Focus points and metering #photography
The focus points on a camera are used to focus on your subject but they are also used for metering.
My Nikon D3200 has 11 focus points and they are useful but I often only use one. Some cameras have a lot more focus points and they cover the whole frame.
Single focus point
This wasn't the best picture I took last night but it demonstrates my point. I took this photo of Santa's sleigh on a very dark street using just a single focus point on the illuminated sleigh. The metering is 'through the lens' (TTL) and so the flash was adjusted based on the light from the sleigh not across the whole frame which you get on matrix metering (or on auto).
You can experiment with a single focus point. Get your focus point on part of your subject that is white and the camera will adjust for the extra light reflected by the white or focus on something black and you get the opposite effect. So you need to focus on a colour somewhere in between. When you're focusing on a person of a group of people with a single focus point then skin colour is a good place to focus.
Using all the focus points
If you're taking a photo of a group of people or objects, then using all the focus points will help you get them all in focus. If you get the middle of your subject in focus then moving your camera very slightly will get the rest in focus. With a group of people, if the middle person is in focus, then the rest will be in focus if they are the same distance from the camera so you can arrange your group accordingly or judge how far they are going to be out of focus. Having the people at the end slightly out of sharp focus isn't usually a problem.
This is the picture of the Rotary club's Santa that I wanted. He even stood up and waved. I did have a diffuser filter fitted and so the image looks quite soft. Notice the flash doesn't light up the background. A few minutes earlier my flash had lit up the whole street at full power, as I waited for the sleigh to arrive, it looked like a lightning strike! I did shoot a little video but that wasn't very good. A DSLR is quite heavy with a 105mm lens fitted. I prefer to shoot video with my 35mm prime lens or using a monopod.
Light and dark
Remember too, that the camera looks for a difference between light and dark when it focuses. I tried to take a picture in the dark and the camera just saw black and wouldn't focus, so I moved the single focus point close to the light from a street lamp and it focused and the flash fired to light up the whole street based on a dark meter reading. Even in landscape photography, you can choose a point using a single focus point that is close to the edge of light and dark to get a sharper image. If the leaves on the trees are blurring into one, a single focus point focused on the edge of the leaf can give a better image.
I hope you now understand that focus points aren't just for focusing but also for metering.
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