The exposure triangle consists of three (I know you wouldn’t have guessed that) main components – ISO, Shutter Speed, and Aperture. With all the other technical details apart, these three dominate how you are going to make the picture. Understanding the exposure triangle is critical for anyone who wants to master the art of photography. I mean really, there is no other way around it. These concepts should be second nature to you. Do not worry though, they are not too difficult to comprehend. I’ll explain what each of these does and then how they are related to each other.
ISO stands for International Standards Organization. It’s an organization which sets standards. Who would’ve thought, right? In photography, ISO is an indication of how sensitive your image sensor to light. When you have very little light available, you have to increase the light sensitivity of the sensor to achieve a reasonable shutter speed to capture the image without causing blur. Seems simple enough. But here’s the catch. When you increase the ISO, the image sensor tends to heat up and produces grainy images (image noise). This is the place where bigger pixels trumps the pixel count. So when possible, shoot at the lowest ISO that allows a reasonable shutter speed for the occasion.
Here’s an example of what ISO does to your images.
f/4.0, 70 mm, 1/60, ISO 100
f/4.0, 70 mm, 1/4000, ISO 6400
Notice that the top image is sharper and much cleaner and the bottom image appears very grainy and not as sharp. These are the two extremes that my camera will go. Any ISO between 100 and 6400 will produce an image in between these two extremes. Also note that I kept the aperture (f number) and the subject distance a constant to achieve the same depth of field. So I had to change the shutter speed to get a properly exposed image. Keep these things in mind because it all comes together to form a good image. I will address the focal length and the depth of field in a future post, to learn more about aperture, shutter speed, and the exposure triangle, keep on reading.
Seems a little obvious but here goes. Shutter speed is the time the film or the digital sensor is exposed to incoming light. A higher shutter speed is required to freeze the motion, especially comes in handy in sports photography, bird photography etc. A lower shutter speed allows to capture the motion. One of the most popular use of low shutter speed, but not limited to, is taking pictures of waterfalls. It gives the picture a silky feeling. Shooting at lower shutter speeds can produce very creative results. In fact, there are specially designed filters called natural density filters (ND filters, my personal favorite) to cut back a portion of incoming light to achieve very low shutter speeds without over exposing the image. A tripod is essential when using lower shutter speeds to avoid blur caused by camera shake. There’s a rule of thumb called the 1/focal length rule. It means, if you’re shooting at 50mm, your shutter speed should at least be 1/50th of a second to handhold the camera. Anything slower will cause camera blur. Most entry level DSLRs have a maximum shutter speed of 1/4000th of a second whereas high end professional cameras have a maximum shutter speed 1/8000th of second. To achieve these speeds, DSLRs employ a mechanism called shutter curtains. There are two shutter curtains and at higher speeds, as one curtain opens up the other one is already closing. So there is a small slit that allows the light to pass through the curtains into the image sensor.
f/1.8, 50 mm, 1/4000, ISO 100
This was taken at 1/4000th of a second and since the shutter speed is very fast, it froze the motion of the droplets.
f/11, 10 mm, 75 sec, ISO 100
This one on the other hand was taken over a period of 75 seconds. I had to use a 10 stop ND filter to cut back the incoming light to capture the motion of the waterfall. If it wasn’t for the ND filter I would never be able to achieve this shutter speed because it was a bright sunny day and it would have over exposed the picture.
Aperture refers to the opening of the lens and is represented by f/number. When the f number is low, for instance f/1.4, the lens has a larger opening. When the f number is high, for instance f/22, the lens has a smaller opening. This greatly affects the depth of field.
Different apertures. Image courtesy – Wikimedia
f/4.0, 70mm, 1/200, ISO 400 – Shallow Depth Of Field (DOF)
Here the background is out of focus where as the foreground or the subject is in focus. This is what we call a shallow depth of field. I will be referring to depth of field as DOF from now on. DOF, in a nutshell, is the area in focus. Aperture is a key factor when it comes to achieving the desired DOF but not the only factor.
f/22, 70mm, 1/30, ISO 1600 – Deep Depth Of Field (DOF)
Compared to the above picture, this one has a cluttered background and the background is in more focus. Notice the apertures used in the two pictures. This one has a deep DOF than the other. Usually a deep DOF is used in landscapes to capture sceneries with both the background and the foreground in focus. At best, this is a crappy DOF but enough to demonstrate the effect of aperture has on DOF. Like I said before, aperture is not the only thing deciding the DOF. Also take note of the different shutter speeds and ISO values I’ve used.
