Part 1 - The relationship between aperture, shutter speed, and ISO

 ISO, noise, and shutter speed

As ISO increases, so does the amount of noise affecting the image.  
To see the original image, scroll to the bottom of the article.

Images from above but enlarged to show loss of details from high ISOs

ISO is the setting you choose on a camera to alter how sensitive the image sensor is to light.  Low ISO values are generally preferred over high ones because details are retained.  However, the cost of low ISO values is that shutter speeds are also low.  You can see in the top image that, as ISO increases, shutter speed does too.  The first gnome photo has a shutter speed of 1/10th of a second with an ISO of 100.  The problem with this is that often 1/10th of a second is far too slow for most hand-held shots, especially with long lenses or subject motion.  

The solution to low shutter speeds (other than to decrease aperture values) is to increase ISO.  By the third frame, the shutter speed is fast enough to shoot using a wide angle or normal lens without involving stabilization technology, assuming that the subjects are not in motion.  To freeze motion, much higher shutter speeds are required; an ISO setting of 3200 would allow a shutter speed of 1/320th of a second which will be enough to freeze most action.  Notice the cost though; the high ISO value results in less details being preserved.

In this case, the aperture was f/2.8, which is very low given what most lenses are capable of.  Using flash is one way to increase shutter speed, but that is not usually an option for most people or situations.  A second way is to wait until more light is available - again not always an option.  The final option is to increase ISO.

AS ISO increases, the sensor is less capable of capturing details because of something called noise, or more specifically, electronic noise.  Noise is somewhat analogous to grain in high ISO film, although they result because of completely different reasons.  So, what is the best way to get decent shutter speeds without risking too much noise?

The best way is to use low ISO values.  However, there are many other factors besides ISO that creates noise.  The second biggest factor is sensor size and well size and spacing.  It takes four wells to produce one pixel.  Smaller wells and wells spaced closer together are more likely to experience noise than wells with larger dimensions spaced farther apart.  In a nutshell, we can say that the size and density of a sensor affects noise.  Small sensors (think cell phone) produce more noise at any given ISO than large sensors.  Sensors with low pixel density (think full frame 20 mp sensors) produce lower noise than sensors with high pixel density (full frame 50 mp sensors).  A very small sensor capable of producing 200 mp, like some cell phones use, will produce a great deal of noise compared to a large sensor as recently mentioned.

There is a caveat here though.  Cell phones using 50 or even 200 mp will average groups out to end up with a 12 mp image.  By doing this a lot of noise issues can be reduced.  However, notice that cell phones often max out at ISOs of 1600 while DSLRs and mirrorless cameras go much higher.  This is simply because noise becomes prohibitive at the higher values in small sensors.

In general, if you want to get fast shutter speeds with low noise, consider the following:

• use a camera with a large physical sensor (APS-C or full frame).
• use lower ISO values and increase them only to achieve necessary shutter speeds. 
• large pixel counts don't always mean less noise, consider sensors that are physically large but have fewer pixels (full frame 20 mp vs 50 mp).
• Keep your aperture set to its minimum value to achieve the desired results.  This allows for the fastest shutter speed under the circumstances.
• Shoot in brighter settings if possible; low light means higher ISO values to keep shutter speeds high.  Lots of light means you can achieve very high shutter speeds at reasonably low ISOs.
• Newer technology will typically have less noise issuea than older technology, all else being equal.
• Consider buying a specalized lens with a very low minimum aperture; sports photographers often use f/2.8 or f/4 lenses as they allow fasher shutter speeds and/or lower ISOs.  And yes, they are pricey.
• Always shoot at the largest size and least compression (best quality) possible as it allows you to resize an image to your specific needs.  An image will appear to have less noise if you don't enlarge it so much.  Consider shooting RAW images with no compression.

Any combination of these will improve noise issues you may have.  Remember that low noise and ISO does little for you if the shutter speed is so low that your images are blurry and useless.  Find the right balance for the situation.

The original image; the third figure from the bottom was used in my above images.

Thanks for reading.

Eric Svendsen     www.ericspix.com     More of this series to follow.