This lesson is equally relevant to both the Sony a7R II and a7S II.
Before we can look into ND filters, we’ll need to know what kind of ND filter to buy. The first important classification, one which certainly affects the price, is whether or not we need ‘hot mirror’ or IR ND filters.
Every camera has some form of IR (infrared) pollution. Many cameras have IR blocking filters right in front of the sensor to mitigate this (and to protect privacy). Under normal lighting conditions the infrared radiation passes on to the sensor along with visible light, and nobody notices anything.
However, when using ND filters (or even other filters that cut light to a certain extent, like polarizers, color filters, etc.), the visible light is cut, but the IR passes through.
This means, when you shoot in sunlight (or tungsten halogen, practically anything that emits heat and therefore, IR radiation) and use a ‘normal’ ND filter, the colors tend to shift due to the IR radiation. This is IR pollution, and the results are undesirable.
This does not happen at all ND strengths, but begin after a certain threshold.
The test for IR pollution
The simplest test for this is to shoot outdoors in sunlight with green foliage, and under tungsten halogen lighting. If there is IR leaking the greens become brown, and black clothing* becomes red-brown.
Here’s a simple result, first with no ND (three different cloth materials):
And the same shot with a 10-stop ND filter, no IR-cut:
The sad thing is, the effects of IR pollution cannot be easily removed by color grading software. Here’s a test I did with the earlier Sony a7S to justify this, “corrected” in Davinci Resolve:
The results are surprising, and very important. You cannot easily eliminate IR pollution effects, especially in the blacks.
Info: Some of the color shifts are definitely due to the poor quality of the ND filters. Some filters are made of resin and not glass, but some are poor glass filters, like this one at only 2 stops:
*Not all black cloth is the same:
- 100% cotton does show the effects of IR pollution, but resists it well.
- Cotton-polyester blends does slightly worse, but is difficult to tell.
- Rayon is worst, and is affected easily.
This is why it’s important to test different cloth types for IR pollution.
So what should we do?
You need to add another filter, called an IR-cut filter. There are different kinds of IR-cut filters, designed to cut slightly different parts of the light spectrum.
I’ll make it easier for you. For the Sony a7R II and a7S II, you need IR ND filters.
What are IR ND filters? These are filters (they don’t have to be ND, but can also be polarizing or others) that also have an IR blocking coating (or whatever) applied to them, so they serve double duty. Typically, they cut from between 680nm to 700nm (the beginning of IR radiation) and beyond. You can also buy pure IR blocking filters.
What’s the difference between IRND and Hot mirror? The end result is the same thing, both stop IR from reaching the sensor, but they differ in their technology. IRND filters, the most common kind, have dyes that “absorb” IR radiation, similar to color filters that absorb light of a particular color but lets the others through. On the other hand, hot mirrors are mirrors – they reflect IR radiation back completely. For this reason, they have mirror-like surfaces, and many say you shouldn’t stack hot mirror filters. The mirror side must face the scene, and be furthest away from the sensor if you’re stacking filters. It goes without saying that you must know which side of the filter is the mirror side, or if both sides are, etc.
Every ND filter shifts color to a certain extent. There have been recent tests with TrueND filters that show minimal color shift. I don’t know if it’s true (pun intended) or not, but both Tiffen and Schneider, the industry standards, have slight color shifts across the image. It is important to distinguish between a color shift and IR radiation. The latter only affects certain aspects of the image, while the former applies a tint across the entire image. A tint is easy to correct, but a color shift in certain regions is extremely hard to fix.
Important! The color tints are not a problem. You must remember this fundamental of cinematography (that many testers on Youtube seem to have forgotten): You must white balance after you put on the IR ND filter. You cannot compare color performance of different IR ND filters with the same white balance!
There is no ‘formula’ to IRND filters. You must test each stop, each filter, each manufacturer, each technology and each sensor combination thoroughly! There is no easy way I’m afraid. Sometimes the results are surprising, especially with cameras that have their own IR blocking filter, such as the Sony a7R II and a7S II.
So, how does the a7R II and a7S II perform?
Very well, actually.
Here’s a test with the Vizelex ND Throttle, at different stops, and you can see for yourself at what point the cloth turns red:
In my eye, the cloth turns red noticeably, second from last. This is about 5.7 stops. At 4.7 steps (the one before it), it’s not so obvious. Therefore, you can say with certainty that the cameras need IR-cut filters once you cross 5 stops of ND.
As a rule of thumb, though, i prefer to use 4 stops. This is how I’ve built my kit. You might always use a camera in the future that has a less powerful IR-filter, like those made by Blackmagic or Red.
So, what happens if you add an IR ND filter? Here’s what happens when you add one at 7 stops:
No more IR pollution effects!
Of course, this kind of result comes at a price. Learn more about which filters I use and recommend in the next lesson.
Takeaways
If there is no sunlight, tungsten light or any IR radiation in the lighting (in short, any light that is accompanied by heat) plain ND filters will work just fine. But we need NDs most under sunlight, don’t we?
Under sunlight or tungsten lighting, there is a noticeable shift. If your talent is wearing synthetic clothing (which a large majority do), then you have no option but to use IRND or hot mirrors starting at 4 stops. If you’re shooting only daylight scenes, I’d say you can ‘get away with’ no IR blocking up to 6 stops, but clearly, the moment blacks become prominent, it starts to show. So, IR filters above 4 stops.
You cannot easily correct it in a grading app, even if you shoot charts.