When the lens is at its lowest possible f number, it is said that the lens is wide open. When you look at the name of a lens, the f number mentioned there is the widest aperture (lowest f number) the lens can go. In other words Canon EF 50mm f/1.8 lens can open up to f/1.8. If there’s a range of apertures given, for instance Canon EF 70-300mm f/4.0-5.6 L IS USM, it means at 70 mm the widest aperture the lens can achieve is f/4.0 and as you zoom in the f number will increase too. At 300 mm the widest aperture the lens can achieve is f/5.6. You can of course control the aperture using a small dial on your camera or on older lenses using an aperture ring. Increasing the f number is called stopping down the lens because it stops portions of light as you increase.
Now, you saw the effects of ISO, Shutter Speed, and Aperture. You need to combine all three of these to achieve your desired goal. Each one of these can throw out the balance of your picture if they are not in correct proportions. you would either get an over exposed image or an under exposed image.
f/14, 10 mm, 1/160, ISO 100 – Properly Exposed
f/14, 10 mm, 1/40, ISO 100 – Overexposed
f/14, 10mm, 1/640, ISO 100 – Underexposed
There are several popular analogies to explain the relationship between these three. I will mention a few.
Equilateral Triangle – Imagine that the perfect combination of ISO, Shutter Speed, and Aperture (perfect exposure) is similar to an equilateral triangle. If you change an angle or a side of the triangle, it’s no longer an equilateral triangle. So you have to adjust the other two accordingly to get it back into an equilateral triangle. Bottom line is that the perfect exposure can be achieved in many different ways. The combination you want to use depends on the result you need. Once you find the perfect combination (cameras have built in light meters to help you with this, although not always very accurate it does a pretty decent job), you can play around with the values. You can lower the f number and increase the shutter speed. You can use a slower shutter speed with a lower ISO etc.
The Window – In this analogy, the camera is similar to a window with shutters. Size of the window is your aperture. Bigger the window, the more light gets in. The time frame you have the window open is like your shutter speed. The longer you have it open, you let more light in. Now, there isn’t exactly an easy way to include the ISO here but imagine that you’re inside the room with your sunglasses on (douchebag alert!). The sunglasses decreases the sensitivity of your eyes to light much like ISO does. Taking off your sunglasses is like boosting the ISO, increasing the sensitivity to light.
Rain Bucket – Unless you do the rain dance, I’m assuming you can’t control the rate of rainfall. This is similar to available light. While there are many ways to manipulate the natural light, let’s just think of a scenario where you’re just shooting as it is. But you can control several other factors to collect just enough (proper exposure) rain water. You can control the diameter of the rain bucket. A wider bucket will collect more rain water in less time whereas a narrower bucket will take much more time to collect the same amount of water. You can also control the amount of time you leave it out there to collect water. Now do you see where I’m going with this? ISO, Shutter Speed, and Aperture are completely under your control to get a properly exposed image.
Here’s the take home message. Lower f numbers (higher apertures) let in more light whereas the higher f numbers (lower apertures) let in less light. This might take some time to get used to because it’s kind of counter intuitive. You can also think of the f/number as a fraction. When the number in the denominator is bigger, the overall value is smaller. This is true for shutter speeds as well. Faster shutter speeds allow less light in and slower shutter speeds allow more light in. Again, think of the number in the denominator. ISO on the other hand is straight forward. Higher the ISO, more sensitive the image sensor is.
Sometimes, you have to intentionally underexpose or overexpose images because the cameras are not just yet advanced enough to capture everything we see in one go. Then you have to combine the best of each image and come up something new.
Combination of the above three images
When you do, it’s called a High Dynamic Range (HDR) image. More on this later though. I’ve dragged this post long enough for now!