How many of us have looked at the numbers that represent our aperture and wonder what in the world they mean? Why those numbers? Why not something more simple?
Yes, the numbers given to aperture size can be a little confusing, so I will explain, because I do not want you ignoring your aperture just because the awkwardness of the numbers.
Understand that those numbers are a fraction: f/4 (sometimes written 1:4). So, just as one half is bigger than one fourth, so f/2 is bigger than f/8. So, a wider, more open aperture will be a smaller number. And note, the aperture is usually called “f-stop”, which is sometimes written ‘f/stop’.
Now, why don’t those numbers follow a more logical sequence, like the film speed does (1/125th of a second, 1/500th, 1/1000th)? A friend and I sat down once to answer that nagging question for ourselves, once and for all. I do not want to dump the math on you (you can look that up yourself, and it is really beyond me anyway), but basically, to halve or double the area of a circle (aperture) you must multiply or divide by the square root of 2 (about 1.4). So, now look at those f-stop numbers and see if they make a little more sense: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and so on. No, I do not expect you to be doing the math in your head as you shoot; just memorize those numbers and life will be easier.
Each number in that list represents a halving or doubling of the light, and each halving or doubling is called a “stop” of light. So, two stops down from f/4—do not think “down” in numbers, but down in the amount of light—is f/8, and that would be cutting the light in half twice. Read that last sentence again, slowly and make sure you have it.
So, if you start at f/2 and you need to “stop your aperture down three stops”, you would be at f/5.6. “Bring it back up a stop” and you are at f/4.
Aperture can be a pain, I know. But it really is one of the most important adjustments you make when taking pictures (and I have not even mentioned the true importance in this entry, for fear of going beyond the original question). We cannot let a little string of numbers get in our way when shooting, so ingrain that data till it is second nature, and especially the idea of stops of light.
What is the point of know f-stop on a digital camera. It always does everything automatically. It’s really frustrating when I adjust my f-stop, then get a blurred image because much shutter automatically slowed down because my camera thinks it knows better.
Well, I do not quite know how to reply, a step at a time, I guess.
“What is the point of knowing f-stop on a digital camera?” Well, learning how to control your aperture makes just as much sense on digital as it did with film…no difference at all.
And your camera only functions automatically if you set it on automatic. 🙂 And when you do that, you are totally enslaved to what some little computer thinks about the lighting in a scene, and camera don’t know squat! They can sometimes light a scene right, but they cannot know if you need to freeze action or add a little blur or adjust your depth-of-field.
Not that there is anything magical about it, but I tend to notice most folks functioning in aperture priority (A on the Nikons). That is my normal setting, though it is not best for every situation, of course. I set my aperture and keep a constant eye on the speed as I am composing the photo.
Many times, though, I switch to manual metering. And now that I think of it, you really have to know WHAT your camera is metering too, meaning, how much of the scene. There are (sometimes under different names with different brands) basically three kinds of metering: full scene, center-weighted, and spot. I usually use center-weighted, and sometimes spot. Cameras just do not have the dynamic range of brightness to be able to choose the right settings when given an entire scene with all the brights and darks, especially when compared to our eyes. I highly suggest using the human brain for that choice.
Now, that is a very simplified explanation and based on some knowledge of how a camera meters, but short of writing an entirely new post, I will keep my answer to that for now.