There are two parts to this question:
- What ND filters for non-S-Log2/S-Log3 modes?
- What ND filters for S-Log2/S-Log3 modes?
ND Filters for non-Slog-2/S-Log3 modes
The Sony a7S II 
Here are some general lighting scenarios from real life (Shutter is at 1/50s):
- Sunny 16 rule (bright outdoors) – f/32 @ISO 100, f/2.8 only possible with an ND 2.1 (7 stops) filter
- Studio environment (1000 lux) – f/2.8 @ISO 100
- General office ambience/golden hour – f/2 @ISO 100, f/2.8 @ISO 200
- Moody home ambience – f/1 @ISO 100, f/2.8 @ISO 800
As you can see, you will probably never need more than 8 stops, which is why the following cameras have these built-in ND filters:
- Sony FS7 – 2, 4 and 6 stops
- Canon C300 – 2, 4 and 6 stops
- Arri Amira – 2, 4 and 7 stops
- Arri Alexa XT – 1 to 8 stops
If the aperture isn’t a concern, in bright sunlight, you can get by with a 2-stop filter that will let you shoot at f/16 – which is what most lenses top out at.
Therefore, you can live with ND filters in the 1 to 6-8 stop range. Specific recommendations are in another lesson.
ND Filters for Slog-2 and S-Log3
In log modes, the lowest ISO is 1600. You can’t go to ISO 800 or 400 or whatever. You must start at ISO 1600. Here are the same common lighting scenarios and f-stops at ISO 100, and what ISO 1600 does to it (shutter is at 1/50s):
| ISO 100 | ISO 1600 | ND for f/2.8 @ISO 1600 | ND for f/1.4 @ISO 1600 | |
| Sunny 16 rule (bright outdoors) | f/32 | f/128 | 3.3 | 3.9 |
| Studio environment (1000 lux) | f/2.8 | f/11 | 1.2 | 1.8 |
| General office ambience/golden hour | f/2 | f/8 | 0.9 | 1.5 |
| Moody home ambience | f/1 | f/4 | 0.3 | 0.9 |
We also need to consider the wolfcrow system of exposure, which is to expose at:
- About 2-3 stops over middle grey for S-Log2
- About 1-2 stops over middle grey for S-Log3.
For simplicity’s and safety’s sake, I’ll calculate for 2-3 stops over. If that’s the case, here’s what our chart looks like:
| ND for f/2.8 @ISO 1600 | ND for f/1.4 @ISO 1600 | |
| Sunny 16 rule (bright outdoors) | 2.4-2.7 | 3.0-3.3 |
| Studio environment (1000 lux) | 0.3-0.6 | 0.9-1.2 |
| General office ambience/golden hour | None-0.3 | 0.6-0.9 |
| Moody home ambience | None | None-0.3 |
If you want the shallow depth of field aesthetic, you have no choice but to use ND filters. Ideally, you will need a set of 1-10 (0.3 to 3.0) ND filters for video work in S-Log2 mode.
How to pick ND filters for S-Log2
There are three ways to go about buying ND filters:
- Buy one or more variable ND filters to cover the entire range. These are circular threaded filters.
- Buy filters in one-stop increments over the entire range. In this case, you’d need 10 filters to cover the 10-stop range.
- Buy four or less filters and then stack them.
Stacking works like this:
- 0.3 – 1 stop
- 0.6 – 2 stops; 3 stops stacked with above
- 1.2 – 4 stops; 5, 6 and 7 stacked with above
- 2.4 – 8 stops; 9 to 15 stops stacked with above in various combinations
It sounds cool, except for one major drawback – if you stack more than two ND filters, you are likely risking image degradation:
- Internal reflections from various filters
- Vignetting
- Loss of resolution
- If you go above a certain threshold, you risk IR pollution (next lesson)
One way to eliminate these factors is to buy really good filters. The second method is to get them as close as possible to the lens with good quality matte boxes or filter hoods. The last system is to use the right size filter and mattes so vignetting is minimized.
Most cheap filters are made of some form of plastic or resin. The really expensive ones are made of glass. If you stack ND filters and don’t want image degradation, you must stack glass filters.
In any case, it is unadvisable to stack more than two ND filters. I’m going to go with this rule of thumb.
Before I give you my advice, let’s also consider ISO. In many cases, you can bump up the ISO rather than remove ND filters, and this is what lets us stack filters. In our lesson on ISO grouping, we found we can comfortably shoot at ISO 1600 (the best) all the way to 12800. Here’s what you end up with:
| Optical Density | Stops | Stops you get by stacking |
| 0.9 | 3 | n/a |
| 1.8 | 6 | 9 |
| 3.0 | 10 | 13, 16 |
| Total Range | 1 to 16 stops | |
How does this work? What if you need to stop down by one stop only? You ND by 3 stops, and then open the ISO by two stops. Same applies for other ‘gaps’. ND to a higher number, and then open the ISO by one or more stops. This way, you can stop down from 1 to 16 stops with just three filters!
If you add one more filter, you can stay within the 1600-3200 range:
| Optical Density | Stops | Stops you get by stacking |
| 0.6 | 2 | n/a |
| 1.2 | 4 | 6 |
| 2.1 | 7 | 9, 11 |
| 3.0 | 10 | 12, 14, 17 |
| Total Range | 1 to 17 stops | |
Not only does this give you greater range, but it would allow you to continue shooting at ISO 1600-3200 permanently, for best image quality, even using the wolfcrow system.
What if you go to five filters:
| Optical Density | Stops | Stops you get by stacking | |
| 0.3 | 1 | n/a | |
| 0.6 | 2 | 3 | |
| 1.2 | 4 | 5, 6 | |
| 2.1 | 7 | 8, 9, 11 | |
| 3 | 10 | 11, 12, 14, 17 | |
| Total Range | 1 to 17 stops | ||
Here, you have the option of sticking to ISO 1600 for maximum dynamic range all the time. So, between four or five filters, it’s your call.
The best option for the a7S II is to either:
- Buy four or five filter sizes – 0.3 (optional), 0.6, 1.2, 2.1, and 3.0, or
- Buy a variable ND filter that goes from 1 to 8 stops, and a 3.0 filter for those ‘occasions’.
In the next lesson, we’ll take a look at IR pollution, so we’ll have a better idea what type of ND filter to buy.