ND filters cut light entering the lens.
To fully answer your question "What do they do, that manually changing the exposure wouldn't also do?", you must understand the basics of what "exposure" is and what settings you have on cameras to adjust that exposure.
There are three things that control exposure in a photograph (applies to video too) and each one has at least one alternate effect that they have as well:
1. Shutter speed - Motion Blur & ratio of strobe to constant light
2. ISO - Noise
3. Aperture - Depth of Field & Defraction
There is no ONE correct exposure setting. There are infinite ones (well infinite with the granularity of what you can set in a mechanical device). Imagine that proper exposure falls in the middle of all three (imagine this as a triangle). If you slow down shutter speed (allowing more time for light to enter the camera) then you must either reduce ISO (making the sensor less sensitive) or close the aperture (thus restricting light through the lens). Its still a correct exposure. If you lower ISO then you must either open the aperture or slow speed the shutter down. Still a correct exposure.
But look at the side effects of the three. Doing something to one setting affects one or two of the others.
Having a fast shutter speed allows you to stop action. IF thats what you want. If you want stop a humingbird's wings then that requires a hella fast shutter speed. And that means lots of light because you are only accepting a tiny sliver of time for the sensor to gather light for the photo. So, you have to raise ISO and introduce noise or open the aperture and change the depth of field, or both. However that type of photo makes the bird look like it was static and unmoving with no sense of life. So slowing shutter speed down to purposefully introduce blur evokes a sense of motion in a photo (video).
The other side effect of shutter speed is it controls the ratio of constant light to strobe light. Strobe light is not affected by shutter speed because every modern strobe emits light in far less time than your camera can run a full frame shutter at (my einstein strobes run at 1/10,000th of a second or there abouts). Constant light is (sun, light bulbs, etc) affected by shutter speed because its always on no matter how fast the shutter runs at. So by raising shutter speed (making it faster) cuts down constant light but leaves strobe light alone.
ISO controls the sensitivity of the media (film or now digital sensors) to light. The more sensitive it is, the more noise (or grain in film) there is. So generally we want to shoot the lowest ISO for the least noise. Normally. Sometimes a certain amount of noise introduces a "real" feeling to a photo and less like a CGI thing. So night photography is a common problem. There isn't much light to work with. So we raise ISO. And we get noise. To combat that we can slow shutter speed down to allow time for more light to enter the camera. But if the camera moves, blurry photo. Its a give and take.
Aperture controls light as it exits the lens and before the media. Closing it down means less light that strikes the media. Conversely opening it up means more light striking the media. Its main side effect is that it also controls depth of field. When closed, you have a deeper depth of field than when its fully open. A frequent theme in portrait photography is to have the background out of focus while the subject is crisp and clean. To do that you MUST open the aperture up and thus narrowing the depth of field. You can only take ISO so low. That leaves you with shutter. Normally on portraits you are frequently using strobes as fill. That limits your maximum shutter speed to whatever your camera is capable of with a full frame shutter before the 2nd curtain falls.
Most people think there is only one shutter, there isn't. There are two. Two curtains. At least with mechanical DSLR cameras. Electronic shutters are a whole different animal mechanically but work similar. The first opens to expose the media, the second one closes to cut off light and then it resets. At some shutter speed (1/250th on my Nikon) the shutter speed becomes so fast that the second curtain begins closing before the first one completely exposes the media. At that point, you can no longer use standard strobe flashes because the whole frame is not ever exposed all at once. If you shoot at faster than the max flash speed you will have a bright line in the image with one or two darker bands where the curtain(s) were covering the sensor (film) at the time the flash fired.
Enter the ND filter. It cuts light. All light. And thats all it does. So that is the reason for ND filters in portrait photography (and others). It cuts light entering the lens without affecting the depth of field. Or in the case of portraits, it allows you to cut light with an open aperture and still keep the shutter speed below the limit for flash use (commonly used as fill lights in outdoor situations). And since it affects all light (which shutter speed does not), it affects strobe and constant light equally.
Here is an example portrait of mine:
That was shot (obviously) in daylight. I wanted the background out of focus because its noisy and would distract the eye from the subject. Shot at f4 with a ND8 which (just) got me to 1/250th (the max for strobes on my camera). She is back lit and rim lit by sunlight and front fill by three strobes in soft boxes in the front. I could not have shot this without an ND filter. The shutter speed at f4 would have been too fast for strobes without the ND filter since ISO was already at minimum. No strobes and she would be dark because the front of her and the tree trunk are in the shade. If I exposed for her in shade then the background and her hair would have been blown out. If I closed down the aperture then the background would be more in focus. So the only choice to keep the background out of focus, use fill strobes and keep the shutter speed slow enough for strobes was an ND filter.
Here is another example:
This time shot at f/10 at 1/800. Because I wanted no texture at all in the sky, I need to open the aperture up a bit. Also, shooting into the sun with a closed aperture would introduce unwanted defraction. This time I was using reflector panels instead of strobes. This was shot with ND8 also. This time for human reasons. Because cameras typically do not show you the image through the optical viewfinder with the aperture in its set position (f/10 in this case) but with it wide open so you can see the brightest image, it was almost impossible to look through it to frame it. So I popped on ND8 so I didn't burn my retina trying to take the photo. This got the aperture opened up a bit and prevented damaging my eyes.
Another factor involved is lens sharpness. Most lenses are sharpest at 1 or 2 stops from wide open. They commonly fall off at wide open and sharpness suffers when the aperture is fully closed. Sharpness is not the same as focus so don't confuse the two. If you focus close on an eye with the lens at near wide open, the eye will be the sharpest. Focusing on the same eye with the aperture closed (probably f/16 or f/22 depending on the lens) the eye is still in focus but not as sharp. And yes, the rest of the face will be more in focus than if shot wide open. Thats depth of field.
In any event, the Phantom 3 series has a fixed aperture and it can't e changed. So that only leaves you with 2 of the 3 points of the exposure triangle to work with. Assuming you are shooting daylight your ISO is probably the lowest it can go at 100. So now you are down to only one exposure point to change, that being shutter speed. Video shot at high shutter speeds (not to be confused by frame rate) looks too crisp to many. We expect a slight motion blur as we have spent a century of being accustomed to it in film shot at 24fps and usually lower shutter speeds due to film's lack of high iso performance and the amount of light needed to light entire scenes. So shooting video at 1/2000th shutter speed simply looks wrong to many folks.
So, given that a typical scene shot from a Phantom has only one possible exposure point to work with (shutter speed), ND filters are the only solution to cutting light entering the camera so that a slower shutter speed can be achieved to reintroduce at least some motion blur back into a video.
Last edited:
