ND FILTERS: ROUND OR SQUARE?
By Roman Martin
Neutral Density filters are one of the most useful filters for photographers. They provide significant advantages when trying to control exposure. They help to reduce shutter speed so you can get smooth water in landscapes. And they help obtain large apertures when you’re shooting portraits in bright sunshine. But when you’re shopping for these useful tools, the common question is – ND Filters: Round or Square?
You see – there are two styles of ND filter available, and they are distinctly different types. There are round filters that thread on to the front of the lens. And there are larger square (or rectangular) filters that slide into a holder attached to your lens. There are advantages to both.
Spoiler alert: we’re going to recommend square filters, but read through and we’ll tell you why. So let’s discuss each type.
Round Style Filters
Most photographers are familiar with the round screw-in type of filters. Most of us have a basic threaded filter on our lenses to protect it from scratches and dirt. Those are simple UV filters, and they have their purpose. And of course, you can also get ND filters in this same form factor.
Certainly, the most obvious advantage of round filters is that of convenience. They are easy enough to operate, simply by threading it on to the front the lens. It can even be done with gloves on, if you need to. Although it’s easy to find low-cost units, you can get some really high-end quality filters too. Using top quality glass versions will ensure that the money you spent on your lens is not wasted, by using cheap glass filters.
And conversely, the most obvious disadvantage of round filters is that one size does not fit all. That is to say – each lens you own most likely has a different filter thread size. So the filter you put on your 50mm lens probably won’t fit the 24-70mm lens. That means you need to duplicate a lot of your filters, if you ever wish to use them on a variety of lenses. You could end up with 3 or 4 of the same filters – one for each lens, for instance. You could also use stepping rings that allow you to use a large filter on a small lens. But that won’t work in the opposite direction.
But be careful when you use round threaded filters on your wide lenses. Sometimes the filter is quite thick, and you could see some vignetting on the edges of your frame. If you do go with round threaded filters, be sure to look for the thinnest versions available.
Square Style Filters
The other style of filter available is the Square type. If you’ve never used one, it may not be obvious how it works. But it’s quite simple – you just slide the square filter into a bracket that is attached to your lens. That bracket can fit any lens you own quite easily, just by having the appropriate mounting ring attached. There are generally three parts to a square filter assembly – the mounting ring for the lens, the bracket that attaches to the ring, and the filter that slides into the bracket.
One of the main advantages of using Square filters is that you really only need to buy one filter of any kind. Even if all your lenses have different filter thread sizes, you can still attach the bracket to the lens. You just slide in a size adapter into the mounting side of the bracket. And each mount adapter is quite cheap, because there is no glass involved – it’s just a ring.
A second advantage is Square filters is variety. There are some square filters that are not available in a round filter format. For instance, a graduated ND filter is only available in a square format. Graduated ND filters need to remain in position, no matter if the lens is rotated, and a round threaded filter can’t do that. If it were a round threaded filter, it might rotate as the lens barrel focuses.
And finally, a third advantage of Square filters is that you can attach them to lenses that don’t even have a mounting thread – like a super wide-angle lens. This is done using a sleeve that can be clamped on to the barrel of the lens, allowing the filter holder to be attached. Very often, super-wide lenses have an extremely large convex front element, and don’t have a filter thread built in.
So What About ND Filters?
This is where square filters work especially well. But first, I will provide some explanations about the types of ND filters that are available. To start with, there is the basic exposure-altering “Stopper” type. This is a very dark – almost opaque – filter. It comes in a few ‘stops’ (like 3x, 6x, 10x) that basically cuts your exposure by 3, 6 or 10 stops. This is great when you simply want to cut your exposure by X amount.
There are a couple of different ways that filter brands name these filters. Some use the ‘Optical Density’ nomenclature, (like 0.3, 0.9, 1.8, etc). And some brands use the ‘Filter Factor’ nomenclature (like 8, 32, 64, etc.). But regardless of how it’s named, as long as you know how many stops it changes, then you’re good to go.
Graduating to Graduated
The next type is the Graduated ND, as mentioned earlier. It’s a filter that starts as black at the top, and gradually fades to transparent at the bottom. Again, these come in varying degrees from very dark, to kind of dark. And they can also have colours like ‘sunset’ or ‘twilight’, etc. The primary purpose of these types of ND filters is to darken the sky in your image, while affecting little change in the land portion. Quite often, the skies are overexposed in landscape photos, and a graduated ND eliminates this problem.
And finally, there is a Variable ND filter. This type of filter only comes in a round threaded version. It’s basically two glass filters that rotate independently. As you rotate the filter, you can “dial in” the darkness of the effect, thereby having several stoppers in one filter. It’s not technically possible to create a filter like this in a square format, well, because it’s square.
The downside with the Variable ND filter is that at some long exposure settings, you will see the dreaded X-Effect, and some odd colours in your image (see the sample image). Even high-end brands can give off a colour cast.
So it comes down to this: if you want maximum control, with the best quality available, and the overall lowest cost, then you need to look at a Square Filter kit.
You would need to buy a bracket, an adapter ring for each size lens you have, and then all the filters you want. One bracket can stack at least 2 filters in it, and some brackets can stack more. That means you can put a 3x and a 10x together, if you wanted to. Brackets and filters come in a few sizes, depending on the camera/lens it would be attached to. The most common sizes are 100mm and 150mm square. But they come in smaller and larger sizes too.
If you were to buy the same amount of items in Round threaded filters, you could end up buying a 3x, 6x, 10x, for each of your lenses, which could be dozens of filters. With a Square kit, you can buy just a few filters, and a few size adapters.
Also note that square filters can be made with resin, or with glass. So you can still spend some significant cash if you buy the best available. The resin versions are still good, and are usually scratch resistant, but glass is always better (although more fragile). As for brands, you should look at Lee Filters, Nisi, Formatt Hitech, and Haida.
The advantages of using a Square Filter kit become apparent when you have them. With the flexibility of being able to use multiple filters in combinations, and the ability to use them on lenses without filter threads is enormous. Maybe the question “ND Filters: Round or Square?” isn’t so tough after all.
There are plenty of opportunities to flex your ND Filter muscles on journeys with Photo Workshop Adventures. Our Iceland Adventures and our New York City adventures are just a couple of locations that lend themselves to ND filter usage. When you are looking for advice on which filter kit to buy, or want to try them out on an adventure with us, be sure to get in touch!