View Full Version : F-Stop, Shutter Speed, Aperture Explained

03-11-2005, 07:04 PM

The concept of the f-stop is very misunderstood among amateur photographers. Most people have some idea of what it's about, but not too many fully understand what's going on and the why behind it. I hope this short piece dispels some myths and clears this up.


Before we talk about shutter speed and f-stop, we must know exactly what exposure is. Exposure is the amount of light needed to properly expose film. Expose simply means to subject film to light.

The best analogy for exposure that I've read about is the famous bucket of water. When we deal with exposure, 2 key components are at play: shutter speed and f-stop. Imagine that you are in control of a faucet that provides water to this bucket. In doing so, you are in control of two things: how fast the water comes out and how long it takes before you shut the faucet.

Your end goal is being able to fill the bucket just to the top without spilling over (overexposing). There are infinitely many ways of doing this, but you must attain this specific value. For example, you can do this by letting the water come out very fast for a short time, or you can let the water drip slowly over a very long time.

Think of f-stop as being a measure of how fast the water come out. Shutter speed is a measure of how long it takes before you have to shut the water off.

Shutter Speed

As I just said, shutter speed measures how long the actual "exposure" takes. Shutter speed works on a double/halving scale.

2 seconds 1 second 1/2 seconds 1/4 seconds 1/8 seconds 1/15 seconds (to align to a decimal scale) etc.


I've said that f-stop is some measure of how fast the light comes in, but what exactly does f-stop mean?

In reality, f-stop is actually a combination of two factors, diving each other to form a ratio.

F-Stop = (diameter of the) aperture / focal length

You don't need to understand why that is because f-stop is a simple ratio and we compare f-stops on a relative basis rather than absolutely.

As most of you know, we're familiar with these common f-stop values.

1 > 1.4 > 2 > 2.8 > 4.0 > 5.6 > 8 > 11

On this scale, something like f/1 intakes twice the amount of light as f/1.4. These values seem very random don't they? Look closer.

If you are math savvy, you will notice that each value differs by a factor of the square root of two. Multiply them out, and you will see what I mean. So where does the square root of two come from?

Aperture is a physical measure of how "open" a lens is. We all know that a lens' opening can expand and contract, much like how our eyes adapt to brightness and darkness. When it's bright, our pupils contract to let in less light. When it is dark, our pupils expand to gather more light. This is why it hurts when somebody suddenly turns on the lights in a dark room!

A lens is circular. We can measure the amount of light entering through the lens by determining its area. We all remember that the area of a circle is Pi * r^2. To double the area, r simply needs to be multiplied by the square root of 2, because the square root of 2 squared is 2! That's exactly why the values above differ by that seemingly arbitrary number.


I hope this short explanation of these terms has cleared up some confusion over this widely misunderstood topic.

Mr. Peabody
03-11-2005, 07:33 PM
I for one am very glad that you shared that. I learned something from it.

I'm a green newbie attempting to learn how to get from the basic zone buttons to the creative zone buttons on my 20D.

Feel free to post more words of wisdom on other subjects. I will read them.

03-11-2005, 10:24 PM
Great Article!, I think the only thing to clarify is exactly why you would want a smaller aperture. If a small aperture will let in less light, what is the reasoning behind it? In short, the wider open the lense, the more blurred the background and foreground are in relation to the subject being photographed. If you were shooting a portrait with f/1.4 the persons nose will be in focus but their ears will not. This technique can be very useful when taking pictures in busy area's (such as market places). By using ~f/4.0 your subjects will be in focus yet the busy market will just be a blur of colors. Landscape photographers use the oposite of this, usually they will use an aperture greater than f/8.0 so that even the smallest details in the background are sharp. In conclusion, Large apertures (Low f #'s) = less DOF (Depth of Field) Smaller apertuers (Higher f #'s) = more DOF.

03-11-2005, 11:16 PM
Darnit, I knew I was missing something. Thanks for reminding me. I'll add that in tomorrow.

03-12-2005, 07:37 AM

Nice to see the same analogy that I used for years when tutoring customers at the camera store I ran for years. It has always seems the best analogy for expalining exposure. Its clear and everyone understands it easily. Just a few notes if you're going to make revisions:

I prefer to avoid "amount", prefering to use "quantity" for more clarity. "Amount" can be taken for "intensity", confusing the discussion.

I also like to add, to the discussion of the bucket, a mention of ISO as being analogus to larger (slow film) or smaller (fast film) buckets.

"f/stop" should _always_ be written with a lower case "f" and a slash (divide sign, "/") as is appropriate when using fraction notation to write a ratio.

Only after _completing_ the basic exposure analogy should you bring mentions of the other impact of various f/stops (DOF, possible sharpness considerations) and shutter speeds (action stopping, intentional motion blur, .. ). Don't get sidetracked with other info before completing the task at hand, "what is exposure".

03-12-2005, 07:42 AM
Great article Rex! I hope we can all show more of such basic photography concepts because many, many people I've met are confused about these things and end up thinking it is the camera that is inferior.

some guy
03-14-2005, 03:49 PM
I like to add this article that explains the above mentioned aspects of photography.

Photography 101 (http://en.wikibooks.org/wiki/Digital_photography)

It has served me well. Actually it still does. :)

06-25-2006, 08:20 AM
I don't mean to bump such an old thread, but I found a link that is very useful in explaining these relationships. It allows you to change the f/stop, shutter speed, and ISO and see the difference.


I finally understand what it means when we say "stop down" (I understoood aperture before, but now I understand the realtionship)