Aperture + Shutter Speed + ISO : Putting the triangle together

Now that we've discussed the three primary elements that make up exposure: aperture, shutter speed, and ISO, it's time to put them together.   I've put together three tables that will help link them to give proper exposure.   For each table we are looking at FULL STOPS just to keep the math fairly simple. Remember, look back at previous posts if need be, how light is affected as we move up or down a stop in each of the elements.  For example, moving aperture value from 4 to 5.6 decreases the amount of light let through to the sensor by 1/2.   Then with ISO, moving from 200 to 400 increases the sensitivity of the sensor by 2x.   With shutter speed, the longer the shutter is left open, the more light (photons) are allowed to hit the sensor.

Let's look at the tables.   In this first table we have a fixed ISO value at 200.   For each row you can assume the amount of light is the same.   Therefore as we increase our aperture our shutter speed has to slow down (remain open longer) in order to maintain the same exposure.   

Let's say we wanted to achieve the fasted shutter speed, given the fixed ISO, we would need to open up our aperture to it's widest point; in this case it's f/4.

In our next table we are going to have a fixed aperture, at f/4.  Once again assume the the amount of light will be the same for each row.   As we decrease our shutter speed we will also be able to decrease our ISO.

In a real world type example, let's say that we wanted to freeze action in a dimly lit room, or outside on a cloudy day.   We would want to have our aperture open to its widest point, at f/4, but then we need to increase our shutter speed to an action freezing speed, at 1/1000.  In this example we would bump our ISO way up to 3200.  

Is the math starting to make sense?   We are doubling values or cutting them in half based on the combination of elements.

In our final table we will shift to a fixed ISO value.   For this example we are using a pretty fast shutter speed at 1/1000 second.  As we mentioned above and in previous posts, this is used to help freeze action.  Once again we are assuming the same amount of light for each row.   In this case, we can shoot with aperture f/4 and ISO 200 to get a good exposure.   But, as we learned before f/4 will give us a shallow depth of field.   What if we still wanted to freeze action, but also wanted a deeper depth of field?   We could close our aperture to f/11 and increase our ISO to 1600 and achieve the same exposure.

I hope this is starting to make sense.  If not, don't worry, it will.   Refer to previous posts to try to solidify what you are learning, but more importantly, try these out on your own camera.    As we move forward I'll give you more things to try and continue to expand on these concepts.   I want to make sure you are comfortable with these fundamental concepts.   Then we can start to get more creative and look at composition and even some basic editing techniques.

To close out this post, I'll share a photo I took recently.  Given I'm up north, we have some pretty cold weather.  I also enjoy macro photography so I took advantage of a recent snowfall to capture individual snowflakes.   Hope you enjoy them.