Aperture is just funky. Well, if you sit around with friends calculating the area of a circle and talk about the next digit of π, then maybe you are just a different kind of normal and aperture is your cup of tea. For the rest of us, though, I highly suggest a time honored method to understanding your camera, though understanding why the numbers are weird will elude you: memorize.
Any of us can calculate that ISO 200 is twice as sensitive as ISO 100, and ISO 400 is twice as fast as as ISO 200. With film sensitivity, a stop of light is easy to figure. Even with the shutter speed, though they inconveniently do not use exact numbers, the key numbers are approximately exponential: 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. There is a little funny business going on in there, but basically, it is easy to figure a stop of light. Aperture, though? Not so much.
Just like the film sensitivity (ISO) and shutter speed, the aperture is made to help us figure our stops of light, whether cutting the light in half one way or doubling light the other way. Obviously, when you make the aperture smaller, it allows less light, when you make it bigger, it allows more light. Well, the reason the aperture numbers are so awkward is that they are based on the area of a circle, precisely the area of your aperture. And since that calculation includes π and a simple equation, the numbers do not change as simply as multiplying or dividing by two.
So, without further adieu, here are the aperture stops, each number one stop of light less (each number indicating a halving of the light): 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and that is what you commonly see on lenses. All the other strange decimaled numbers you encounter are simply 1/2 or 1/3 stops between those. You can, of course, do the math, but unless you are pretty savvy calculating π in the head, which is only to prove the concept anyway, you will most likely still end up memorizing that list if your aim is to use it in real life photography.
So, there is my tip for the day, gleaned from the technique course I am about to finish up in a couple days. This may seem like another overly technical piece, but I assure you, it is fiercely practical. If you memorize that list, and think a bit more about the shutter speed and film sensitivity (ISO), you hold the keys of knowledge to unlock every camera ever made, from the thousand-buttoned sport-shooters-delight of the sidelines to the wooden box with polished silver plates.
…that’s what it’s all about! [ clap, clap ]
Hey there! Do you know any tricks or tips to determining aperture in the context of depth of field? For example, if I have 2 or 3 subjects at a set distance from eachother, and I need all 3 faces in focus but everything else out of focus, how would you quickly determine the depth of field?
Whew! That is a bit tricky, because every model of lens is different. I have an simple and a more detailed answer. As for the detailed, all of my Nikkor lenses have a depth of field scale on the lens. So, I can look and see how to get what amount of depth in focus, set my aperture according to that scale, then figure out what other settings need to change to reach the proper exposure. That sounds nasty, and it is, especially when the distance to the subjects will change that scale. I honestly have never actually figured it backwards that way; it is certainly possible, but you would have to have a little spare time on your hands.
I just have a feel for my lenses and the depth-of-field. It is far from precise, but I am usually fine. Now, I am usually shooting wider angle lenses, so I have a bit more depth-of-field to work with anyway. If I only have one subject, I rarely think about it. With multiple subjects, lets say a group of five people but fairly well packed together, with my 35mm lens, I would feel safe-ish at f/4. But if that was a wedding where I really needed to nail it, I would not feel safe with any larger aperture than f/5.6. Now with a bigger group or a small group spread out a little deeper, I would probably go with f/5.6 or a little smaller at f/8. Now, a 35mm lens at f/8 is going to give you a lot of depth. That might be overkill. And if I was shooting with a 24mm lens, the wider angle gives me more depth-of-field, so I would feel comfortable with a big group at f/4.
Oh, it just dawned on me, I switched out the focusing screen in my Canon 5D and mine basically shows a constant depth-of-field preview, so I can SEE if I have them all in focus. Which brings me around to the much easier answer which I had not thought of earlier…use your depth-of-field preview button and skip the calculation and guesswork. However, that preview is going to be fairly dark at f/5.6 and smaller apertures (f/8, f/11, etc), and seeing your depth-of-field is not such an easy thing.
Hm…talk long enough, and I guess I was destined to find the easy answer!