Let’s talk about ND (neutral density) filters and still photography. ND filters are like sunglasses- they restrict light getting to your sensor. That’s it. They don’t change color. They stop light. While it is beneficial in some ways, it could hurt in others- for example, in aerial photography. First- extremely brief explanation of how your camera’s light sensor works. An exposure is made up of three things- shutter speed (measured in time open, in fractions) ISO (the speed of the film,) and aperture (how big the hole is that lets light in, measured in f/stop.) Balancing those will give you (what the camera believes) is a good exposure. So if you take away from this one, you have to add to this one. While this works well most of the time, other times you want to get creative. They also have their downfalls- shutter too long, you can induce shake. ISO too high, you can get grain. Aperture too low, not everything is in focus. Here is how it’s helpful. I took picture number 1 with my DSLR.
I wanted to get that blurred water effect on the waterfall. So I used a ND 4 and a very high aperture. This made my shutter speed extremely long. Had I not used a ND filter, I wouldn’t have been able to keep the shutter open that long to give the blurred water look. So how does it hurt? Move onto picture number 2.
I took my P3P and set it to auto. The light meter in the camera read this scene as 1/25th, ISO 100, and f/2.8. The aperture (f/ number) is set to f/2.8. You can’t change it. So the only two we can use is speed and aperture. Next for picture number 3,
I put on an ND 4. The P3P saw this scene as 1/12th, ISO 168, f/2.8. So to compensate for let light hitting the sensor, it reduced the length of time it was open and kicked up the ISO. Picture number 4 is our base line of our original shot-
I put this on manual and shot it the same as number 2. It is at 1/25th, ISO 100, and f/2.8. You can see it is darker. Finally, look at picture number 5.
I put on a ND 8 for this shot. I let the camera choose what it thought the scene should be. It chose 1/12th, ISO 431, f/2.8. It really hit up ISO in this case. Picture 6 is again our base line- shot in manual at 1/25th, ISO 100, and f/2.8. You can see what the ND filter does. So what the heck does all of this mean? When you take a picture, you want to keep the camera as still as possible. While the P3 is incredibly stable, it is still a vibrating hovering mess of goodness up in the air. How can we help it take better pictures? Well, the faster the shutter, the less we are going to induce shake. So we want a high shutter speed. How do we not make it grainy? We want a lower ISO. Sticking an ND filter on with change one, if not both, of these settings. This does not help when we take still photos. I would strongly suggest you only use them for video. I hope this explanation helps. If anyone wants to add to this, feel free. I am by no means an expert; I just like taking pictures. There is much, much more to the application of the exposure triangle, but I wanted to keep this already comically long explanation as short as possible.
Thanks to Jim Messerfish. Who submitted this Tutorial.