What is ISO? (part 1)

Hopefully you've had a chance to test out shutter speed and aperture with your own camera and began to see how they work together.   Now I'd like to add the third item that makes up the photography triangle, ISO.

Simply put, ISO is a measurement of how sensitive your camera's sensor is to light.   This exists with film as well.  For those who did shoot film, you probably remember 100 speed, 200 speed, and 400 speed film.   You could, typically through specialty shops, buy 800 or even 1600 speed film.   One of the large advantages of today's digital cameras is that they extend way beyond these values.   And they shoot fairly cleanly at high ISO values.   We'll get to what all this means.

In the last post we talked about stops.  We said that f/4 to f/5.6 is considered one stop.   Well ISO also follows stops.  Most cameras start at either 100 or 200 ISO, then they stop up from there.   Just as with Aperture, there are fractional stops, but for now we'll just look at FULL stops.   Here is a list of full stops:

100  200  400  800  1600  3200  6400  12800  25600  (we'll stop here, but some cameras now go a few stops higher than this.)

Just like aperture, each stop up is twice as sensitive as the previous level.  So ISO 200 is twice as sensitive as ISO 100.   And ISO 1600 is twice as sensitive as ISO 800 and so on.   I bet you can guess what this will mean when we see how this works with aperture or shutter speed... more math.

There is a very important thing to understand when it comes to ISO.   As you increase your ISO, you also introduce more noise.   Noise??  Noise can be similar to film grain.   It is basically imperfections in the image caused by the sensor.  At lower ISO levels, this may be very unnoticeable.  At very high levels it may cause undesirable images.  

Let's take a look at two images.  One was shot at ISO 100 and the other at ISO 3200.  Both were shot in RAW and the only adjustment made was to crop the images so you can see the noise better.  (Make sure you click on the images to see them full size)

82mm  f/4  ISO 100  8 second exposure

82mm  f/4  ISO 3200  1/4 exposure

Pay close attention to the background of the image.   In the first photo at ISO 100 the background is clean without any noticeable grain (noise.)  In the second photo at ISO 3200 the background displays a good amount of grain (noise.)     One thing here is that since this is a DSLR, noise isn't too terrible.   For giggles, I took the same shot with a point and shoot camera.   In this example the ISO is ONLY 400.  

You can clearly see how noisy this image is with all the grain and even false color (green & red blotches.)  You can also see what looks like hot pixels.   This is not uncommon for point and shoot cameras (or smart phones) which use very small sensors.  

Ok, now we know what noise looks like and we know that as you increase ISO the amount of noise will increase.   As you might have known, there is a way to control the noise in your images.   Cameras have built in noise reduction software that will have an impact on your outputted images.   You can also use software on your computer, like Lightroom, to reduce the noise in your photos.   One drawback to noise reduction is that it will 'soften' your image.  You will lose sharpness in your photos.   As you can imaging, the higher ISO, the more noise reduction applied (also longer shots require more noise reduction) and hence you are likely to lose more sharpness.   

In the next post, we'll take a look at how we use ISO along with aperture and/or shutter speed to get the images we want given the lighting conditions we are facing.