Long exposure photography is the technique of keeping the shutter open for a longer than usual duration and capturing an image. Usually, when you press the shutter release, the shutter curtains inside the camera open and close extremely fast. This depends on the shutter speed you’ve selected, or the camera selects based on the ambient light and the other parameters – aperture and ISO.
Let’s say that the shutter speed is 1/100 sec. That means the shutter curtains open and close at a speed of 1/100 of a second. That’s pretty fast. But there are faster shutter speeds, too, such as 1/2000 sec and higher!
One thing is clear, though. You cannot shoot a long exposure photo using such fast shutter speeds. You need to slow the shutter speed to something like ¼ sec or slower. Some long exposures last for several seconds, and some long exposure photos can take several hours to shoot!
Anyways, coming back to the point, it’s ordinarily difficult to shoot with a slow shutter speed during the day. This is because, during the day, there is a lot of light around and if you use a slow shutter speed, you risk burning your exposures. In other words, your exposures will be blown away, and there will be no discernible detail in them.
Photographers use a Neutral Density (ND) filter to negate this situation. An ND filter is like a sunglass for your camera. It screws onto the front thread of the lens. Its job is to stop the amount of light that passes through. They come in various strengths of light-stopping power.
A one-stop ND filter is the most basic ND filter, it halves the amount of light entering a camera. So, if a normal exposure takes 1 second, that same exposure will take 2 seconds with a one-stop ND filter attached to the front of the lens.
Similarly, with a 2-stop ND filter, that same1 second exposure will take 4 seconds. So for every incremental light stopping power stop, the exposure time will keep doubling.
The highest strength of an ND filter is 10-stop. With that ND filter on the normal exposure that took 1 second will take 1024 seconds to complete! That’s a whopping 17 minutes!
By now, you may have guessed this – all those breathtaking photos of creamy mushy waterfalls, brooks, seascapes, etc., all these images are shot using an ND filter.
You can also shoot daytime exposures using a long shutter speed. For example, you can use a 5 stop ND filter and shoot a busy tourist spot; the image will be devoid of people. Because the people moving will not register in the exposure, which will be several seconds long.
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