By John Siskin
There is no other subject in teaching photography that is at the same time so basic and important as understanding aperture and at the same time so confusing and hard to explain. I want to start with a simple comparison to shutter speed: as the shutter speed number increases the amount of light let in by the shutter decreases. This is because the number is a fraction and we are talking about the bottom number of the fraction, called the denominator. The number4 we use to discuss the aperture is also the bottom number of a fraction and as this number gets bigger the amount of light transmitted by the lens gets smaller. So for instance f16 lets less light reach the sensor than f8. 2 stops less light, but we will get to that soon.
The aperture number is actually the focal length of the lens divided by the width of the lens. So if you have a simple one-element lens that is one inch wide and has a focal length of 8 inches the lens aperture would be f8. If the lens had a one inch diameter and had a focal length of 4 inches that the aperture would be f4. Please don’t make me do this in metric, the relationships work but the math is more annoying. That wasn’t so bad was it? Here’s where it gets tricky, if you want to decrease the aperture of the lens by one stop you have to cut the area of lens in half. If you halve the diameter (distance across the lens) you will decrease the area of the circle by four times or two stops. Instead of doubling the diameter of the lens you have to multiply by the √2 (square root of two) that is approximately 1.4. This is similar to converting a focal length from your sensor size to what it would be in full frame 35mm. So 8X1.4=11.2, which we refer to as f11. If you want to increase the amount of light reaching the sensor by one stop you need to divide by 1.4, remember the number must be smaller to let in more light. So 8÷1.4=5.7, which we refer to as 5.6, in this case it really is closer to f5.6.
Please forgive me for all of this, but I didn’t make it up. As you may find it difficult to multiply by 1.4 in your head you may want some help on how to manipulate and use these numbers. There are a number of important applications that we will get to next week. In the mean time you might want to try to remember the full stops: 1, 1.4, 2, 2.8, 4, 5.6, 11, 16 and 22. You may notice that every other number is double; this helps to memorize the numbers.
I also want to mention that small numbers isolate focus and large numbers increase the area. This is called depth of field. I wanted to mention it so that I can put some pictures into this blog.
 Collin with Trees © John H. Siskin All rights reserved |  Bath with Stained Glass Window © John H. Siskin All rights reserved |
 Grand Centtral Station Interior #1, New York © John H. Siskin All rights reserved |
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