This exercise was again in two parts. The first part was to take similar shots at both normal and high sensitivity. All modern DSLR's have the ability to select the ISO prior to taking a photograph. The second part was to show how the ISO affects the quality of the photograph.
What is ISO I hear you cry?
With film cameras you would have had to change the film roll to adjust your cameras sensitivity to light. With digital cameras it is just a matter of altering a setting on the ISO dial to increase or decrease the cameras sensitivity to light. Its a numerical scale, if your camera is set to ISO 200 and you change to it to 400, it is now twice as sensitive to light as it was at 200.
ISO is only one third of the equation when it comes to taking a well exposed photograph. The other two, which we have spoken about in previous exercises are aperture and shutter speed. These two work in conjunction with your ISO setting to capture a photograph that is the correct brightness.
To demonstrate how ISO can be used, the next photograph was taken at a fixed aperture of f16 with ISO set to 100. Then the same photograph was taken at a higher ISO of 800 , by doing this it allowed the shutter speed to be increased.
This photograph was taken on an overcast day, the shutter speed of 1/30 using the focal length shown and hand holding the camera. This was far too slow to capture these cyclists without motion blur, or camera shake however minimal.
By increasing the ISO to 800 I was able to select a shutter speed of 1/250. As you can clearly see the motion blur has been greatly reduced and the detail on the cyclist's shirt is clearly legible.
I could have set a larger aperture of say f4, and left the ISO set to 100 and obtained the faster shutter speed that way, but I wanted to demonstrate the effect of increasing the ISO. Remember the ISO is just one part of the three things; shutter speed and aperture are the other two that we can adjust to obtain the correctly exposed photograph.
A busy street scene taken at 1/15sec, with the selected shutter speed; any movement shown within the image is blurred.
Again by increasing the ISO to 800, thereby increasing the cameras sensitivity to light eight fold. A shutter speed of 1/90 freezes most of the blur within the image.
By increasing the ISO still further the shutter speed has risen to 1/180sec eliminating any motion blur or camera shake within the image.
With the overcast conditions, coupled with the late afternoon light, has resulted in a shutter speed of 1/10sec. Any movement within the image is blurred.
Similar scene, same lighting conditions. Increasing the ISO, shows an increase in available shutter speed.
Increasing the ISO, like the previous photographs has given a suitable shutter speed to freeze any movement within the image.
So why don't we just use a high ISO all the time when taking photographs? Well the higher you boost the camera's ISO, the more noise you introduce into the image. Noise is akin to film grain, great if you want that grainy, arty look, but bad if you want a sharp landscape or portrait photograph.
This photograph has been been cropped and I have i've zoomed in so that it is possible to see if there is any noise in the darker areas of the image.
As the image was taken with an ISO of 100, apart of camera shake and motion blur, no noise is apparent.
By increasing the ISO I have eliminated any camera shake or motion blur, but the consequence of that is an increase in noise.
This can be seen in this photograph as a speckled like texture over the entire image.
Using the Nikon D4, which is fantastic at high ISO's the noise only became apparent at an ISO of 1250 and above and only when the image was enlarged dramatically. When using my D300 the noise becomes noticeable at ISO 800, but again only when zoomed in.
If you are going to print your photographs as 6"x4" prints, then the noise will not be an issue to worry about, however anything bigger than A4 size then noise will certainly have an effect on the image.
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