Check out these three articles first before you read the rest of the post.
- Part 1: Understanding Aperture
- Part 2: Understanding Shutter Speed
- Part 3: Understanding ISO
BASIC DAYLIGHT EXPOSURE (BDE)
This is also referred to as the SUNNY 16 rule.
To achieve proper exposure on a bright sunny day, your ISO and your shutter speed are going to be the same when you’re aperture is set at f16. So for example if your ISO is 200 at f16 your shutter speed will be 1/200th of a second. This will ALWAYS be the same as long as it’s a bright sunny day.
Basically what “equivalent exposures” is referring to is achieving the same exposure while manipulating the depth of field in different ways.
Let’s say you’re out at the beach on a bright sunny day. So you think to yourself, “Hey, Natalie taught me all about Basic Daylight Exposure! I’m gonna try it out!”
So you set your ISO to 100 and thus you’re going to have a shutter speed of 1/100th, because remember when using Basic Daylight Exposure (Sunny 16) ISO and shutter speed will match at f16. So you’ve got your shutter speed and your ISO matched up and you shoot your aperture on up to f16 and SNAP!
You've got a great, perfectly exposed beach scene. You can see mom and dad clearly in the foreground. You can also see the kids playing in the water behind them, also in clear focus. You can see the boogie boarders in the background in clear focus as well. Your image is perfectly exposed, not too bright, not too dark, JUST RIGHT!
So you’re thinking you’re hot stuff until you think, “You know, there’s a lot going on in this photo. There’s really no focal point. It’s TOO BUSY!” You decide you’d like to focus only on mom and dad and throw everything else in the image out of focus. . . you’re going for a shallow depth of field. SOOOOO what’s a girl to do? . . . or a boy as the case may be. Well, clearly you need to open up your aperture, right? Open that baby up to give you a more shallow depth of field.
Now we understand from the post Understanding Aperture that in order to OPEN our aperture to let in more light and cause a shallow depth of field, we’re actually going to be setting the camera on a LOWER aperture number. So we open that baby on up to f4 and SNAP! . . . uh oh, what the heck happened?? We have an image that is just pure white! All light. Completely blown out. NOW WHAT? This is where Equivalent Exposures come into play. . . this is also the point where you really need to have read the other articles in the series. Part I, Part II, Part III. GO! For the rest of you, let’s break this down:
- You had a correct exposure of the beach scene when your camera was set to ISO 100 at 1/100th of a second f16.
- When you dropped the fstop down to f4, you let in WAY too much light and ended up with an image that was completely blown out.
- The obvious answer would be to increase the shutter speed, right? To let in less light? You got it! BUT, by how much? How do we know how fast our shutter speed should be? Equivalent Exposures. That’s how.
Since you started at f16 and went down to f4, how many stops did you change your aperture by? Look at the chart below, that you SHOULD have memorized if you did your homework from Part I. . . and see how many FULL STOPS you had to move to get from f16 to f4. Go on and count ‘em.
In the chart above you see the TRUE apertures. Those highlighted in blue are the common apertures for most lenses.
That’s right, 4 stops. You went down 4 FULL F STOPS to get from f16 to f4. . . now here’s the secret of life. . . you simply have to match that number of stops with your shutter speed to achieve the same exposure with a different depth of field! In other words, since you opened your aperture by 4 stops, you simply need to speed up your shutter speed by 5 stops in order to ensure that the same amount of light hits your sensor as it did at f16. . . THUS ACHIEVING THE SAME EXPOSURE WITH A DIFFERENT DEPTH OF FIELD. “Bing!” That was the sound of the light bulb going off in your brain. It’s OK to be excited. It really IS that simple. Don’t be scared to go back and read it again if you need to, but don’t make it harder than it is.
The chart above is an illustration of Equivalent Exposures. Each setting above will allow the SAME amount of light to hit your sensor. The exposure will be the same in each instance, the only difference would be the depth of field.
Let’s do another little quiz:
You’re at a horse race. . . Anyway, so you’re at a horse race and it’s bright and sunny out, but with a few clouds in the sky. You decide to use an ISO of 200. So, using BDE, what’s your camera going to be set at? ISO 200, 1/250th (250 because it’s the closest shutter speed to your ISO of 200) at f16.
You take a shot of Ocean Muffin, the fastest horse in the race, check your LCD and decide that you’d like to see more movement in the shot. You want to show just how fast Ocean Muffin really is! What are do you need to do?? Slow down your shutter speed, that’s right. So let’s say you decide to drop your shutter speed by 2 full stops. So you’re now at what? You should have these memorized by now if you’ve read the other posts in the series. You’re now at 1/60th of a second. But remember, when you slow down your shutter speed, you’re going to be letting in more light, so what else do you need to do? Close down your aperture (higher fstop) by 2 stops. Now your aperture would be at what? F32, right.
That way you can keep the same exposure and still show Ocean Muffin in all his magnificent glory. HOWEVER! What if your camera only goes up to f22? OH NO NO NO! What ever will you do now? You need to drop your shutter speed by 2 stops to catch O.M’s movement, but you can only close down your aperture by one! What else can you do? Here's a hint, check out Part II in the series on shutter speed (the link’s up top). That’s right, your ISO also affects your exposure, right? So, go ahead and move your aperture by one stop and adjust your ISO by one stop, taking it from 200 to 100. . .that way you've adjusted your exposure by 2 stops to compensate for slowing down your shutter speed by 2 stops! YAY, the world is right again!!
NOTE: If you don’t use a light meter, always start out using BDE and then adjust accordingly until you find the right exposure for the light you are in. This is one of the many reasons to hail the LCD screen as king of the world and all things that are good. . . you can SEE if your exposure is dead on and tweak it bit by bit until it is! Start out with BDE and then see that your image is too dark. . . so work with the fstop first. Open it up until the image is properly exposed and from there use Equivelant Exposures to find the right exposure and depth of field you are looking for! Easy cheesy.
Some of you are ready to rip your hair out, because this can be really confusing. HOWEVER, go back and read the first 3 posts (all the links are up top) . . . then read and re read this one, and it will become clear.
