something that’s far away appears to move more slowly than motion that’s nearby.
So it is with capturing motion blur. Look at these fairground shots.
- As we’d expect, the 500 shutter setting gives nice, sharp images – no motion blur here, my dears!
- With the 60 setting, the horse is blurred, but the figure behind is less blurred – it is moving less quickly.
- At the 30 setting, the horse is very blurred, but some of the poles are not as blurred – they are further away and coming towards the camera.
- The 8 setting is most interesting: it is mostly a mush but one pole is amazingly sharp: it must have been travelling straight at the camera for that fraction of a second.
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