Rex914

03-11-2005, 07:04 PM

Introduction

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.

Exposure

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.

F-Stop

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.

Conclusion

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

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.

Exposure

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.

F-Stop

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.

Conclusion

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