This guy here:
pretty much knows all about the use of filters, explained in a simple way. Now - he's a Phantom flying guy, but his explanation goes for the use of ND filters in general:
" Start at ISO 100 on a sunny say. Set WB to daylight or sunny. Put an ND16 on and bring up your histogram. With camera on Manual and shutter speed set to 60 (or 2x your frame rate), adjust your f-stop for a good even distribution. Your exposure should be in the neighborhood of f5.6 with this setup. +/- astop or so is fine depending on your lighting.
Our Phantoms have basically three settings:
ISO - Camera sensitivity to light
Shutter Speed - length of time your ‘shutter’ is open. 60 = 1/60th of a second.
F-Stop - essentially the iris of your camera.
We’ll talk about all these and how they relate to each other.
The ISO tells the camera how sensitive it is to light. The higher the number, the more sensitive. ISO 100 is the bottom setting. It’ll give you the best quality results as far as noise in shadow detail. If at all possible, you should strive to start with this setting. If there isn’t sufficient light (i.e., you can’t get the shutter speed you want or some other reason) you can adjust this upwards. ISO 200 is twice as sensitive as 100. ISO 400 is twice as sensitive as ISO 200 and four times as sensitive as ISO 100; and so on. There is a trade off. The higher the ISO you go, the faster the sensor has to collect light and the more noise is introduced into the darker regions of your image. Only your eye can be the judge if it’s suitable. On some occasions, you may want to set a very high ISO on bright days to allow for a very fast shutter speed. This will allow you to stop action. Noise wouldn’t be as big a factor as you’re shooting in a bright environment.
The shutter speed simply is the length of time the sensor catching light. The speed is actually the reciprocal of the number you see on screen, i.e. 200 = 1/200th second. 1000 = 1/1000th second. The higher the number, the shorter (faster) time the ‘shutter’ is open. You’d want a fast shutter to stop action. A slower shutter to create blur or to use a smaller f-stop.
The f-stop is the iris of your camera. In Auto mode, the computer determines what it thinks is the best combinations of settings to get an exposure. It adjusts all these parameters and comes up with an EV value. This value is displayed on your screen as well. The f-stop value starts at f2.8. This is the widest opening your camera can have. It lets in the most amount of light for a given ISO and shutter speed. When you set the f-stop to f4, you let in exactly 1/2 the amount of light as f2.8. An f-stop of 5.6 lets in HALF the amount of light as f4 and ONE QUARTER the amount of light as f2.8. An f-stop of f8 lets in HALF the amount of light as F5.6 and ONE EIGHTH the amount of light as f2.8. F11 = 1/2 the amount of light as f8 and ONE SIXTEENTH the amount of light as f2.8. You can see the relationship in the little graphic I attached (see attached).
If you’re starting to see a pattern here, you’re right. Let’s look at some examples.
For the sake of argument lets say you have ISO set to 400. Shutter speed set to 800, f-stop set at f5.6. We’ll call this an EV of 0.
You want to change the shutter speed to 400. That lets in twice as much light because it’s 2x slower. To keep the same EV=0, you’d have to change the f-stop to f8, which lets in half as much light as f5.6. (Yes you could also have changed the ISO to 200 but it’s good practice to keep the ISO fixed for each shoot). If you wanted to change the shutter speed to 200, you’d have to change the f-stop to f11, again as 1/200 is twice as long as 1/400th, letting in twice as much light, thus f11 lets in half as much as f8 and your EV stays at EV=0.
Well what now if you want to get to a shutter speed of 100 (or 60?) Remember cinematic blur? F11 is the uppermost (smallest opening) f-stop. Since we can go any further on the f-scale (we could change the ISO in this instance to 100, letting in 1/4 the amount of light via sensitivity), this is where ND filters come in. An ND4 filter lets in 1/4th (2 f-stops) the amount of light than without. An ND8 lets in 1/8 (3 f-stops). ND16 (4 f-stops) and ND32 (5 f-stops)
In this instance, we could use an ND16 filter, set our shutter speed to 50 (or 60) and leave the f-stop at f11 keeping our EV=0. The ND16 effectively reduces the amount of light coming in the camera to give us more latitude to set our camera for our creative needs.
There are other reasons in photography to use a higher (smaller opening) f-stop that affects some of what we do but not as much as a handheld still camera. It’s called depth of field. The smaller the opening of the iris, the more light refracts and the greater the depth of field. Let’s say you take a picture of your loved one, and you focus on their face. At f2.8 Their face is nicely focused and the background beautifully blurs into a colorful, non-distracting canvas. If you used f-11 (or f16-f22 on those type cameras) the background would be in sharp focus. Maybe what you want, but maybe there would be something like a trashcan you didn’t see in the background demanding to be emptied.
On your Phantom, I’ve found that if you keep the f-stop at around f5.6, you have two stops up and down latitude while in flight. It’s normally the sharpest area of the lens and your depth of field in the air will normally be suitable to keep from constantly focusing as you fly. Also note, the Phantom allows you to adjust the f-stop in 1/3 increments. If you see EV +.3 or EV -.7 you’re 1/3 stop over exposed in the first example and 2/3rds stop under exposed in the second. It’s simply giving us more creative latitude to work with.
Remember, Strive for ISO 100. Use ND filters to get your shutter speed to 2x your video frame rate and an f-stop somewhere around f5.6. Look at your histogram to ensure you have a good distribution of light-dark (you can see this even in bright light) and you’ll be happy with nice even exposure."
Also, see his video on using the histogram here: