Great BEF Question

Great BEF Question

Great Question:
In your announcement at the beginning of the week you mentioned to reach out to you if we were having trouble understanding BEF.  I have tried going over it so many times in my head but I just can’t seem to understand.  Could you help me please?

 

Answer:
Let’s get you squared away now. 🙂 I am happy to help. Go get your camera so that you can dial in the settings as you read if you like. Take each part in stages. Understand each stage before moving on. Email me for questions.

We are going to start by going over a few things that you probably know but will assure we are on the same page.

As you know, your camera allows you to control how much light is going into it by controlling the ISO, shutter speed, and aperture. The ISO determines how sensitive your camera is to light. The lower the ISO the brighter your light should be. This means ISO 100 is good for bright light like full sun. When you are shooting inside, you will want to bump up your ISO to something like 400 or 800. Every time you double the ISO (100 to 200 for example) you double the sensitivity of your sensor. You can think of it like having one light on and then turning on a second light of the same wattage on in your home. This is double the light. If you were to go from 200 to 400 ISO, this would be like having two lights on and then turning two more lights of the same wattage on to have a total of four lights. (Yup, it’s that easy.)

How long your shutter stays open is based on your shutter speed. So if you go from 1/2 a second to 1 second, you have left the shutter open twice as long on one second from 1/2 of a second. By going from 1/2 a second to 1 second, your photo will be brighter by one stop. (This is the same as going from ISO 100 to 200. The photo will be brighter by one stop.)

Our last tool is the aperture – a variable sized hole inside your lens. It works in stops values too by being able to brighten (or darken) your exposure. For example, if you are at f/11 and you open your shutter to f/8 you have brightened your exposure by one stop. Again, this is like going from ISO 100 to 200. When you go from f/11 to f/8, your photo will be one stop brighter.

You may have noticed with the aperture that the larger the number is the smaller the hole is in the lens. This is due to the aperture being a fraction. Think of a delicious pie/cake. If sugar and calories were not an issue, would you rather have 1/8th or 1/11th of a slice? Most people would want more pie and choose the 1/8th (larger) slice.

Naturally, the converse of this is true. If you go from ISO 200 to 100, or shutter speed from 1 second to 1/2 a second, or f/8 to f/11 your photo will be one stop darker because half the amount to light is getting to your sensor. Of course as you go up and down each control’s values, you will be jumping by one stop.

NOTE: Your camera is most likely set up allow you to control your exposure by a 1/3 of a stop. This means you will have to click each dial three times to make a full stop. This is why you want to learn the FULL STOP range for each control.  Below is an image of the full stop range for each with examples of how each setting might affect your photo. A larger aperture hole will give you less in focus. A slower shutter speed will allow blur to show in your camera. Higher ISO settings can create visual noise. Your camera and lens may not have ALL of these settings shown but that’s OK. It’ll have enough for you to work with.

Whew, with all that knowledge as our base, let’s move on to your question – how do you make all three of these controls work for together to create a properly exposed photo? This is where BEF can come in to help.

The basic exposure formula (BEF) is more commonly known as the Sunny 16 Rule. This means that on a bright, sunny day with sunlight hitting your subject your exposure will be f/16 @ 1/ISO. To dial this into your camera, let’s have your ISO set to 100. This means your shutter speed would be set to 1/100 of a second too. As we are working in full stop scales, we can see that the closest shutter speed setting in the attached image to 100 is 1/125 of a second. That’ll work just fine. So…

Bright sunny day with ISO set to would 100 = f/16 @ 1/125 of a second.

Does this make sense? If no, email me. (wadempsay@sbcc.edu)

Now, let’s change the ISO to 200. What would your shutter speed change to?  Again, here is the full stop settings to help or use your camera to figure it out.

….

Don’t read down for the answer. Work to figure it out yourself. 😉
….
….

Did you come up with a shutter speed of 1/250th of a second? If so, then you have figured out the correct exposure for a sunny day. (Yay!)

Of course, life does not give us only bright sunny days. Sometimes haze, clouds, and night rolls in or we could be photographing inside. This is where the BEF Cheat Sheet comes into help – http://wfs.sbcc.edu/Departments/GDP/photo109/htm/befcheat.htm

If you have a look at it, you will see that haze in the air will reflect back one stop of light. This means we need to add one stop of light (make it brighter) to our exposure Sunny 16 Rule exposure to compensate. You can do this with the ISO, or the shutter speed, or aperture. Choose one. For example:
We start out on a sunny day with an exposure of f/16 @ 1/ISO. This means that if our ISO is set to 200 our exposure will be f/16 @ 1/250. Then we notice haze is rolling in so we need a brighter photo by one stop. Let’s look at ways of correcting the exposure:

Sunny Day:
ISO 200 @ f/16 @ 1/250 of a second

Hazy Day:
ISO 200 @ f/11 @ 1/250 of a second – Opened aperture one stop
ISO 200 @ f/16 @ 1/125 of a second – Slowed down the shutter by one stop
ISO 400 @ f/16 @ 1/250 of a second – Increased the sensitivity of the sensor by 1 stop

You only need to make ONE of the corrections above to correct for the exposure. You choose which one by determining if you want less depth of field (less in focus) or potentially showing more motion blur or possibly showing more noise respectively.

Got it? If not, email me. (wadempsay@sbcc.edu)

We are going to move on to a brighter cloudy day. Looking at the cheat sheet, we see we need to open up (brighten) our exposure by two stops (+2). This means you can choose two of the above settings and dial then into your camera to get the correct exposure. For example a new exposure could look like:

Sunny F/16 Day:
ISO 200 @ f/16 @ 1/250 of a second

Bright, Overcast Day:
ISO 200 @ f/11 @ 1/125 = opened the aperture one stop and slowed the shutter one stop
ISO 400 @ f/11 @ 1/250 = make the sensor more sensitive by one stop and opened the aperture by one stop

I won’t bore you with all the combinations because I also want to go let you know that you can also make two stop changes to one of the controls. Here are the examples:

Sunny F/16 Day:
ISO 200 @ f/16 @ 1/250 of a second

Bright, Overcast Day:
ISO 200 @ f/8 @ 1/250 of a second – Opened aperture two stops from the Sunny 16 Rule
ISO 200 @ f/16 @ 1/60 of a second – Slowed down the shutter by two stops
ISO 800 @ f/16 @ 1/250 of a second – Increased the sensitivity of the sensor by two stops

OK, I have thrown a lot at you. Let me know where you need more clarity.

Best,
Say