Proper Exposure and F-Stops (For Manuel)

 

First thing’s first: What’s an F-Stop? In the simplest terms it’s a unit of light. Add an F-Stop, you double the light onto the sensor (or film). Remove an F-Stop, and you HALVE the light that goes into the sensor. Simple enough, right? Right.

Here’s where it becomes a little more complicated, there are three factors that can change the amount of light that goes into the camera (f-stop):

  • ISO
  • Aperture
  • Shutter Speed

 

SHUTTER SPEED

We’ll start by explaining the easiest to understand, shutter speed. The shutter opens and closes very quickly to let light into the camera, you can think of it like blinking, except that you can change how fast the shutter (blink) moves. Shutter speed is measured in seconds, or fractions of a second. On your camera you’ll see a shutter speed notated by a number (ie 125, 30, 8), this number is actually a FRACTION of a second. 125 means 1/125 (or 125th of a second), 30 means 1/30 of a second, etc. The larger the number, the shorter the exposure. An example: 1/4000 of a second is smaller (a smaller amount of time) than 1 second. If the exposure is 1 second or larger, the photography notation is a quote mark, for example 30 seconds would be 30″. This allows us to differentiate between 1/30 of a second (30), and 30 seconds (30″). It seems confusing at first, but you’ll hardly ever use an exposure longer than a second.

Now that we’ve got an idea about shutter speed, imagine what that speed would do to the amount of light let in. If you had a 2 second exposure, a one second exposure would let in half the light, right? In the same way a 4 second exposure would let in TWICE the light, RIGHT??? Right. The same with 1/250. 1/500 lets in half the light (it’s twice as fast), and 1/125 lets in half the light (it’s half the speed of 1/250). This idea of being able to double or halve the light is the basis of the f-stop. Each “stop” allows in either half as much or twice as much light, allowing them to be interchangeable for exposure control (making sure the picture isn’t too bright or too dark).

 

ISO

While we’re on a roll, let’s move onto ISOs. ISO used to denote the film’s sensitivity to light, the higher the ISO, the less light you’d need. The problems are:

  1. The higher the ISO (the more sensitive to light it is), the more grainy the picture.
  2. If you use a high ISO film, pictures in bright light could be easily over exposed.
  3. With film, you’re stuck with one ISO for an ENTIRE roll of film… Have to plan ahead.

With digital things are MUCH easier. Your camera can be set to adjust the ISO automatically for each picture (that’s its default behavior), so “problems” #2, and #3 are negated entirely since you can/will have a different ISO for each picture. Problem #1, however, still exists. The higher the ISO, the more digital noise you’ll have. On your camera, the noise level is pretty low up until about ISO 800, so I’d try to keep it under that if I could. In regard to f-stops, ISOs are simple, double the number and you double the amount of light the sensor gets, halve the number and you halve the amount of light! So if you start at ISO 200, then ISO 100 lets in half as much light, and ISO 400 lets in DOUBLE the light. Are you starting to see how ISO and shutter speed can be interchangeable for adjusting the amount of light the sensor receives?

 

Aperture

This is where things will get a little complicated since the numbers don’t double and halve… The aperture. Also known as the iris (DON’T call it this when talking about photography…), it operates much like the iris in your eye, opening to let in more light, closing down to let in less light. Not that hard, right? What makes it difficult is the numbering system used to convey the size of this opening… f numbers will be written like: 2.8, 5.6, 11, etc. What do they mean??? They’re actually a fraction (like the shutter speed), so f2 would be f1/2 or HALF the focal length. If you have a 50mm lens at f2, the aperture will be 25mm. This means that as the number gets bigger, the hole gets smaller. f11 is 1/11th of the focal length… f22 is 1/22 of the focal length. The f-number on a zoom changes as the lens zooms even if the opening stays the same because it’s a ratio of the FOCAL LENGTH (which zooming changes) to aperture size… This though, is not important for this discussion, and if you want more specifics (including formulas and equations to figure out the area of the opening at a given focal length), you can ask, that’s definitely a topic for another time.

The important things to know in this discussion about exposure (in regard to aperture) are:

  • The smaller the number, the LARGER the opening (it lets in more light)
  • The numbers don’t halve and double to indicate one stop like ISO and shutter speed

Standard f numbers are:

  • 1.0
  • 1.4
  • 2
  • 2.8
  • 4
  • 5.6
  • 8
  • 11
  • 16
  • 22
  • 32
  • 45

(It can keep going, but you probably won’t see larger than 1.0 or smaller than 45) These are the f numbers in ORDER, f-stop order. So, if you have your aperture set to 4 and you stop it down to 5.6, you’ve just halved the light. If you start at 4 and stop it up to 2.8, you’ve doubled the light.

