I do not recommend that you purchase a variable strength neutral density (ND) filter. If you are familiar with these filters, that might surprise you. It is not that I don’t agree that they are a wonderful idea, it is just that I don’t think they work properly. Instead, I think you should buy normal neutral density filters. Start with a 6-stop filter and add others if you feel the need. Yes, it might mean buying multiple filters, which will cost more. But variable strength neutral density filters are fools gold and simply won’t help you in most cases.
What Is A Variable Neutral Density Filter?
Let’s back up and talk about standard neutral density filters. What are they? They are filters that restrict the amount of light that is allowed into your camera. The filter won’t change anything else (like color), but it has a dark coating on it that restricts the light allowed into the lens by a certain number of stops. Why would you want to restrict light into your camera? Because often you will be in scenes that are so bright that you are forced to use a shorter shutter speed than you want. In a bright scene, you might reduce the aperture to its smallest setting and reduce the ISO to the lowest value on your camera, but you still might be forced to use a fast shutter speed to achieve a proper exposure. In some situations – notably where you want to slow down the shutter speed to blur water or the clouds – that is a problem. In fact, it happens more often than you might think.
Neutral density filters come in different strengths. Common strengths are 3-stop, 6-stop, and 10-stop filters. It is common for landscape and other outdoor photographers to buy several different strength filters in order to have complete control over shutter speed. But what is there was a way to vary the strength of these filters so you only had to buy one? That was the idea of the variable neutral density filter, and the idea itself is pretty great.
Enter Variable Strengths
A variable strength neutral density filter is actually not a neutral density filter at all. Instead, it is two polarizing filters attached to each other. If you have ever used a polarizing filter, you know that they get lighter or darker as you twist them on your lens. Polarizers restrict the light allowed into the camera by about 2 stops at their maximum, or almost not at all at their minimum. With two of them fitted together to form one filter, you have a lot of flexility. For example, you can twist the filters to allow the most light possible, which results in only a mild neutral density filter. Or you can twist the filters to their darkest settings to make it a much stronger neutral density filter. Of course, you can also work anywhere in between, and that provides great flexibility. In the end, they usually reduce the amount of light allowed into your camera by somewhere between 2 and 8 stops, depending on how you twist them. As a result, they are usually billed as “2-8 stop variable neutral density filters.”
At first blush, it seems this is an ingenious solution to the problem of having to buy multiple neutral density filters. And for some photographers, it works. But for reasons I’m about to get into, I have no use for them and I don’t recommend them for most people.
The Problem With Variable NDs
The problem with variable strength neutral density filters appears when you use them at wide angles. When you do so, you will get large strips or even a big X running across your picture. I’m not talking about some mild gradient that only a pixel-peeper can see. I’m talking about dramatic stripes running through the frame. It makes the pictures completely unusable.
In the past, I have been sort of sheepish about this issue. I personally saw the problem, but I heard from other photographers who loved their variable ND filters. I thought perhaps I was just doing something wrong. As time has passed, however, I have talked to more and more photographers who have experienced the same phenomenon. I believe it occurs in every variable strength ND filter given the nature of how they are made. It is a fact that using polarizing filters at extreme wide angles is problematic, so using two of them as a variable strength neutral density filter should have the same result. I admit I have not tested every variable ND on the market to see if this occurs, but I believe it does, as it is a simple function of them consisting of two polarizers.
As mentioned above, I have heard from some photographers who love their variable strength neutral density filters. I reconcile that in my mind by assuming that they are not shooting at wide angles. In any event, I don’t use variable neutral density filters and I don’t recommend them. If you are someone that shoots mostly at moderate to longer focal lengths, they might make sense for you. But most people using neutral density filters are using them because they are trying to capture a scene with water or clouds in it, and that usually means they are using a wide angle lens. Therefore, for most people, these filters are a waste of money.