Back to The Basics - The F-stop

Tuesday, December 13, 2011

Back to The Basics

(The F-Stop)

    When the DE (Digital Era) finally came into its' own, the biggest loss was knowledge and understanding. The automatic camera in the FE (Film Era) started this decline, and the DE killed it off. You didn't have to understand anything to take a photo. Was this good or bad? From my point of view it was bad. Why? Because you don;t have to think as much, and in the studio, thinking about a "shot" is most important. So for today, and intermittently in the forthcoming future, I'm going to try and help you, the reader understand the basics that seem to have been lost.

    An f-stop is an abbreviation for focal stop, or as it's called in another way - the lens opening. There is a definition correlation (relation) between each f-stop to the other. 

Basically, it's the formula of a circle - Pi R squared. 

Pi is the number found (and it's the same for every circle) is the circumference divided by the diameter.

R represents the radius and squared means multiplying any number by itself.

The exact relationship of each f-stop to the other is the size of the lens opening. Each radius becoming larger or smaller is increased or reduced by 1.4. If you multiply 1.4 x 1.4, you get 2. Why this number? Because 2 as the number allows 2x  more light or 1/2 as much light as the lens opening is changed. Of course, digitally speaking things got a bit more complicated, as you could have an f 19 or f 27 rather the familiar specific numbers of rf-stops below:

1.2

1.4

2

2.4

4

5.6

8

11

16

22

32

45

64

90 

128

Now since a specific lens opening lets in more or less light by a factor of 2 (2x or 1/2), correspondingly shutter speeds are the other numbers that give "the same exposure result".

The symbol  " " "  represents the distance "inch" as well as  unit of time - the second.

16"

8"

4"

2"

1"

1/2"

1/4"

1/8"

1/15"

1/30"

1/60"

1/125"

1/250"

1/500"

1/1000"

1/2000"

1/4000"

1/8000" 

If you are having trouble understanding all of the above, then ignore it.

But here's a simple analogy that might help you understand.

Imagine you have a large bucket, and hoses of different specific diameters. Obviously, if the have a bigger diameter hose, it will take less time to fill the bucket, and if you have a smaller diameter hose it will take longer. 

When we fill our swimming pool in the spring time it takes 48 hours to fill the pool with  a 1/2" diameter hose with a volume of about 60,000 litres (15,000 gallons) running almost constantly. When our swimming pool maintenance person empties the pool in the fall, he has  6" hose attached to a pump. It takes him 20 minutes. I thing that should help you get the idea! By the way, the hose is the f-stop, and the  time is the shutter speed. The 60,00 litres is the exposure on the film.

Now what different f-stops do is change what is called the depth-of-field". This simply means  that as you make the diameter of the lens opening smaller, there will be an increase in the focus from the front to the back of your photo. 

What I've done is take 3 of my new toys (Lesney steamrollers that look very recent), and change the lens opening from large (f 4) to small (f-22).

So without further talk, have a look at the series of exposures and toy images.

f 5.6 @ 1/125"

f 8 @ 1/60"

f 11 @ 1/30"

f 16 @ 1/15"

f 22 @ 1/8"

f 32 @ 1/4"

Notice how the back steamrollers are getting into more and more focus as the lens opening gets smaller.

The back 2 toys are still "unsharp", but in the future, I'll explain to you how to get them into focus.

You used to be able to do that by setting the focus on the depth-of-field markers on the old FE (film Era) lens. The manufacturers "dumbed down" or better still decided to save money by eliminating this. If you have a Nikon camera, form what I understand, you can still use your older "smarter" lens on your digital cameras. Smart move, Nikon!