What is the go with ISO?

Back in the days of rolls of film. The sensitivity of the film to light was measured using ISO (or ASA). The higher the number the more light sensitive the film. So ISO 50 had low sensitivity (good for bright sunny days) with good colour saturation, where as ISO 800 was sensitive to light and could be used in low light situation and leaving the image with a "grainy" look to it.  

Now in the days of digital cameras, CMOS and CCD sensors why do cameras still have an ISO feature and how can I use this to improve my photography?

To answer the first part of the question, the sensitivity (ie number of pixels used) of the sensor can be adjusted to meet the lighting needs. To answer the second part of the question, the lower the ISO number, the less light sensitive the sensor is (ie using less pixels to render the image and record light as colour). So ISO 50 or 100 is far less sensitive than say ISO 800 or ISO 1600.

So far so good. Now as we double the ISO number (ISO 50, 100, 200, 400, 800, ...) the exposure is also doubled. This needs to be taken into account when setting the correct exposure.

As an example, let us  say that the correct exposure when ISO 100 is dialed in is 1/20 sec at f8. Now if  I dial in ISO 200 instead, we could correct this increase in exposure by halving the shutter speed so ISO 200 1/40 sec and f8 will produce the same exposure. This has the advantage of using a faster shutter speed (if needed to reduced camera shake, or to get an action shot). Of course we could have adjusted the exposure by making the aperture smaller, but it is often useful to think of increasing the ISO to speed up the shot (ie use faster shutter speeds) and lowering the ISO number to slow down the shutter speed (longer exposures are needed since less pixels are used).

The advantage of understanding the connection between ISO and shutter speed has lots of useful applications. For example let us say that a scene needs 1/8 sec and f32 at ISO 800 to get the correct exposure but you would like a longer exposure time to blur some motion in the shot, so we could lower the ISO from 800 to ISO 50 which is halving 4 times, so we double the exposure time 4 times (now 2 seconds). The correct camera setting would now be ISO 50, 2 seconds and f32.

On the other hand, during night photography (without a flash) ISO 800, f4 and 30 seconds is a good starting point and adjusting the ISO (and shutter speed) we can become creative in what and how things can be captured.

So have fun playing with ISO,  just a final word of warning. The higher the ISO the more noise is introduced into the shot, which needs to be removed in post editing. I learned a very cool and deadly quick method for removing noise using Photoshop which I am excited to share with you another time.

have fun! :-)


Subscribe via RSS