This subject gets often mentioned by beginners at Bristol Photography Meetup, so I thought I try to write my own little introduction. I tried to make it as general as possible and camera make and model agnostic. Any polite feedback is welcome.
My Dad used to say that photography is about capturing light. Not enough light captured and your shot ends up too dark, aka as underexposed, too much light and it is overexposed. So how do you control the amount of light which gets inside your camera? Broadly speaking you have two parameters to play with. The light, for any camera I am aware of, has to pass though an adjustable opening aka aperture in the camera body (or strictly speaking the lens attached to the body) . The larger the aperture the more light can get inside the camera body and onto your camera’s sensor, and correspondingly the smaller the aperture the less light gets though. The size of the aperture is usually called an F stop followed by a number, and confusingly the larger the aperture the smaller the number. So F2.8 is a larger aperture than F4.
The other parameter affecting the amount of light that falls on the sensor is how long you let it to flow inside the camera. Most cameras still have a mechanical shutter, ie., some sort of a curtain which covers the sensor from incoming light. When you press the shutter button on your camera that curtain, i.e, shutter is pulled aside to expose the sensor and is then pulled back. If your camera doesn’t have a mechanical shutter, the same functionality is performed electronically by powering on and off the sensor automatically. That duration of time, the length of time the sensor is exposed to the light is what is called shutter speed. The shorter the duration the faster shutter speed is called, conversely, long duration exposures are called slow shutter speed. To add yet another source of confusion is that shutter speed is given in units of time, typically a fraction of a second. And the smaller the amount of time the faster the shutter speed is considered. So for example 1/500 is faster shutter speed than 1/30.
Luckily, modern cameras have automation which helps to insure our photos get the right amount of light while still allowing a fair amount of artistic control. And no, I am not talking about the Full Auto mode.
Now what is the difference between the two methods of controlling the amount of light, commonly called Aperture Priority and Shutter Priority? In Aperture Priority Mode you select the aperture and the camera selects what it thinks the correct exposure should be. In Shutter Priority Mode you select the shutter speed and the camera selects what it thinks should be the correct aperture setting.
Ideally, when taking photos with a hand-held camera, you would in most circumstances want a reasonably fast shutter speed. This is because our hands shake and if we let the camera gather light for 1 whole second rather than say, 1/500th of a second, the resulting image will be quite blurred. The often quoted rule of the thumb is to have the shutter speed faster than the inverse focal length of the lens you are using. For you average zoom lens with focal range 24-105 mm that would correspond to keeping shutter speed above 1/25s at the wide end to 1/125s at the long end. In most circumstances, when shooting most types of subjects outdoors during daytime the sufficient shutter speed for hand held photography would be guaranteed. If you can’t get the sufficiently fast shutter speed with the chosen aperture setting, just open up the aperture. So, from my experience at least, I would say Aperture Priority mode is fine for 90% of my photography.
There are however cases when you explicitly want to control the shutter speed and have to use Shutter Priority mode. For example when you are trying to photograph something that moves very rapidly, like for a example a hummingbird flapping its wings and you want to specifically freeze that motion. You want the fastest shutter speed in this case. Conversely, there are cases when you specifically want to achieve a special look that does require slow shutter speeds. For example that smoky smooth water surface look or car lights trails.
Beware: Inappropriate use of fast shutter speed results in levitating cars. Canon 5D with 24-105 F4 zoom, shutter speed 1/1600s, aperture F7.1, ISO100.
Same setup as above but with slower shutter speed and panning (tracking the subject with your camera) blurs the spinning wheels and the background.
Slow shutter speed on tripod. Canon 5D, aperture F18, shutter speed 2.5s.
That typically calls for shutter speeds of several seconds and a tripod to mount your camera on. There are also some even more special cases when you may need to use slow shutter speed while not only hand holding you camera, but also tracking the moving subject you are trying to photograph. That is sometimes called “Shoot and Pray” method, for success rate is not very high. For most cases you are ok for hand held photography if your shutter speed is above 1/30 sec.
Slow shutter speed with panning (tracking the subject with your camera). Canon 5D MkII with 24-105 F4 zoom, shutter speed 1/50s, aperture F6.3, ISO100.
So how would you use the Aperture Priority mode and why is it there in the first place? Simply speaking, all lenses have a so-called sweet spot, that is an aperture at which they produce the sharpest image. Generally it is somewhere around f7.1 setting. If nothing else matters, and the shutter speed is acceptable this is where you want your aperture to be. Again, generally most lenses perform worse the larger the aperture gets, improve towards the sweet spot and then worsen again as the aperture gets smaller. So why move way from the sweet spot? There are two reasons:
- your shutter speed is getting too slow for hand held shooting and you need to gather more light for properly exposed photograph. Then you need a larger aperture (that is a smaller F stop, say, F4 rather than f7.1)
- you want to control the depth of field. If you are shooting with a compact camera, or a mobile device, unfortunately, you can only do it to a limited degree.
Depth of field is the how far (or near) into the distance from the camera your photo is going to be sharp. If you are photographing a landscape you would typically want everything, from that boat on the lake in front of you to the sun on the horizon to be sharp. You want maximum depth of field. That is achieved by making the aperture smaller (that is increasing the F stop to say F13 or above). On the other hand if you are shooting a portrait you might want to blur the background to isolate your subject from the background clutter, that is decrease the depth of field. For that you need to increase the aperture (that decrease the F stop to F4 or lower). The smaller the F stop the shallower the depth of field would be for any given lens.
Canon 5D MkII with 50 mm F1.8 lens, aperture F2.0, shutter speed 1/2000s, ISO100
Canon 5D MkII with 16-35 mm MkII F2.8 lens, aperture F9.0, shutter speed 1/160s, ISO100
Neither Aperture not Shutter Priority modes are terribly smart however and can’t cope with every shooting situation you can possibly throw at them. Switching between different metering modes (normally the camera would average out what it sees on the whole frame) can help your camera to select the proper exposure, but I find it quicker just to override exposure in one direction or the other. This is usually done with a control wheel somewhere on your camera. For example if you are shooting a portrait with a bright sky in the background (and person’s face is correspondingly in the shade) you might need to tell your camera to use longer exposure than it would normally calculate to use to properly expose the person’s face (and consequently overexpose the sky in the background, but hey, I am not interested in the sky for that particular shot). Or if you are photographing something white, like a dog or a bird, you might need to tell your camera to use shorter exposure to prevent that dog from being over exposed.
There’s a third parameter involved which I haven’t mentioned yet. Though, strictly speaking it doesn’t affect the amount of light that gets in the camera. ISO number instead controls how that light is processed by the camera. The higher ISO the number the more electronic signal created by the light is amplified by the camera’s electronics. Correct me here if this is an inaccurate description. You can select the ISO number on you camera, or in most cameras allow the camera to select it on you behalf. The more the signal created by the light is magnified, the less light your camera needs to gather to properly expose your photograph, so you can select a smaller aperture, or faster shutter speed or both. There’s however no free lunch in this case either. As the ISO number goes up, so picture quality goes down. More so in some cameras than the others. You get less details, more grain and general awfulness. So you would normally want to keep ISO as low as possible and normally only start to increase it as it gets darker and you can’t achieve fast enough shutter speed you need.
There are still some other, less important, topics I haven’t intentionally covered here. I wanted to keep it as short as possible.