You’ll notice on your camera that ISOs, shutter speeds, and apertures don’t exactly match up to what I’m using as example. That’s because your camera can make adjustments by 1/3 of a stop (a good thing!). For the purpose of this discussion, I’m using only whole stops, but once you get the idea how it works, it will be easy to use the same logic with smaller stops. This is even easier if you keep your camera in Av mode because it will AUTOMATICALLY adjust the shutter speed and ISO to give you a proper exposure, all you have to do is set the desired aperture (I recommend staying at 2.8).

 

Exposure Examples

Perfect Exposure

For the rest of this we’ll be using the above example. In this scenario, a perfect exposure is achieved with an ISO of 800, an aperture of f8, and a shutter speed of 1/60. In these examples, that is the PERFECT amount of light.

 

Underexposed by six stops

In the example above, it’s UNDEREXPOSED by SIX stops. First, the ISO is reduced (reducing the amount of light the camera gets) by 2 stops, from 800 to 200. (800 to 400 is one stop, 400 to 200 is one stop). Next, the aperture is stop down twice (8 to 11 is one stop, 11 to 16 is another). Finally the shutter speed is increased (faster shutter, less light gets in) by two stops (1/60 to 1/125 is one, 1/125 to 1/250 is another). Understand? At six stops underexposed, the image will probably be entirely black. Not so good.

 

Overexposed by six stops

This example (above) is OVERexposed by six stops. It’s the same idea as the last one, but in reverse. This picture would probably be almost completely white (WAAAAY too much light).

 

Underexposed by two stops

The above example is only underexposed by two stops (only the ISO changed), it would be very dark, most details lost.

 

Underexposed by one stop

To compensate for the two stops of underexposure from the ISO, we’ve now adjusted the aperture. By adding a f-stop from the aperture, we’ve allowed in twice as much light as before, so now we are only underexposed by ONE stop. It will be dark, but probably usable.

 

Perfect Exposure

PERFECT EXPOSURE!!! Achievement get! Continuing from the last example, to let in more light (since we were under exposed by ONE STOP), we added a stop by lowering the shutter speed from 1/60 to 1/30. Now even though the ISO, aperture, and shutter speed have all changed, we still have a perfect amount of light hitting the sensor.

 

Perfect Exposure

To change it up a bit, this time we reset out shutter speed to 1/60, but to still allow enough light, we opened the aperture up to f4 (from f5.6). Notice that f4 in these examples is two stops from the line (f8 is at the perfect exposure line). Since the ISO at 200 is TWO stops underexposed, we need to make up those stops with the other choices we have (aperture and shutter speed).

 

Perfect Exposure

Last example. This time notice that we’re two stop underexposed because the aperture is so tiny (f16) that it’s not letting in very much light. To compensate for these two stops of underexposure, we added a stop of light with the ISO (by bumping it up to 1600 from 800), and we added the other stop by slowing down the shutter (from 1/60 to 1/30) to allow it to stay open longer.

 

Closing

A couple more thoughts. When talking about exposure, ISO, aperture, and shutter speed are all interchangeable in order to get a good exposure. This works for 1 stop, or 100. The caveat is that a change in any of these has a direct effect on the quality of the picture. High ISO adds noise. Low shutter speed (typically below 1/30 for handheld) adds blur. Large aperture makes the depth of field (the distance that’s in focus) smaller. On your camera, f2.8 should always be fine. You’ll find that it’s very difficult to get a shallow depth of field because of the size of the sensor in your camera, but this will make it easier to keep things in focus when you’re just getting started. I’ve mentioned it before, but I’d probably try to keep the ISO below 800, and disable flash whenever possible. It is better to get a noisy (high ISO), washed out (flash) picture than none at all, so weigh that into your choice to take or disregard this advice. One other thing that will effect exposure is the metering… This gets a little complicated and is beyond the scope of this discussion, but depending on which metering mode you use, you could have wildly different exposures. I suggest looking at the histogram occasionally to see if the exposure is right or not. I’d also recommend allowing the camera to use image stabilization, turn on sharpening (in camera), and use the highest quality JPG as the picture format. Finally, for aspect ratio, if you plan to print these for relatives, use 3:2, if you plan to send to people with older compys, 4:3, if you plan to add the pics and video together for some sort of HD slideshow, choose 16:9. These aren’t entirely necessary, but if you use a different ratio, you’re gonna have to crop later to use the pics as you intend. You’ll get the most pixels at 4:3 (12MP, 4,000 x 3,000), but honestly, even in 16:9 (9MP 4000 x 2248) you have enough resolution to make VERY large prints, even after re-cropping.

Leave a Reply