Getting constructive: The basics of exposure.

So I’ve gotten a bit of grief about just telling people to go read books instead of actually trying to teach them things. Here’s my attempt at teaching something.

If you have no real interest in photography, you can stop here. This post turned out to be a lot longer than I had anticipated.

We’re going to look at exposure. First, we should try to define exposure. Exposure is the amount of light allowed to hit your sensor (or film if you’re old school). That’s it. Notice I didn’t say the “correct amount of light” — just “amount of light.” That is because you can have underexposure (too little light) and overexposure (too much light) and exactly what is or isn’t “correctly” exposed is entirely up to the photographer.(1)

Background.

Exposure used to be a really tough thing to calculate. People had to follow this rule called the Sunny 16(2) or the Moony 11(3) and had to calculate exposure in their head according to the situation. With the advent of light meters inside of cameras, we’ve forgotten all this stuff.

Normal-speak first.

To help us along, we’re going to use a popular analogy to describe exposure.

Light travels in a stream of packets so it’ll help if we imagine light not as a mysterious, ubiquitous force but as a bunch of tiny water droplets constantly pouring down in a steady and uniform manner.(4) Now pretend we have a device out in the rain with a circular door above a bucket and we can control the size of the door and the amount of time the door is open. Let’s also pretend that the goal is to fill the bucket to the brim without spilling.

Let’s say we have to open the door so that its area is 1 ft^2 and we have to leave it open for exactly 10 seconds to have the bucket filled to the brim without overflowing at all.

Now, this is just one of a countless number of possibilities. We could cut the size of the opening in half and leave it open for twice as long and still get the same amount of water or we could open the door 50 times larger and leave it open for 1/50th the amount of time. Mathematically, there’s an infinite number of combinations we can use to reach the same exact amount of water in the bucket but to get the same amount of water, the size of the hole and the amount of time it is open is always inversely proportional.(5)

(If I’ve lost you at this point, you should really reread all that.)

To bring it all back to camera-speak, the bucket is your sensor. Filling it to the brim (or whatever other goal) is the correct exposure for that circumstance. The size of the door is your aperture. The amount of time the door is open is your shutter speed. And the rain is light.

As we can see, the way to control for exposure is the aperture (size of the door) and the shutter speed (time it is open). There is a third way and we’ll get back to that later.

Here’s the part where we get technical.

f/this.

Aperture is the size of the hole in your lens. It is measured by a ratio, called f/stop,(6) and comes in strange numbers that make absolutely no sense at first.(7)

F/stop is a ratio because the the focal length (F) changes according to different lenses and different amounts of zooming in. F/stop gives us the convenience of having a set of numbers which will always be constant regardless of the type of lens we are using.

So… what are these strange numbers?

f/1.4   f/2   f/2.8   f/4   f/5.6   f/8   f/11   f/16   f/22

This is the standard set of numbers up to f/22 (the normal max of most common, modern-day lenses). Every number up or down from the next is a halving or doubling or the area (and therefore the amount of light). This is referred to as a “stop” of light. So if you are told to “stop down”, they are telling you to go up in f/number but down in terms of the size of the hole because you are trying to go down in the amount of light.(8) Also, remember that three stops is not equal to three times as much light but eight times as much light (2 * 2 * 2).

To further confuse you, these aren’t the only numbers cameras use. Cameras divide each stop into thirds so you can go up or down by 1/3 steps instead of jumping entire steps.(9) So even though the above set of numbers is the standard measure of f/stops, most cameras have two steps between every standard number. That is to say, if you are at f/4 and wanted to go to f/5.6, you would first have to go to f/4.5 and then f/5.0 because stops are divided into 1/3 increments.

I know, it’s confusing and stupid, but that’s the way it is and if you’re still with me at this point, you’re as good as golden. The rest is really easy.

Now shutter up.

Shutter speed is much easier to understand. Shutter speed is the amount of time you allow your aperture to stay open. The base measurement is one second and the measurements go up and down the same way f/stops do. That is:

1 second   1/2 seconds   1/4 seconds   1/8 seconds   1/15 seconds   1/30 seconds

Here, it is more evident that each step up or down is a doubling or halving of light. And, just like with f/stops, these aren’t the only values.(10) Cameras (again) go up and down in 1/3 increments.(11) Most cameras go from 30 seconds to 1/4000th of a second.

