If you look at the exposure triangle, there is one tool that photographers feel is the most important of all and that is the aperture(the other two are shutter speed and ISO). Many photographers tend to control the aperture most of the time and leave the shutter speed and iso to the camera.
What is Aperture
Aperture is the opening in the lens that lets light enter and fall on the sensor. It can be adjusted to let the correct amount of light fall on the sensor. This also has a big impact on your depth of field and your bokeh. To know more about what is the depth of field and bokeh click here.
There is an iris in the lens that helps the aperture to open and close. The larger the opening the more light enters and the smaller the opening lesser the light that enters the camera. This is how the aperture affects the exposure of the image.
Note: Different lenses can have different aperture settings, pay close attention to this when buying a lens. It is written as f-stop or f-number, for example, f/2 or f/8. You will see this number written on the lens. But when reading your f-stop value you should understand that the smaller the value the larger is the opening and the opposite is also true.
Fast lens and Slow Lens
These two terms can be deceiving. Why? The word fast and slow does not have anything to do with your aperture instead it is your shutter speed. A fast lens is a lens that has a very low aperture which can be below f/3 or f/3.5 the lower the value the faster it is. Why am I mentioning fast? Because the lower the value the wider you open this allows more light to come in, hence you need to use faster shutter speeds to compensate for the exposure.
The exact opposite is true for the slow lens. The reason why you should know this is because it will help you with the next purchase of your lens and what kind of lens you will require in which situation.
As mentioned earlier the aperture priority mode is the mode on your camera that lets you control just your aperture and the shutterspeed and ISO is left to the camera. This is a semi-automatic mode that enables you to control only the aperture of your camera. If given an option to control any one of the options it should be aperture (for most of the cases anyway).
How aperture affects Depth of field
The most predominant effect of aperture on your image other than exposure is the depth of field. The depth of field is the region in front of your camera that is acceptability sharp. Any object not in the depth of field gets blurred out.
When to use a lower F-stop and when to use higher F-stop
The most important question in your mind will be when will you use higher or lower F-stop values.
The first thing is exposure if you need more light to come in or faster shutter speed you will want a lower f-stop value or when you want lesser light to fall on your sensor you will go with a larger f-stop value. But remember making these kinds of changes can have an adverse effect on your depth of field.
Depth of field
When you want a shallower depth of field say as in a portrait shot or something like that, then you will go for a to with a lower f-stop value, which will help you get better bokeh background and foreground. But when you want an image to have a deep depth of field then you could go for a higher depth of field say landscape photography.
Tip: When you have a lower f-stop value (your aperture is wide open) take extra precaution on where you are focusing, since if your focus point is not perfect then you could ruin your image where the point where you want to focus could go out of focus.
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