In our explanation on how to correctly expose a picture, we have previously talked about the importance of aperture (I and II) and the sensor sensibility to light, and how they affect the artistic properties of our pictures. Now it’s time to talk about the shutter speed, the third and final pillar of the exposition process.
The exposition time is the amount of time that our sensor is receiving light. A longer exposition time allows a higher amount of light to enter the sensor. If the exposition time is too long, we will get an overexposed picture where everything will appear brighter and paler than it should, probably losing the details in the highlights. On the contrary, if the exposition is too short, we will get an underexposed picture where everything will appear darker than it should, and losing the details in the darker parts. In order to control the exposition time we change the shutter speed. Slower shutter speeds increase the exposition time while faster shutter speeds decrease the exposition time.
Shutter speeds follow the same principle as the ISO value: duplicating the time also duplicates the amount of light, while halving the time halves the amount of light. Many times the speeds are so small that representing them as a decimal number (for example 0.01 seconds) is not practical. Usually, if they are lower than 0.25 seconds they are represented as a fraction. 0.1 seconds as 1/10, 0.02 seconds as 1/50, etc. This means that if we want to halve the light we need to divide by two the number if we are working with numerators, but we will have to duplicate the number if we are working with denominators:
We have taken a correctly exposed picture of a flowing river at ISO 400, f/4 and 1/200 s. The overall aspect is correct, but water seems static and unnatural. To obtain a better result we want to decrease the shutter speed in order to capture the water motion in a soft way. We reduce ISO from 400 to 100 (2 steps) and aperture to f/8 (2 steps). The total reduction is 4 steps, which means that we have reduced light 16 times (2^4 times; we halved 4 times). In order to compensate this movement we need to increase the shutter speed 4 steps, from 1/200 to 1/13 seconds (1/200 –x2-> 1/100 –x2-> 1/50 –x2-> 1/25 –x2-> 1/13). For a camera configured in thirds mode it means 12 “clicks” (4·3 = 12). Multiplying by 2 a numerator for doubling the light is the same as dividing the denominator by two. If we had moved to the opposite side (faster speeds) we would have divided by two the numerator, which translates to multiplying by 2 the denominators.
Working with the camera is really easy, though, as moving the corresponding dial will always increase or decrease the shutter speed without having to think if we are working with fractions or decimal numbers. In a camera configured in thirds mode, three clicks in the reducing direction will reduce the amount of time to the half whichever the original value was.
The shutter speed also has importance from an artistic point of view. When we are photographing still objects you can use any speed needed, but when the object is in movement some considerations arise: If the object moves fast enough, it will change its position relatively to the sensor while it is open, leaving a trail in the image instead of a clear image. A fast shutter speed will decrease the period of time the sensor is exposed, not giving the object time to move enough, and it will appear frozen in our picture. A fast shutter speed is the option desired when we want to get a sharp picture of a football player kicking the ball or a rally car in the moment it takes the decisive curve. On the other side, a slow shutter speed allows more light but will not freeze the action. We will use slow shutter speeds when we want to record the trails the cars’ lights leave at night or the movement of a person inside a dark room.
The “trail” effect at low shutter speeds can even be more exaggerated. Using a very long exposure time (low shutter speeds, low ISO values and small apertures) we can capture everything that is still all the time. Everything that moves, like cars or people, will reflect so little light to the sensor while they move that they will simply disappear from the picture, as the amount of light received reflected by the moving object is too small compared to the amount of light reflected by the background while the object is not there. Using this you can “empty” places without having to care about people, but only if the place is not very populated. If the place is very populated this technique won’t work, as the place occupied by one person will be soon occupied by another and on average the spot will be more time occupied than free, giving an “average” trail, which also has artistic possibilities.
An important consideration with shutter speed is related with motion-blur, caused by the shake of the photographer’s hand. Usually, at high shutter speeds the exposure time is so fast that the shake doesn’t affect the quality of the picture. But when you go slower than, usually, 1/50 seconds the shake of the hand starts to appear in the picture as a general blur that affects all objects in the scene. It is also worth knowing that this effect also depends on the focal length of the lens used. 1/50 seconds is the lower safe for a focal up to 50 mm, but when the focal increases (for example, with a telephoto) the safe speed increases with the reciprocal of the focal, which is the wordy way to say that for a 100 mm lens the safe speed is 1/100 seconds and for a 300 mm is a 1/300 seconds. Nevertheless, this is only a reference value, as a photographer with good pulse or standing (or using an image stabilized lens) can go a little bit slower without being noticeable in the picture, while other conditions like hard wind can make faster speeds needed in order to correctly take the picture.
Finally, another artistic consideration of the shutter speed is panning. This works on slow shutter speeds in an opposite way as explained before. With a slow shutter speed and a fast moving object what we will usually get is a sharp background and a trail of the trajectory of the object. Panning consists on following the moving object exactly at the same speed that it’s moving (i.e. to keep the object exactly on the same point of the viewfinder while you move the camera following it). When it is correctly achieved we will get the object perfectly on focus, as it hasn’t moved relatively to our sensor while the shutter was open, but the background, which has changed all the time, will appear as a trail, providing the desired sensation of speed while our main object is frozen. This technique is quite complicated for beginners as it requires following the moving object very precisely, and for a bold effect requires very low speeds, which means an steady pulse for a higher amount of time, but when is perfectly dominated provides very interesting and spectacular pictures.
With this is mind, the three pillars of exposition are completed, and now you have all the basic information needed for haunting anything the way you want. Now is time to prove it: go outside and start practicing. Do you have any doubts or remarkable experiences? Share them on the comments!