Making Sense of Variable ND Filters
Filters are one of the most common accessories that people look at when they purchased a camera and/or a lens. There are many different types of filters out there to choose from. The most common ones are the UV filters, the CPL filters, and the ND filters. Sometimes it’s a bit hard for people to make sense of all of them at once, so we’d like to write a few blogs to share what these filters do and how to use them properly.
Today, we’ll focus on the ND (Neutral Density) filters. You can think of them as sunglasses, but for your lens. 😎 It helps you to get the right exposure for your photo or video even in bright sunlight. I’d say it’s one of the most cost-effective ways to improve the image quality of your work. It can be overwhelming when it comes to choosing the right one for your purpose, and I hope this brief article can help you pick the right one.
Why do you need them?
ND filters come in different shapes/sizes/grades, some can even take it a step further and create various effects. Ignore those details for a moment and let’s first talk about how they actually help us with our photos/videos.
One of the most common uses for ND filters in photography is for landscapes. Especially if you want to get those long exposure shots during the day, where you will usually get over-exposed without these ND filters. When shooting long exposures, often you will need to compensate with another camera setting to maintain the right exposure.
First, you would try to lower the ISO as much as possible and if that is not enough, then you would try to decrease the size of your aperture. Sometimes those settings alone will still not be enough, especially when doing extra long exposures. You might even want to maintain a relatively shallow depth of field by keeping your aperture wide open or some lenses just don’t maintain their peak image quality at extreme settings.
This is where a ND filter comes into the picture. They will assist you to get those silky smooth clouds or seamlessly flowing water in your landscape shots; that you wouldn’t normally get without using an ND filter. For this kind of photography, you would want a high level of ND and fixed ND filters would be your best bet for quality. If you are predominantly a landscape photographer then you may want to consider using square filters for further control and accuracy. When you take landscape photos, you will often experience bright sky and darker landscapes. The graduate square ND will help you resolve this issue and make sure the whole image is rightly exposed.
For video, often we are aiming to get that shallow depth of field look; especially with the onset of full frame cameras. As a rule of thumb, video shooters need to keep their shutter angle at 180 degrees or shutter speed at approximately double the value of the FPS (Frames per second) to maintain natural motion blur. For example, if you are shooting 25 FPS then your shutter speed will need to be 1/50th, 30 FPS 1/60th, 50 FPS 1/100th, etc. This means that we have one less camera setting that we can employ to get the right exposure which is your ISO.
Typically you would want to use variable ND filters so that you can precisely set the exposure and quickly make “on the fly” adjustments when the lighting changes. Although with using variable ND filters, there are a few precautions that we need to take into consideration as they are not perfect. More on that soon.
How do variable ND filters work?
In short, variable ND filters are basically two polarizers stacked on top of each other. One of the polarizers can rotate to allow you to incrementally adjust the level of ND.
There are different grades of ND and units of measurement. Effectively you’ll need to know how many stops of light the ND filter can reduce. This way you can determine what level of ND you’ll need to achieve your exposure.
When you are looking at purchasing one of these variable NDs, often you’ll come across labels such as, “0.9” or “ND8”. In this case, both are equal to 3-stops. For variable NDs you might see, as an example, “ND2-1000” in which they are referring to the ND factor. You can refer to the table below as a reference. ND filters are not limited to these values. This is just a reference to the most common range.
What’s the Catch?
Variable ND filters are very convenient especially for videographers. Usually they have ranges of about 2-8 stops. However this convenience comes at a cost. Typically if you want to preserve the highest quality, you’d want to avoid variable ND filters and use solid ND but this is starting to change. With variable NDs, at the heavier end of the range, you will start to notice a ‘X’ shaped pattern or some weird dark patches in your image. You might notice some vignetting or even some colour shifting. These are the side effects of how variable NDs work and the only way around is to dial it back a little bit until the side effects go away.
With all that said, variable ND filters in recent day have improved quite significantly in quality and there are now options that help mitigate the side effects. These days you will find options that have limited or reduced ranges to help avoid these issues. You’ll likely find that the ranges are separated into 2-5 stops and 5-10 or 6-9 stops.
Syrp Variable ND Filters
PolarPro Variable ND Filters
Freewell Variable ND Filters
Beyond Variable ND Filters
Variable ND filters are more popular than ever and there are now many different manufacturers. Consequently, we are starting to see some innovations in the filter market. The Tiffen black pro-mist filters are currently very popular and trending in the market. Now we are seeing brands like PolarPro and Freewell integrating mist filters into their variable NDs. This means you have a hybrid filter that can do both instead of using two separate filters. There are also hybrid ND and CPL (circular polarizer) filters as well from brands like H&Y. H&Y also make the ‘RevoRing’ series of filters which is essentially a variable filter thread system. This saves you from having to buy separate step-up rings for each of your lenses or separate filters all together.
Freewell Mist Variable ND Filters
PolarPro Mist Variable ND Filters
H&Y RevoRing Variable ND + CPL
There are many different types of ND filters out there and it can be very confusing to know which one to go for. First, start by identifying how many stops of ND you need or at least the ranges you’ll be working in. Then find out what filter thread sizes your lenses are. This will quickly narrow down your options and then you just need to think about what additional features you might need in an ND filter.