Learning about shutter speed (Part 2)

On to part 2...

So from part 1 we learned that fast shutter speeds are good for stopping movement (action) whereas slow shutter speeds help bring in more light and can help create dreamy water (amongst other things.)  That certainly sounds pretty easy and straight forward.   Conceptually, it is pretty easy, but unfortunately reality adds some challenges.   Remember we have 3 things to worry about: Aperture, Shutter Speed, and ISO (sensitivity.)    These three things have to work together in order to get the right exposure, while also accomplishing your other objectives... like freezing action or creating dreamy water.

For now, since we haven't started to talk about ISO, we'll assume we have a fixed ISO.   Let's start with the basics (and a little math.)   Later we'll add some of the challenges which sometimes are out of our control.

Recall that aperture values start with small numbers, like f/2.8, which represents a large opening and goes up to large numbers, like f/22, which represent very small openings.    Whereas with shutter speed it is measured directly in seconds; 1/1000 is 1/1000 of a second and 20 seconds is, you guessed it 20 seconds.

These two settings work very closely together.   Let's assume that at a fixed ISO and an aperture of f/4 we have a perfect exposure with a shutter speed of 1/500.  What if we wanted a shallower depth of field?   To do that we would open up our aperture to f/2.8.   Remember that f/2.8 allows 2x as much light in as does f/4.   Here comes the math.   Since our shutter speed of 1/500 gave us a perfect exposure... what do we need to change our shutter speed to at this new aperture?

The answer: 1/1000       If you're scratching your head a little, it's completely ok.  This can take a bit to make sense.

Let's dissect this a little further.   ISO is consistent so we don't care about this right now.
-- f/2.8 allows twice (2x) as much light to pass through the lens as f/4.
-- 1/500 allows the sensor to be exposed for 1/500 of a second.
-- Since we are now letting 2x more light through we need to reduce the time that light is hitting the sensor.   So to combat the 2x more light we need 1/2 the shutter speed or in this case 1/1000.

Now, to hopefully help pull this together, let's change the problem.   We'll start with the same settings:
aperture fixed ISO, aperture f/4, shutter speed of 1/500.   What if we want to increase the depth of field?   This time we need to close down the aperture to f/5.6 (to keep the math easier.)   Since the aperture of f/5.6 will only allow 1/2 the light in that f/4 does, we will need to adjust the shutter speed again.   Since less light is coming through the lens (due to the aperture) we need to keep the shutter open longer.  So to combat the decrease in light passage by 1/2 we need to also slow down the shutter to keep it open 2x as long.   In this case 1/250 of a second.

Dissecting the problem.   Still an unchanged ISO is begin used.
-- f/5.6 allows 1/2 as much light to pass through to the sensor as f/4
-- 1/500 allows the sensor to be exposed for 1/500 of a second
-- Since we are letting in 1/2 the light, we need the shutter to be open 2x as long for the correct exposure... 1/500 x 2 = 1/250

Good grief... nothing like a wordy blog post.

200mm  f/4  ISO 200  1/2000 exposure

I had to throw in a picture.. The Great Roger Federer!  Obviously needed a fast shutter speed to capture this action.

Happy New Year!!!