Guide to ND Filters to use on the Sony A7s

There are two parts to this question:

  • What ND filters for non-S-Log2 modes?
  • What ND filters for S-Log2 mode?

ND Filters for non-Slog-2 modes

The Sony A7s records 1080p video in Rec. 709 under an ISO range of 200-102400 (expandable to 409,600). 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. If you’re staying under 6 stops, you won’t need IRND or hot mirrors. However, if you’re shooting between 6-8 stops over, then you need them, no doubt.

Specific recommendations are at the end of this article.

ND Filters for Slog-2 mode

In S-Log2 mode, the lowest ISO is 3200. You can’t go to ISO 1600 or 800 or 400 or whatever. You must start at ISO 3200. Here are some common lighting scenarios and f-stops at ISO 100, and what ISO 3200 does to it (Shutter is at 1/50s):

ISO 100 ISO 3200 ND for f/2.8 @ISO 3200 ND for f/1.4 @ISO 3200
Sunny 16 rule (bright outdoors) f/32 f/196 3.6 4.2
Studio environment (1000 lux) f/2.8 f/16 1.5 2.1
General office ambience/golden hour f/2 f/11 1.2 1.8
Moody home ambience f/1 f/5.6 0.6 1.2

We also need to consider the wolfcrow system of exposure, which is to expose at 3 stops over middle grey. If that’s the case, here’s what our chart looks like:

ND for f/2.8 @ISO 3200 ND for f/1.4 @ISO 3200
Sunny 16 rule (bright outdoors) 2.7 3.3
Studio environment (1000 lux) 0.6 1.2
General office ambience/golden hour 0.3 0.9
Moody home ambience n/a 0.3

The low light ability of this camera is phenomenal (Check out the results here). If you want the shallow depth of field aesthetic, you have no choice but to use ND filters. Most users will be satisfied with 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 five or less filters and then stack filters.

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
  • 4.8 – 16 stops; 17-31 stops stacked

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

Refer to the previous lesson to see what I mean.

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. I have found you can comfortably shoot at ISO 3200 (the best), 6400 and 12800. You can stretch to 20000 if you like, but I wouldn’t advise it unless you’re desperate. 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 stop down by 3 stops, and then open the ISO by two stops. This way, you can stop down from 1 to 16 stops with just three filters!

If you just want to stay within the 3200 to 6400 ISO range, then you’ll need four filters:

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

If budget is a concern, there’s nothing wrong with option one. What tilts the balance is the fact that:

  • You’ll need the ND filters for future cameras as well, and
  • You get a total of 10 “steps” with four filters, while only 6 “steps” with three filters.

What am I planning to do personally? I’m going to get 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

I get all the steps from 1 to 12, and then 14 and 17 as well for those really rare scenarios. I get 14 “steps”, and I’ll never have to stack more than two filters. Therefore, as a recommendation, I suggest you get this kit (if you’re not going for a variable ND solution, that is).

It goes without saying that all of my filters will be IRND or hot mirror filters. I must test further, because these are expensive things! If you want me to test Tiffen filters, I’ll do so, but I must get enough requests!

Recommendations come next.

ND filter suggestions

There are three options:

  • Variable ND filters
  • Threaded ND filters
  • Square filters (4×4, 4×5.65 or 6×6)

Variable ND filters

I recommend the following:

As you can see, these are not cheap. If you’re getting 6 stops for $444, that means you’re getting 1 stop for $75. I will never recommend cheap vari-ND filters. Variable ND filters are more convenient to carry, and you don’t need to change them around as much. You could buy one large size and then use step down rings for the rest of your lenses.

Here are four choices in the 82mm size. Buy one and be done:

Threaded Circular ND filters

The good ones are not cheap. Hoya makes cheap ones that are passable, but I can’t recommend them for professional work.

For Non-S-log2, we can have three filters – 2, 4 and 7 stops. I recommend you buy one large size, 82mm, for example, and then use step down rings for various lenses:

For S-Log2, here are the four options: 2, 4, 8 and 10 stops (notice how it’s not very different from the previous choice? But these need to be IRND filters):

One way to mitigate cost is to buy just one IR blocking filter, and then put all the ND filters behind it. Test!

Square filters

This one is easy. Go Tiffen or Schneider:

These expensive filters are used with matte boxes. They are made of high-quality glass, are thick, and offer the best possible image quality. Test! You can’t afford to buy expensive filters without testing. Remember, every combination is different!

That’s it. I hope this makes your filter buying easier. Or did I make it harder?

Click on the link below to the next lesson or head over to the main menu (above). If you need help with something, feel free to send me an email. I’ll try to get back to you as soon as possible.

FAQs

These are important topics raised by subscribers that shed more light on this lesson.

Q. How do you get an ND filter on a Samyang 14mm?

A. Check out the Samyang SFH-14 14mm Filter Holder!