Good question! Exposure relates to how much light you let into your photo. Too much or too little is inadequate for a good photo
Check the two images below: one has too little light and the other has just about good light and it’s the same location
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Th impact on quality is clear. In photography you want a phot that has good exposure, not too little but enough for the purposes of your photo.
How can you make tour 2mp…there’s little you can do seeing as most phone cameras are auto, they self adjust for exposure when you select a focus point. However, some phone cameras allow you to adjust exposure which can be great e.g an iPhone.
ISO usually works together with exposure and is especially useful at night when you want to increase exposure of your photo. However too much ISO (e.g 1600) makes your picture grainy. So you’ve got to get a good balance between ISO, exposure, and shutter speed to reduce the grains and still get a properly exposed photo.
You have to remember that exposure is how wide the shutter opens (usually denoted with an f e.g f14)
What you are talking about (which ranges from -3 to +3) is exposure compensation. It boosts or reduces the amount of light that can enter the shutter. Though it could be kwa phone that is what they have for exposure.
A negative (e.g -3) will significantly reduce the amount of light entering your shutter especially if combined with a high shutter speed. Such a setting would allow you to take a photo of the sun by blacking out the sky and remaining with only the sun. Such a photo would would look like its night. I once uploaded photos of the lunar eclipse sometime ago; they are a good example.