While not the most exciting thing, ND filters are a useful tool, especially when using them outdoors. An ND filter is essentially “sunglasses” for your camera. It is a dark tint that goes over your lens, allowing you to shoot at a higher aperture than without one. This is critical for getting shallow depth of field videos in the bright sun. It can also come in handy for taking long exposure photos, such as getting that “cotton candy” shot of a waterfall. When purchasing an ND filter, there are 2 important things to pay attention to: thread size and filter strength.
What Size Do I Need?
The thread size is simple. It is not the focal length of your lens. For example, the thread size of the Canon 50mm STM f/1.8 lens is not 50mm, it happens to be 49mm. Finding the thread size varies by lens, but on my 50mm Canon lens, it is directly marked on the front with a symbol and “49mm” next to it. If all else fails, you can always Google your lens and the online manufacturer specs will tell you what it is.
What Strength Do I Need?
Unfortunately, the strength of an ND filter is not as simple as determining the thread size. It is honestly trial and error. I’ve seen ND filters from 0.3 all the way to 3.0. What do these numbers mean? Basically, the ND filter will dim the light by one F-Stop per 0.3. So if you have a 0.9 filter, it will dim the light by 3 F-Stops. If you’re shooting at f/10 on a lens and put a 0.9 ND filter on, you could theoretically shoot at f/7 and have the same exact exposure (more on this later). The best way to determine what ND filter strength you need is to go in the environment you plan to shoot in, and shoot a properly exposed photo without an ND filter. Use this as your base. I discovered that I normally have to shoot at f/10 on my lens outdoors. After that, I went inside and did a shot at f/10, and at f/7, using my ISO to compensate for the exposure. I compared the 2 images to see the benefit that a 0.9 ND filter would add. I was able to see how much depth of field I would gain with a 0.9 ND filter, and decide if I was satisfied with it or not. Using this method, you can determine what ND filter is right for you.
The Strengths Are Sort of Accurate
So, I mentioned previously that an ND filter will “theoretically” dim the light by 1 F-Stop per 0.3 in strength. This is sort of true, but I actually find that these ratings are underrated. I find with my 0.9 ND filter, it dims the light by 5-6 F-Stops, depending on the scenario. This is much more than the advertised 3 F-Stops, but this is actually perfect for me, as I’m shooting in bright, outdoor conditions. In my experience, a 0.9 ND filter is great for casual outdoor shooting, but if you’re in extreme bright conditions, such as beating down bright sun, you might want to consider a 1.8 ND filter.
“What About a Variable ND Filter?”
If you’ve done any research at all, I know what you’re thinking. What about a variable ND filter? Can’t I just buy that and be good? Well, yes.. but you will sacrifice quality. Much like a zoom lens vs. a prime lens, the performance of a variable filter simply can’t match that of a fixed one. Variable ND filters often cast excessive blue and yellow tints. While any ND filter casts a light tint, the tint on a variable one is much more extreme. In addition, variable filters are much more expensive. While a variable ND filter is certainly an option, you might not get the best quality. I personally chose the Tiffen 49mm 0.9 ND filter. Tiffen is a great brand, and B + W is also an amazing brand (although expensive). Before buying any brand, just make sure to check the reviews first.
The “One Stop” Solution
If you’re looking for a “one and done” filter solution that will work in any conditions, with any lens in your portfolio, you will want to purchase a variable ND filter for the biggest lens you have. For any smaller lenses, purchase step down rings. This is a simple converter that will let you use a bigger filter on your smaller lens. It looks goofy for sure, but functions perfectly normal. I would not recommend this solution, as variable filters just don’t offer the quality of a fixed one (as previously mentioned). If you’re looking for a one stop solution, though, this is the best way to go.
TL;DR
Hopefully you now know more to help you choose which ND filter to buy! I did several hours of research to decide on mine, and there just wasn’t much helpful information out there. If you skipped past the previous 750 words, that’s fine! The moral of the article is that I recommend buying a fixed filter instead of a variable one, this offers higher quality and less distortion. For the strength, find the best number that works for the scenario you’re in (see the section in this article for more details). I recommend 0.9 for general outdoor shooting.
