What's with those numbers? Understanding aperture and shutter speed numbers.

1.4, 1.8, 4.0, 5.6, 25 or 6 to 4*... What's with all those numbers on your camera?

Shutter speed is usually easy to define and understand. The progression is logical. 1 second, 1/2 second, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. OK, 1/15 and 1/125 aren't exact half/double, but close enough to understand. Each step represents either twice the exposure or half the exposure of the previous number. They are fractions. So 1/2 is larger (longer exposure) than 1/4 or 1/8. Pretty easy to follow.

But it does sometimes get confusing when you get down slower/longer speeds around 1 second. When the shutter speed dial says "2" is that 2 seconds or 1/2 second? On Canon cameras 2 seconds is shown as 2" and 1/2 second is shown as 0"5 (5/10 of a second). 1/4 second is shown as 4, but there is also a 0"4 if you are set to 1/3-stop exposure level increments in your camera's custom functions. If set to 1/3-stop increments you will see 1", 0"8, 0"6, 0"5, 0"4, 0"3, and 4. At 1/2-stop increments you will have 1", 0"7, 0"5, and 0"3, and 4.

Whew! I thought that shutter speed was going to be easier than that. And compared to aperture, it is pretty simple. Now let's look at the standard aperture scale:

1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32, 45, 64, 90

Looks kind of simple at first. Every other number is double or half. But unlike shutter speed, 2.0 does not give twice the exposure of 4.0. It gives four times the exposure. And what's with 2.0 being greater than 4.0?

Well, apertures are fractions. The full values are 1/1, 1/1.4, 1/2, 1/2.8, 1/4, etc. So that should help understand that 1/2 is larger than 1/4. But what about those numbers in between? 1.4, 2.8, 5.6, etc? Aperture is a measure of area. And these numbers are based on the square root of two (1.4). Each full stop is 1.4x the previous. 1.0 x 1.4 = 1.4, 1.4x1.4 = 1.96(2.0), 2.0 x 1.4 = 2.8, 2.8 x 1.4 = 3.92 (4.0), 4.0 x 1.4 = 5.6, etc. Each step in the full list (above) is one stop more/less than the next number. so 2.0 is twice as much light as 2.8, which is twice as much light as 4.0, which is twice as much light as 5.6.

But, what happens in the lens when you change these numbers? Back in the good old days of film you could open the back of your camera and look through the lens as you changed the aperture and see what happens. For those who haven't seen what changing aperture does, take a look at this illustration:

Here you can see what happens at f/5.6, f/8, f/22, and f/32. I'm hoping that seeing these various apertures, along with knowing that they are fractions, helps understand that "smaller" numbers (1.4, 2.0, 2.8) are larger aperture values (openings) than "larger" numbers (11, 16, 22).

Once you have a good understanding of the progression of shutter speeds and aperture you are well on the way to being able to use your camera without having to stop and think about the numbers. Eventually they should become second nature and not interfere with the process of photography.

The next step is to learn how these numbers affect your photos. Shutter speed affects your ability to stop motion or introduce motion blur. Aperture controls the depth of field (the area between the nearest and furthest objects in the scene that appear to be in focus). But that is for another blog entry.

Please feel free to send questions.

Thanks!


*25 or 6 to 4 isn't photography related. It refers to a song by the band Chicago that is talking about 3:35 or 3:34 (25 or 26 minutes until 4am) in the morning and not being able to sleep. Sometimes these camera numbers keep people awake worrying about them.