[yelvertoft]

Having covered fully manual exposure, exposure compensation and aperture priority, it’s time to round off this set with an article on shutter priority mode.

Ok, you’ve got your first camera that goes beyond “point and shoot”. There’s a lot of buttons and dials on it that you really would like to know about but feel a little intimidated in case you break your new toy or get stuck. Your pictures seem ok when you stick to “Program” mode so you stick with what your comfortable with. Why should you change? Read on……

A lot of users, when they get their first ‘serious’ camera find themselves in the situation described above, there is nothing wrong with using Program mode, but you’re going to find it a lot more difficult to get the picture you see in your mind’s eye if you don’t understand the settings the camera is choosing for you.

Let’s discuss shutter speed priority mode, more commonly referred to simply as shutter priority. This mode of operation is usually marked on the camera’s mode dial as Tv. Tv stands for Time Value.

What is the shutter speed? Put in its simplest form, it’s the amount of time that the film/sensor exposed to the light. All SLR cameras have shutters (or curtains) to block the light from the film or sensor. These shutters are opened for a given amount of time to expose the film/sensor to the light. In many compact digital cameras the sensor is exposed to the light all the time, but the sensor is not in “capture” mode. In this case, the shutter speed is the amount of time the sensor is actively capturing/recording the light falling on it.

Shutter speeds are typically measured in fractions of a second ½, ¼, 1/8, 1/16, 1/32…… etc. down to speeds as fast as 1/8000th of a second on some models. The speed is normally displayed on modern cameras just as 125 instead of 1/125th (for example), so a bigger number means a faster shutter speed. Long exposures, measured in whole seconds, are usually displayed with a “ suffix, so 8 seconds would be shown as 8”, whereas 1/8th of a second would simply be 8.

Why do you need to change the shutter speed? Because it affects many things, most notably, it can have the effect of freezing the movement of the subject (or movement of the camera). A faster shutter speed is capturing a shorter period of time, a slower shutter speed is capturing a longer period of time. Remember, each time the shutter speed is halved, the amount of light getting through to the film/sensor is halved. To compensate for this either the sensitivity (ISO speed) needs to be increased or the aperture needs to be opened up.

So what effect does changing the shutter speed have on the movement of the subject or camera? Any relative movement between the subject and camera whilst the shutter is open will cause some blurring. This is not always a bad thing, this effect can be used to emphasise any movement of the subject. It can tell the viewer that the subject is moving, impart a sense of speed to a moving subject. Likewise, this subject blur can ruin a shot where the intended image needs to be sharp.

Use the shutter speed to vary the effect of freezing a moment in time. A longer shutter speed is often used to blur moving water, giving a smooth flowing effect. Another example may be a picture of a slalom canoeist where the exact opposite effect is desired. A high shutter speed could be used to freeze the movement of the water, enabling the viewer to see the dramatic shapes of the water splashes.

It all depends on the desired effect. Using shutter priority mode allows you to control the sense of movement in the picture you want.

Unless a tripod or other support is used, for 35mm film, the rule of thumb always used to be that a shutter speed of less than 1/focal length of the lens was needed to prevent blur due to camera shake. For example, if a 50mm lens was used with the camera hand-held, it was inadvisable to use a shutter speed of less than 1/50th of a second. This rule is still useful to know, but it should be adapted if used with today’s digital sensors – if your camera has a “crop factor” of 1.5x then the 50mm lens becomes an effective 75mm lens and it is inadvisable to use a shutter speed of less than 1/75th if blur due to camera shake is to be avoided. With practice, there’s a bit of leeway in this rule, and if you are using a lens/camera with image stabilisation then there’s considerable latitude to go slower than the rule suggests.

Right, now having outlined the basic effects of changing the shutter speed, how do you put this into practice? When you switch your camera to Tv mode, you have control over the shutter speed. You can set the shutter to a large number (slow speed), providing less movement “freeze”, or a small number (fast speed), providing more movement “freeze”. The camera will then automatically set the aperture to a value that it considers will give the correct exposure, providing the lens aperture and ISO speed can cope with the shutter speed you have chosen given the amount of light available.

I hope this helps to demystify the Tv mode on your camera and gives a greater understanding of the effect of shutter speed even if you are using Program mode.

Regards,

Duncan.