I'm just looking for a simple reminder when it comes to aperture, such as: when there's a lot of light in the atmosphere where a photo is exposed, the aperture should be low, or something. The more light, the lower the aperture - something like that, but in the correct sense because I'm still not sure about it.
While the aperture does control the amount of light being let through the lens, I'd say it's primary purpose it to control for depth of field.
For example, let's say that you're using the Sunny 16 rule. Assuming ISO 100 to start, you'd have:
- Aperture: f/16
- Shutter: 1/100 (or 1/125)
- ISO: 100
This would stick you with a very large depth of field. Let's further assume that you want to shoot a shallow depth of field, say f/2.8. Your settings would change to:
- Aperture: f/2.8
- Shutter: 1/4000
- ISO: 100
As you can see by the above example, a f/2.8 aperture can be used just as easily as a f/16 aperture in big, bright sun. As you can also see - changing the aperture resulted in a necessary change in the shutter speed (or ISO).
So, there is no rule that anyone can give you for aperture. Instead, you should refer to learning more about the Exposure Triangle.
If ISO/sensitivity and shutter time both remain constant, you want a smaller aperture in brighter light and a wider aperture in dimmer light.
We usually express the aperture in terms of an f-number. An f-number is the focal length of the lens divided by the width of the entrance pupil (the aperture as seen through the front of the lens) expressed with the numerator as "1" over the f-number. Smaller apertures have higher numbers, such as f/16 or f/22, and wider apertures have lower numbers such as f/1.4 or f/2. This is because 1/16 or 1/22 is smaller than 1/1.4 or 1/2.
But why limit yourself to only one aperture for the same shooting condition? The nice thing about manually controlling exposure is that you can select which thing is most important to you for a particular shot, choose the value for that variable, and then set the other variables to allow proper exposure. The is what is often called the exposure triangle. Some would say calling the exposure model a triangle is a misnomer as it is at least three dimensional, whereas a triangle has three lines but they are contained in only two dimensions.
If you are in bright daylight and want as much as possible to be in focus, set the aperture (Av - for aperture value) at f/16, ISO at 100, and shutter time (Tv - for time value) at 1/125 second. If you desire a faster shutter time to freeze camera or subject motion, use a Tv of 1/500 and set the ISO to 400 to get the same total exposure with a faster shutter time. In this case we decreased the Tv by two stops and increased the ISO by two stops.
So what's a stop? Each "stop" on a photographic scale is a power of two. If we increase exposure by one stop we double the amount of light we capture. If we decrease exposure by one stop we halve the amount of light we capture. ISO, Tv (shuttert time), and Av (aperture value) all have scales that are measured in "stops." ISO and Tv are straightforward - each doubling or halving is one stop. ISO values go like this: 100, 200, 400, 800, 1600, 3200, etc. Shutter times (Tv) go like this: 1, 1/2, 1/4, 1/8, 1/15¹, 1/30, 1/60, 1/125¹, 1/250, 1/500, etc. Av is a little different. Remember when we said the f-number is the focal length divided by the aperture diameter? Well, the amount of light allowed into the camera is determined by the area of the aperture's opening rather than the diameter used to calculate the f-number. So on the aperture scale, each full step is a power of the square root of two: 1, 1.4¹, 2, 2.8¹, 4, 5.6¹, 8, 11¹, 16, etc.
¹ Photography often rounds numbers. 1/15 second is really 1/16, 1/30 is really 1/32, 1/60 is really 1/64, 1/125 is really 1/128, and so on. The same is true of the f-numbers based on the powers of the √2. For more about this, please see Is there a sane reason why ¹⁄₁₂₅ is not, instead, exactly half of ¹⁄₆₀?
On the other hand, if you want very shallow depth of field, select an Av of f/2.8, ISO 100, and Tv at 1/5000. If you have a lens that opens up to f/1.4 and you wish to use it in bright sunlight at ISO 100 you would need a shutter time of 1/20,000! The problem is that not very many cameras can shoot that fast. The top pro bodies usually max out at 1/8000 second. To get around that hurdle you could use a neutral density filter to reduce the light coming into the lens. A two-stop filter would get you back to 1/5000, but many entry level or mid-level cameras can't go past 1/4000 so you would use a three-stop filter to use 1/2500 or a four-stop filter to use 1/1250 and so on.
There are other scenarios where the shutter time might be the most important factor. Shooting sports or action under artificial lighting would be one such occasion. Most of the time we have to open the aperture as wide as it will go, set the Tv to what we need to stop the motion of our subjects (1/500-1/1000 is a good starting point for human athletes), and then set the ISO to whatever we need to get a decently bright image. At my hometown's high school stadium I shoot at f/2.8, 1/800, ISO 3200.