Scribbling with light…

Photography for dummies :)

ISO Speed

The ISO in ISO speed stands for International Standards Organization. In analog camera, it measures the senstivity of film to the light. If the film is more sensitive to light, the right exposure can be attained at faster shutter speed, hence it will have a faster ISO speed.

In digital camera, the ISO speed simply refers to the light senstivity of the image sensor. It is essentially the ISO equivalent of the film.

As in the case of aperture and shutter speed, the ISO speeds also follow the factor of two rule. That is if the ISO speed is doubled, say from 100 to 200, the light senstivity of the film or sensor is increased by a factor of two.  The following example shows the change in exposure with change in ISO speed. The aperture and shutter speed was kept constant for all these shots at f/3.5 and 1/20 s. Note the increase in darkness/ decrease in exposure as we change the ISO speed.

Photo 1:

Photo 1: Change in exposure with ISO Speed.

Hence a high ISO speed is recommended when shooting in a low light condition. However, the disadvantage of shooting at high ISO speed is that an increase in noise is observed. The result of this increased noise is seen as grainy texture of the photo. The second example illustrates this point. Observe the back ground and the stem of the flower to observe the increase in grainy texture in the photo.

Photo 2: Change in noise with ISO Speed

Photo 2: Change in noise with ISO Speed

Hence, there one need to consider the trade-off between right exposure and reduced noise while choosing the right ISO speed.

Most photographers (as much as I know) prefer to keep the ISO speed at ‘Auto’, hence reducing the number of parameters to be chosen at spur of moment. At the ‘Auto ISO Speed’ setting, the camera chooses the lowest ISO setting for the given light conditions. However, there are times when you might need to choose the ISO speed manually, hence it’s important to understand this trade -off.

This covers the basic of ISO speed. However, there is one other factor that gets affected by the choice of ISO speed: Depth-of-field. The DOF increases as the ISO speed increases. This is illustrated by Photo 3. Note how the lines on the wall next to the window comes in focus as the ISO speed increases.

Photo 3

Photo 3: Change in depth-of-field with ISO Speed (Image from: Cameratown.com)

Hence there is another trade-off factor that you might want to consider. By increasing the ISO speed, you can get a greater depth-of-field and hence can compromise with noise. This becomes relevant for macro (close-up) and night photography.

Just one more thing before I sign off. As I discussed earlier, the size of image sensor varies from DSLR to DSLR. A full-frame DSLR has sensor of size 36 mm X 24 mm while all the other DSLR have sensor size smaller than that. A camera with a larger sensor, unargubly, have a larger light senstive area and hence gathers more light. So ideally, the noise levels generally decrease with larger digital camera sensor. Also the DSLR with larger sensor have small pixel size compared to DSLR with smaller sensor. Hence the noise in higher resolution becomes less enlarged so we observe less noise in the image. It’s relevance to current discussion, you ask? Well DSLR with larger image sensor can allow a higher ISO speed (and hence higher shutter speed) for same amount of noise.

This basically sums up the ISO speed. Catch you next time.

February 21, 2009 Posted by | Basics | Leave a comment