Shutter Speed and ISO

Over the last number of weeks, DigiKnow has looked at photography as a means of Making Images. This has covered a number of things including:

• Composition in automatic mode
• Program mode (includes exposure compensation & white balance)
• Depth of field
• Using apertures in photography
• Aperture/shutter relationships in semi-automatic modes

This weeks post continues with Shutter Speed and ISO. This post won’t go in to all the technicalities of which shutter speeds or ISO to use in different situations.

Shutter Speed

Shutter speed is a control that allows you, as a photographer, to manipulate movement within your images. Shutter speed is how long it takes for the shutter curtain to open and close whilst letting light in to the camera and capturing the subject.

Shutter priority mode (S or Tv) on your camera is a semi-automatic mode, i.e., you set the shutter and the camera sets the F-number as per the lighting condition. Fast shutters freeze movement. Slow shutters show movement.

Where might fast shutters be used?

Photographers may want to freeze the movement of sports, action, portraits and documentary shots (i.e., war / weddings, etc.). The faster a subject moves, faster shutter speeds are also needed to be used.

For example, someone walking at an average pace, 1/250 shutter speed may be fast enough to freeze their movement but this will not freeze the movement of a running horse or racing motorbikes.

Running horses may need shutter speeds of 1/1000 or faster. Racing motorbikes, faster still. Needless to say, the larger number at the bottom of the fraction (the denominator), the faster the shutter speed (and vice versa).

The first question is ‘Do you intend detail to be crisp?’

If the answer is yes, use faster shutter speeds to ensure subjects within the frame are not moving.

Consider the subject. Is it moving? How fast is it moving and is it close or faraway? Someone running past you will seem much faster than someone viewed running at a distance. These are some things you might want to consider before deciding on settings.

When we talk about the shutter speed 1/250, it’s one 250th part of a second, i.e., one second divided by 250. Most cameras can go as fast as 1/4000 with newer models 1/8000.

Using too fast a shutter speed in the wrong lighting condition will lead to underexposure (a dark image).

Where might slow shutters be used?

In many instances, landscape photographers use slow shutter speeds to capture the movement of water in a scene (as seen below) or the movement of the stars at night around Polaris (North star).

Other photographers might paint with light or want to give a sense of movement in their images by showing the movement, such as the dancer shown earlier. Here, the camera stays static whilst a subject moves through the scene.

The question is ‘Do you intend movement to be evident?’

If the answer is yes, slow shutters are needed which requires some additional equipment also. A tripod is a must. Filters may be required along with a remote shutter release (although self-timer will suffice).

Slow shutters can be 1/30 of a second right up to 30″ (30 whole seconds). The “ is important in-camera as this denotes a whole second of time as a shutter speed.

Using slow shutters in bright conditions results in overexposure (a really bright image).

Task 1

In this task, use running water from a tap into a glass to see the difference of fast and slow shutter speeds with water.

Choose Shutter Priority on the mode dial (S or Tv).

Dial in 1/500 (note: some shutter speeds may not show as a fraction).

Photograph the water coming from the tap in to the glass.

This picture above is detailed which indicates a fast shutter speed. If you achieved anything like this where you can see water droplets, well done.

Task 2

Still using running water and a glass but also using a tripod. Set the camera to a one second exposure, it’ll be 1″ on the dial and take the same shot.

A slow shutter speed would make the path of the water smooth and the water ‘milky’ looking. You wouldn’t see water droplets. However, everything else static in the scene would continue to be static, i.e., tap / glass, etc.

Using slow shutters in bright conditions can lead to overexposure, whilst fast shutters in dim conditions can lead to underexposure. This can be correct partly through using ISO and learning how to shoot manually.

What is ISO?

ISO relates to how sensitive the film was to capturing the scene, i.e., how quickly the scene ‘imprinted’ on the negative or how quickly the film reacts to light. ISO is still used in digital terms today and the ‘film’ is replaced by a digital sensor.

ISO has a range of numbers: 100, 200, 400, 800, 1600, 3200, etc., (other numbers are available).

Smaller numbers have finer grain and are used in better lighting conditions (i.e., the film is less light sensitive). Whereas, duller lighting conditions requires ISO numbers to be increased (thus making the sensor more sensitive to light) to allow for better exposures and faster shutter speeds. However, this also adds more grain to the image.

Higher ISO allows faster shutter speeds in poorer lighting conditions. Lower ISO will require shutter speeds to be slower, however, if using a tripod and if showing movement isn’t important, lower ISO is more favourable.

A rule of thumb is to use ISO 400, this is considered a daytime ISO. This is a fairly low grain option and should allow for photography in the majority of lighting conditions through the day. At night, higher ISO may be required but this really depends on what is being photographed and the outcome to be achieved, i.e., star trails, etc.

Summary

In summary, shutter speeds control whether movement is frozen (crisp) or shown within the scene. The faster a subject moves, faster shutter speeds are required if the intention is to freeze movement.

Slow shutters can be used for creative merit and in landscape photography. Using slow shutters requires the use of a tripod and some other accessories but it depends on the situation and intention of the photographer.

Even using the Shutter Priority as a semi-automatic mode, overexposure and underexposure can still be achieved by using too slow or fast a shutter speed in the wrong lighting conditions.

ISO is how sensitive the film/sensor is to light (regardless of the length of shutter speed) and how quickly the scene imprints to the film/sensor.

ISO is variable but a good rule of thumb is ISO 400. The higher the number, more grain develops in the image which probably isn’t a big issue but it can make images slightly soft and noisy.

Next week…

DigiKnow will continue with shutter speeds and show some techniques of using slow shutters for creative photography. This will be over the next three weeks.

We look forward to you joining us next Monday for the next post.