I can already sense the panic, that this topic sounds way to mathematical and has some people already scrambling for back arrow.
Bear with my because if you can nail this aspect of photography you are well on your way to opening up the true ability of your DSLR.
The term "exposure triangle" is used to describe the relationship between the three main components of the exposure of a photograph: aperture, shutter speed, and ISO.
You must balance the three of these to achieve the type of result you are looking for in your photo. Not only do these three elements change effect the exposure of an image but they are also used to determine the overall appearance of an image and therefore they are super important to understand how they work, to enhance our ability to deliver the types of images we have in mind.
ELEMENT ONE - APERTURE
Basically, aperture is the size of the lens iris or shutter.
Bigger aperture (or lower the f-number) = More light entering the camera
Lower aperture (or higher f-number) = Less light entering the camera
Playing with the aperture helps us to not only control the light but also the depth of field.
Bigger aperture (or lower the f-number) = Shallower depth of field
Lower aperture (or higher f-number) = Greater depth of field
ELEMENT TWO - SHUTTER SPEED
Shutter speed is exactly that, the speed the shutter takes to open and close, and therefore how long the sensor is exposed to light. Faster shutter speeds give the sensor less time to receive light and slower shutter speeds more time.
Shutter speed is primarily used to highlight or prevent movement in an image whether that be the subject, camera shake or elements in the image (eg waterfall or clouds).
Remember that whilst the shutter is open, the camera sensor is essentially recording the movement of light or elements in the frame; if elements move then the result will show blurring in the image.
Many photographers would argue that shutter speed is the most important element to photography, saying if your shutter isnt fast enough to capture a sharp image nothing else will save the image.
ELEMENT THREE - ISO
Thanks to the developments in technology, ISO is now easily adjusted and the term is used to basically describe how sensitive the camera sensor is to light (it actually applies a post image gain to the signal received).
To break it down simply, by increasing the ISO on your camera allows you to work with less light. As we have learnt above, when one thing changes, there is always another change.
In this case, the higher the ISO we use, the more grainy and less detailed our images will appear. Also, we will lose what is called the dynamic range in our images which reduces our ability to process the image on our computers.
WHY DOES NOISE CHANGE? At lower ISOs, the size of the signal received by the camera (or light) is large relative to the noise (signal to noise ratio), meaning the noise is usually negligible. The higher the ISOs, the image signal received (or light) is less and the signal to noise ratio increases.
READY FOR AN ANALOGY TO HELP UNDERSTAND? Think of the image signal and noise like this. If my image signal is strong and equals for example 1,000 bricks in a wall (just like megapixels in a camera), i wont notice too much if the noise adds 5 bricks to the wall.
However, if my signal is weak (poor light), we can use higher ISO to bring the exposure up. In this example, the poor signal equals 10 bricks being used to build the wall (less input signal=smaller wall/less bricks), those same 5 bricks of noise will become more noticeable. Then, when the ISO setting amplifies that signal back to the usual 1000 bricks, the amount of bricks will increase to 500. A huge jump from the initial 5.
YOU MAY ASK, WHY USE HIGH ISO THEN? When working in low light, it is important to amplify that signal and boost your ability to expose in low light situations. For example, you have the aperture of your camera wide open (most light entering) and the shutter speed as slow as possible to freeze motion but your images are still on the darker side. The ISO is your only choice left (or a flash - but I am not a big fan though). Your lens cannot open any wider to let more light in. If you slow the shutter speed down further your subject will not be sharp and begin to blur.
You should then increase your ISO to amplify the light signal received based on the current scenario you are trying to capture and ensure your image is correctly exposed.
Putting it all together: EVs and Stops
The specific combination of aperture, shutter speed, and ISO is called the EV or Exposure Value and often refers to a change that either halves or doubles the light entering your camera as a stop.
Math sort of takes over here and I don't want to get too technical but for those that are interested continue and let's get our geek on.
Shutter speeds and ISO respond numerically how you would expect: a change from ISO 200 to 400 is an increase of one stop; a change from a shutter speed of 1/30s to 1/120s (most cameras will make this 1/125s) is a decrease of two stops. However, f-stops, which correspond to aperture, are arranged in a geometric series that roughly approximates powers of the square root of two (The reasons for this make sense, but it becomes a bit complex to explain in a blog post). In other words, in the following sequence, each new f-stop represents a decrease of one stop: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. So, how do you intuitively work with “doubling” or “halving” an f-stop? You don’t. Just memorize the sequence. It’ll become second nature very quickly and you’ll have no problem jumping between stops and understanding exactly how many stops you’ve added or subtracted to your exposure. For example, moving from f/2.8 to f/8 represents a decrease of 3 stops, so we're now using 1/2 * 1/2 * 1/2 = 1/8 as much light.
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