The exposure triangle is a common way of associating the three variables that determine the exposure of a photograph: aperture, shutter speed, and ISO. One must balance all three of these to achieve a desired result, an adjustment of one requiring adjustments of at least one of the others.
The Aperture
A space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters a camera.
The ISO
ISO is the sensitivity of your sensor to light. The ISO setting you use depends on the amount of light in the scene you are photographing. The more light you have to work with the lower you can set your ISO. As settings go, ISO is one of the key camera menus you need to learn.
What are ISO camera settings and what does ISO mean? ISO stands for International Standards Organisation and it refers to the industry norm for sensitivity of emulsion based film, with 100 ISO being not so sensitive (and the standard ISO used by most people) to 1600 ISO which is extremely sensitive to light.
How does ISO affect a photo? ISO is one of three determining factors of the exposure of a photo, along with aperture and shutter speed. These two affect the lens and exposure time respectively, with the ISO affecting the sensor (or film). To be more specific, the ISO determines how well exposed a photo will be by changing the sensitivity
The shutter speed
Shutter speed is the nominal time for which a shutter is open at a given setting.
What happens when you change the shutter speed? If you are using aperture priority mode and dial the exposure down 2 stops for example, the aperture will stay the same but the shutter speed will change to a faster speed so that less light enters the camera and the picture is made darker.
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I've used different apertures, shutter speeds and iSO for each picture. Some of them are overexposed and some are underexposed, some look more better than others.