ISO confused now…

If you’re still with me at this point, there’s one more factor to complete the photographic triangle. ISO speed. ISO speed is the sensitivity of your sensor. Going back to the water analogy, imagine that your goal is still to fill the bucket up to the top but your bucket is now half as deep (therefore, requires half the volume of water to get to the same target). You now cut the amount of water necessary in half even though the water still fills up to the same height. That is to say, if you needed a 1 sq ft hole to stay open for 10 seconds in order to fill the original bucket, now you’ll only need either a .5 sq ft hole to stay open for 10 seconds or a 1 sq ft hole to stay open for only 5 seconds or any mathematical equivalent. The apparently level of the water is the exact same (top of the bucket) but the bucket itself now only requires half the water to reach that level.

ISO speed is measure by nice round numbers and goes like this:

ISO 200   ISO 400   ISO 800   ISO 1600

Despite popular belief, ISO200 is considered good working film speed.(12) Cameras can go as low as ISO 100 and as high as ISO 6400. These ISO measurements can be further manipulated to mimic ISOs as low as 50 and as high as 25,600. The general rule of thumb is that the lower you ISO speed, the better the quality, saturation, and noise control of the photo.(13) That is to say, a photo shot at ISO 200 will result in a clearer photo with less grain and better color saturation than a photo shot at ISO 1600. However, the reverse of that is that a photo shot at ISO 200 will require three stops more light (eight times more) than the same photo shot at ISO 1600.

Putting it all together.

So, at its very core, exposure is nothing more than juggling these three tradeoffs to fit the situation. If you need to catch a shot of a bullet going by perpendicular to you, you’ll need to shoot at a really fast shutter speed. This will require either a really wide f/stop (lower number) or a really high ISO or a combination of the two. On the other hand, let’s say you want to take a photo of star trails. You’re going to need to find the right balance of keeping your shutter open for long amounts of time (some of the best star trails are 6-8 hours long), ISO low, and f/stop high (small hole).

There is, however, a lot more than this. When I find the motivation, I’ll go over the power of aperture (depth of field, circle of confusion, lens sharpness and diffraction) and shutter speed (implied motion, exacting sharpness).

There we go. The basics of exposure. This, of course, is just a general overview of what exposure is and how to control it. In the future, we’ll go over why you want to control it.

Tired,
Mathew

Footnotes:
  1. That is not to say there isn’t a mechanical “correct” exposure, but that people purposefully deviate from that point for artistic reasons. [↩]
  2. The Sunny 16 rule states that on a clear, sunny day, the typical exposure for a scene will be f/16 and your shutter speed should be 1/(ISO speed) or any mathematical equivalent. This rule will work no matter where on earth you are — on the equator or on the north pole, it should still hold true. [↩]
  3. Moony 11 is like the Sunny 16 except it only applies when you are shooting the moon itself. The moon is just a big rock reflecting the (very bright) light of the sun and so in order for it to be properly exposed and not show up as a big white ball, you should be shooting at f/11 and shutter speed 1/(ISO speed). [↩]
  4. We should note that this is just for simplicity of the explanation. In real life, light is not steady or uniform and scenes more often than not contain multiple light sources emitting packets of light at different speeds, sizes, and colors. [↩]
  5. Bigger hole requires less time. Smaller hole requires more time. [↩]
  6. Note that this is a ratio. F = focal length of the lens and the stop is a constant number. The quotient of this number is the aperture. Since it is a ratio, the larger the f-number, the smaller the quotient and therefore the smaller the hole. [↩]
  7. The technical reason for this is that f/stop measures the diameter of the circle and not the area. The diameter of a circle increases exponentially as the area increases. A=(pi)r^2. [↩]
  8. It should be noted that you can also stop up and down using the shutter speed. [↩]
  9. Why 1/3? Turns out the human eye can’t detect differences in light unless it’s greater than 1/3. [↩]
  10. Also like f/stops, these aren’t perfect measurements. That is, 1/15th of a second is used instead of 1/16th of a second, etc. It helps keep round, easy to calculate numbers. [↩]
  11. So, to go from 1 second to a half second, you must first go through 1/1.3 and then 1/1.6 seconds before arriving at 1/2 seconds. [↩]
  12. Don’t believe people when they tell you ISO100 is where you should be at. [↩]
  13. It should be noted that with modern technology and the advent of advanced digital processing and logarithms, ISO speeds below a certain threshold (let’s say below ISO 800 on more expensive cameras) all have the same quality and being able to tell the difference between ISO 200 and ISO 800 would be incredibly difficult on normal sized prints. [↩]