MenuSo what is this Trinity I’m talking about? As you can see in the diagram there are three variables what these variables do is well defined.
First up is Aperture. At least for those who work with landscapes, portraits and other slow moving events, it can also be used for sports or bar gigs in low-light conditions. So what is the aperture? Here’s a simple experiment I want you to close your eyes and gradually open them. You noticed the wider you opened your eyelids the more light that you saw? That’s similar to the aperture, the wider the aperture the more light that you can see. There’s a catch, and this is where the negotiations come into play. Aperture is primarily about Depth of Field, commonly known as DoF. The wider your aperture the less scene depth will be in focus. This is a very complex topic but in essence if you’re looking for that dreamy out of focus scenes then you’re looking for telephotos with fast apertures (f/2.8 or faster). If you want to photograph a landscape then you’re looking at f/8 to f/16. Beyond f/16 you’re going to hit microcell diffraction a topic for another day.. Lens type will also have an effect on DoF such as Wide Angles have a wider DoF than Telephotos. There are numerous DoF calculators available online to work out the maths behind all this.
Second is shutter speed. This variable is about capturing speed and stopping action. It’s also about making sure you avoid blurry shots from slower shutter speeds. Cameras have the ability to open and close a shutter extremely fast, some going up to 1/8000th of second or faster. However unlike DoF you have two complications to negotiate. First is hand shake, no this isn’t a friendly how do you do it’s the main reason for blurry shots. Hand shake occurs when your shutter speed is too slow for the light conditions. For example I can hand hold a 50mm lens 1/60th with little problem but my keeper rate drops quickly when I hit 1/30 or slower. The rule of thumb is to handhold a lens you keep your shutter speed equal to or faster than the focal length. Remember to factor in your crop factor to the equation! Alternatively you could side step this point by using a tripod. The second point to negotiate is the amount of light you can see. Imagine you are on a street corner at night and a car turns round the corner slowly, it illuminates your area for a long period, however when a car comes quickly round the corner then light is there for less time. So shutter speed is a balancing act of light and freezing movement. Also with studio lights and flash guns you will often see something called Shutter Sync, this is the maximum speed you can go before you get what’s called Shutter Creep. Shutter creep is where the lower third of the frame is darkened, or has a black bar.
The final option is ISO this should always be chosen last. Out of all the variables this has the greatest impact on image quality. Simply put the higher the ISO the more noise/grain you will get. It also mean the images will fall apart quicker when retouching as there is less data in the channels to work with. The only time you will need to ramp up the ISO is when you fail to negotiate with the Shutter Speed and Aperture to get good exposure. The other downside of ISO is that the images become softer and lack as much detail as you would get with ISO 100.
To summarise in essence:
Aperture is about how far can you see and how much.
Shutter speed is about image sharpness and avoiding motion blur.
ISO is like going on a carrot diet to see better at night, but just like seeing at night you see less detail.
Of course all this is is simplification of the exposure process but it's all you really need to figure out the basics.
An edited version first appeared on Blueprint.
For discussing or debating Exposure and how to deal with difficult scenes please read and contirbute to this thread.
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