Why moving subjects are usually blurred in photos, but not always?

Question

Why moving subjects are usually blurred in photos, but not always?

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Answer 1

Assuming you are talking about motion blur (where a moving object in a photograph appears blurred), rather than something just being out of focus.

It comes down to the camera settings used for that particular photo. To get a good exposure, you have to have the right balance of shutter speed and aperture to allow just the right amount of light into the camera. You can alter the two, and still get a good exposure, as long as the right amount of light gets in – but the effect on the image will be different.

Let’s say you use a wide aperture setting (this means the hole through which the light passes to enter the camera is open wider). Because a wide aperture allows more light in, you don’t need to have it open for very long, so you would use a fast shutter speed. If the shutter speed is very fast, you can capture a moving object in sharp detail, like it’s frozen in time.

But now let’s say you are using a much narrower aperture setting. Because less light is getting in, you need to use a slower shutter speed (so the shutter stays open for longer, and more light comes in). Because the shutter is open for a longer time, your picture will record moving objects as they move across the field of view, and they will leave a trail or look blurred.

You can think of the relationship between aperture and shutter speed like turning on a tap to fill a bucket of water. If you turn the tap on hard (wide aperture), you don’t need as long to fill the bucket. But if you only turn the tap on to allow a trickle of water, you need more time to fill the bucket with the same amount of water. By opening the aperture wider or narrower, you need less or more time to let the same amount of light into the camera.

There are other effects as well – for example, having a narrow aperture allows you to have everything in both the background and foreground in sharp focus, while having a wider aperture allows you to focus on one point of interest and have the background blurred. But for moving objects, it’s more about the shutter speed.