And here's the one we've been waiting for. This is where things start to be important. The others were necessary, but this is the most important of the three when it comes to photographing an object.
an f/stop is a unit of measurement that defines how MUCH light is allowed to get to the image. The opening is like the iris of your eye. And it gets weird from now on.
the numbers for f/stops were (and still are) 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 64. Some lenses had smaller and larger numbers, but these are the most common ones.
Weird #1. Each number is half as much or twice as much as the one next to it. I didnt make da rules, i just repeat them.
Weird #2. Each number refers to the size opening. THe SMALLER the opening the LARGER the number, and vise versa.. So F2.8 has a bigger opening than f11. And F 90 has a teeny opening where f4 is a big hole. The smaller the opening, the less light can come in, and the larger the opening, the more light is able to enter.
so what? There is a nifty rule of physics (or photography, not sure which) called 'depth of field.'
Imagine, if you will (rod sterling impersonation) that you are photographing a persons head. THe camera can only focus on a point, let's say the ear... and if the person is facing you dead on, the ear on the other side is in focus, but the nose, which is closer, is out of focus. You're basically focusing on a sheet of glass that is 90degrees perpendicular to the camera. And it's paper thin.. so anything in front of it or behind it is out of focus.
depth of field refers to how thick the glass can become. The rule is, the smaller the number, the bigger the opening, the thinner the plane of focus. As you decrease the opening size, make the number bigger, your sheet of glass gets thicker, and thus the more in-focus your subject is.
To put into perspective...
when you photograph a vessel, bowl, 1932 packard, empire state building, whatever... your camera will focus on a small part of it. anything within that plane of focus is sharp. Anythig not in the plane is out of focus. But, if you use a smaller opening, bigger number, the more your focus will be, back to front.
Normally when i photograph a person, i use a mid-level fstop around f8 or 11. THis gives me enough focus without having to loose too much light, because i'm outside and only have the sun or a portable flash, ie no control.
when in the studio, and the subject is still, if we were using just the light of the room, we could slow the shutter speed way down to gather enough light. So that we could use a small aperture to have adequate depth of field. When in the studio, we have more control over the aperture, shutter speed, and also we're probably using a flash.. which comes later...
and so on..