Photo lesson of the day: aperture

The first thing you need to understand about aperture is the logic behind the odd looking numbers that we call stops.

The aperture number is simply the relationship between the diameter of the aperture opening and the focal lenth of the lens. A 50mm lens with an aperture of 1 would have an opening of 50mm. Yes this lens does exist but it's not really affordable. A 50mm lens with an aperture of 2 would have an opening with a diameter of 25mm. You get the idea. So, why the odd numbers? consider a sequence of numbers:

1, 2, 4, 8, 16, 32, 64, 128 etc.

Let's say that represents units of area. A "stop" either doubles or halves the light allowed onto the frame (film or sensor).

Remember basic geometry? if you double the linear size of a shape then its area increases fourfold. If you don't understand the concept then take a simple square post-it note. It has an area of 1 square post-it. Now double the size (linear) and maintain the shape (square) you don't just put another post-it next to the first one. That makes a rectangle. You need to put three more with it to maintain the square. As you can see, the linear dimension doubled from one post-it to two post-its but the area quadrupled. If you triple the linear then you'll have nine times the original. 4x would be 16 square. etc...

If you double the diameter of a circle then you quadruple the area. It's the area of the opening that matters to your exposure value. An aperture stop means double (or half) the area so you need the square root to get the linear comparison.

Now take the square root of that series of numbers:

(just ignore the minor rounding errors)

1 = 1

2 = 1.4

4 = 2

8 = 2.8

16 = 4

32 = 5.6

64 = 8

128 = 11

256 = 16

Do these numbers look familiar to you? The old timers found out that marking the aperture ring of a lens this way would allow you to ignore focal length and reduce your light metering to the two basic functions of shutter speed and aperture.

That's about it. You only need to understand two things:

1. The"f-stop" is a the ratio of focal length to the aperture diameter.

2. A stop is double (or half) the area of the opening.

Any questions? Let me know.