Using ND filters for iPhone video can greatly improve the cinematic qualities of your video, but bad ND filters can ruin it completely.
You may have heard about using ND filters for iPhone video, but there are many different ND filters available in different sizes, densities, types and prices. It is easy to be overwhelmed by choice, so I’ve put together some information to hopefully make it easier.
The “ND” in the term “ND filter” stands for Neutral Density. The job of a ND filter is to uniformly reduce the amount of light passing through it in order to control exposure.
I don’t want to see you waste money on bad ND filters just to save a few dollars. You’ll have nothing but regret and colors you can’t correct.
A neutral density, or ND filter should reduce all visible wavelengths (colors) of light equally, producing no color imbalance in the image, only a drop in brightness.
IR Pollution and Color Shift
However, camera sensors don’t just pick up visible light, they can also pick up invisible light, especially infra red or IR. Infra red light is emitted by many light sources, primarily the sun, and so most camera manufacturers build IR filtration into their cameras.
If an ND filter only reduces visible light, but lets IR pass, you will likely see color imbalance and muddy brown/red tinted shadows and dark areas in the image. This color shift, and “IR pollution” is very difficult, and most often impossible to correct later in post production.
Depending on the camera, IR pollution may be more or less of a problem, but how uniformly a ND filter reduces the visible wavelengths of light is always important.
iPhone ND Filters FAQ
I’ve tried to answer some common ND filter questions below. If you have a question which isn’t covered, please contact me and I may add it.
There are two common ways that manufacturers use to describe the densities of an ND filter.
One is with a ND number, such as ND2, ND4, ND8 etc. and this refers simply to the ratio of light it will allow to pass. A ND 4 allows 1/4 of the light to pass. This is common with photography filters.
The second is by optical density, this will be 0.3, 0.6, 0.9, 1.2 etc. which is most common for ND filters used in cinematography.
Click here for a table of common ND filter ratings.
Unfortunately you will rarely find actual transmittance data given by manufacturers for specific products. The two most important things that should be mentioned is minimal (or no) color shift, and an IR coating.
Typically the more expensive the filter, the better it will perform, although price isn’t always a determining factor. It is also worth looking for independent user reviews.
Many popular iPhone lenses also have filter adaptors available. A filter adaptor may not come with the lens, and may have to be bought separately. Look for the filter thread diameter that matches the lens filter adaptor.
Click here for a table of common iPhone lens filter sizes.
There are now a few good ND filter systems available for mobile filmmakers that fit directly over the phone camera lens.
I have been using the Polar Pro Iris filter system for some time and love it. I can’t comment on others as I haven’t used them yet.
Take a look at the solutions below.
PolarPro Iris ND Filter System
NiSi P1 Proseries Mobile Phone Filter Kit
When shooting video in bright conditions you will usually aim to keep your shutter speed value at double your frame rate. For example, if your frame rate is set to 24fps, you’ll aim for a shutter speed of 1/48th second.
You can calculate which ND will reduce your shutter speed easily by dividing the auto shutter speed value by two until you reach close to your desired shutter speed. Count how many times you had to divide, and you’ll know how many “stops” you need to reduce. Refer to the table by clicking here.
In bright daylight, you will usually need a 6-7 stop reduction, which is a ND64 (Optical Density 1.8) or ND128 (Optical Density 2.1).
Using FiLMiC Pro:
1. Long press on the exposure reticle (circle shape) to open the manual exposure control, or swipe in from the left side of the screen. In the centre of the arc are two numbers, the top is ISO value, below it is the shutter speed value.
2. Adjust the exposure by moving the left arc up or down. This will change both the ISO and shutter speed in correlation with one another. Adjust until the ISO value is as low as it will go. Tap the ISO value to lock it.
3. Adjust the exposure again by moving the left arc up or down. The ISO value will no longer change, only the shutter speed. Adjust until the exposure on screen looks correct.
4. Read the shutter speed value. Shooting 24fps in bright sunlight, this is usually 1/6144th second.
5. Divide this value by 2 until you reach your desired shutter speed, count how many times you divide. For example 1/6144th divided by 2 is 1/3072, divided again is 1/1536, and you must divide a total of 7 times to reach 1/48.
6. Refer to the table. A 7 stop reduction requires an ND128 (Optical Density 2.1).
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