Mastering your camera’s exposure is not the key to being a master photographer; anyone can go out and take a picture on “auto mode” and it will usually come out how they wanted. Your cameras auto exposure system is very good these days and will get the exposure right 95% of the time. However, that 5% that the camera gets wrong could include a shot that you really wanted like a rare bird flying over your head or that moment when you wanted that rock in the foreground and the mountains in the back ground to be in focus. If your camera gets it wrong, you may have to wait a long time to see that bird again or you may never see the same light illuminating that mountain side. This is why I feel that understanding and mastering exposure is key to getting the full enjoyment out of photography, no one likes getting home, loading your photos onto your computer and realising that your camera messed that shot up for you. You did all the work getting there and your camera let you down.
Firstly, let’s get the basics clear, exposure is what it is. It is the exposure of that delicate sensor to the world in front of it through a lens. The lens is as vital in the whole camera system as the sensor itself, without it your pictures would just be a high megapixel block of random shapes and colours. Try it; take your lens off your camera and take a photo with it. That high megapixel block of random shapes and colours is the world in front of your sensor. The thing is, the sensor will just take the light that is presented to it and make it into a picture. Without a lens none of that world (or light) is being focused onto the sensor, that is why your photo is just a high megapixel block of random shapes and colours. Focusing is a whole discussion in itself, so I’ll leave that for another day. Focusing is not the only vital function of your lens though, your lens controls the aperture.
Aperture is one of the three elements in the exposure triangle. The definition of aperture is “a hole or opening through which light travels”. It is the same as the pupil of your eye. Aperture can control how much total light hits your sensor as well as how much of the photo is in focus. To understand aperture, you must understand the maths. You will see aperture represented as “f” followed by a number. This is called “f/stop”. For example, f/1.8 or f/16. The larger the f/stop the smaller the opening in the lens but the more of the scene is going to be in focus. Confusing isn’t it! This is because the number after the f means 1 over that number like 1 over 1.8  or 1 over 16  this explains why the opening is smaller with a larger f/stop, the larger the denominator is the smaller the fraction. It gets even more confusing because in photography a stop is a doubling or halving, so why is f/8 a stop higher than f/5.6? 5.6*2 isn’t 8? This is because the number is actually an exponent of the square root of 2. So, f/1.8 is actually 1 over the square root of 2 to the power 1.8. But it isn’t! The numerator of the fraction (i.e. the number on top like the 1 in the previous example) is your focal length, so if you were shooting at 50mm, f/1.8 is actually 50 over the square root of 2 to the power 1.8. I’ll stop now!
Let’s talk about light. The smaller the opening in the lens the less light hits the sensor, logic tells us this! The less light hits the sensor the darker the image. The darker the image the higher the ISO must be to get a properly exposed picture if you want to stay at the same shutter speed (more on which later). The higher the ISO (and it’s not pronounced I.S.O it is one word, ISO not an acronym, it comes from the Greek word “isos” meaning equal) the more image noise you get. Noise is the same as the old film grain. It is always present all photos no matter how low the ISO. Some people say “my photos have no noise when I’m shooting at ISO 100”. This simply isn’t true! If you take a photo taken at ISO 100 or even ISO 64 and you zoom into an area of solid colour (especially blue) you will see little grains of colour. Noise occurs because ISO is the camera boosting the light received by every sensor pixel, if there is little or nothing to boost you will see that pixel as noise. It is all down to Signal to Noise Ratios or SNRs but I won’t go into them.
High noise photos just don’t look good, they seem to lack detail and are covered in these tiny speckles of colour and darkness. So, it is a matter of getting the right aperture for your light situation. If you chose an aperture that is too wide such as f/1.8 then your image may be over exposed (i.e. too bright), however, if you chose an aperture that is too small such as f/22 the image is more than likely to be under exposed (i.e. too dark) or noisy as a higher ISO has been used.
Aperture also controls how much of the scene is in focus, the larger the f/stop number the more of the scene in focus. The blurry effect in parts of the scene that aren’t in focus is referred to as “bokeh”. So, for example f/1.8 will have more “bokeh” than f/22. You might use a low f/stop like f/1.8 if you wanted to create an emotional image with sole focus on the subject. Whereas f/16 or f/22 for landscape images where you have really close foreground elements and far background elements. If you are not sure which f/stop you should be at just keep around f/5.6 to f/8 (or 2 stops above the wide open for more experiences photographers) as this is where your lens tends to be sharpest.
So, we have discussed 2 elements of the exposure triangle. The third is shutter speed. The shutter of a camera is a thin open slit that runs across the sensor when you push the shutter button. A faster shutter speed means that this slit runs over the sensor quicker than a slow shutter speed. For example, 1/1250 sec is a lot faster than 1/30 sec. So, when would you use different shutter speeds? For shots where you want to freeze motion such as sport or wildlife photography you want to use a faster shutter speed. However, there is a trade-off, a faster shutter speed means that the sensor is exposed to the outside world for a shorter amount of time so less light is gathered forcing you to increase the ISO and thus more image noise is present in the final image. Slower shutter speeds such as 1/10 sec allow you to take clean images in dimmer environments like inside a house at night. Even slower like 30 seconds allow you to take beautiful images of a scene only lit my moonlight or the stars. Anything below 1/30 sec you really want to be using a tripod as your hands cannot hold your camera still for that length of time. This is called camera shake and will result in un sharp images. A general rule of thumb is called the reciprocal rule which tells us that you should be shooting at the reciprocal of your focal length (and a bit more). For example, if you are shooting at 50mm you would want to be at about 1/60 sec where as if you were at 450mm, if you were shooting wildlife, you should at least be at 1/500 sec. As you get more into photography and the science of it you begin to realise that the reciprocal rule becomes less and less useful, but for a beginner or the casual shooter you should be fine following the rule. A good shutter speed for general shooting is around 1/60 sec, this should usually be fine for your out and about quick snaps.     
So, what have we learnt? That camera auto exposure systems are rubbish? No. You generally want to let the camera do most of the work. People generally think that good and experienced photographers work just in manual mode and nothing else. That is wrong. They will change the mode depending on what they are shooting. For example, if you are shooting in a situation in which you need a fast shutter speed such as sports of wildlife just put your camera in shutter priority mode, choose a fast shutter speed and the camera will decide the aperture and ISO. For any other type of photography, I would keep my camera in aperture priority mode so I can quickly switch between the emotional “bokehlicious”, subject focused shot or the all-in-focus landscape shot.   
The best tip I can give you is to just play around with the exposure settings of your camera, if you get it wrong, so what? You lose that shot but you will learn from your mistake and hopefully, you won’t make the same mistake again!

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