September 22, 2021

Square filter or round?

Should you mount a filter holder on your camera and drop filters in or screw on round filters? What's the right system and when?

It’s getting to that time of the year when sunrises and sunsets happen at more sleep-friendly times of the day. That means we can get out and do landscape and seascape photography and still have a good sleep. I am often asked about different filter options because I target my Introduction to Seascapes & Landscapes workshop to people new to shooting seascape photography. One question continually which crops up is the difference between filters that screw on the front of the lens (round filters) and those that slide into a filter holder (square filters).

Why do we use filters?

It is worth understanding the type of filters we use in seascape photography and why to answer the question. There are three types of filters used in landscape photography. Neutral Density filters that look like sunglass lenses. They are used to slow the light coming in through the lens to give longer exposures, for example, to smooth water. Circular Polarising filters that can make water either reflective or see-through. Finally, Neutral Density Graduated Filters are used to balance the exposure where one part of the image is much brighter than another, such as a sunset.

Square or round for landscapes?

Now we know the filters we will be using for seascape photography, let’s consider which work best for landscape photography. The ND filter would work both as a square or round as it will affect the entire frame. While it is possible to buy a square circular polarising filter, they are pretty rare, as, by their nature, they work by rotating the filter, so are best suited to being round.   The Graduated filter is the one that makes me plump for a square filter system rather than a round filter setup.

The position on the filter where the dark part of the filter graduates to clear, always remains in the same place on the physical filter. However, that isn’t always going to be the case in the scenes we are shooting. If we use the rule of thirds in our composition, the horizon could either be on the top or bottom third of the shot. When using a square filter system, we can move the filter up or down to make the graduation point mirror the scene we are shooting. With a round graduated filter, this can’t be the case. The graduation point usually happens straight down the middle on round filters so that some image elements will be unnecessarily graduated.

Another issue with round filters is that they are usually thicker than a single piece of square glass. This is because they need to have a thread built into each filter. It is often the case that we will shoot with all three filter types at once, and because of the thickness of the combined filters, we often see vignetting at the edges of the image, caused by the edge of the stacked filters being visible in the photograph.

My last reason for choosing a square filter system is not peculiar to landscape photography. Lenses come in different sizes, and the way manufacturers express the lens’s size is by stating the diameter of the front part of the lens. Common sizes would be 58mm, 72mm, 77mm or 82mm. If you have lenses with different diameters, you need to have a separate filter for each lens. The costly part of a filter is the glass, so it becomes an expensive business to have ND filters for each lens diameter. By contrast, square filter systems just require a different ring that allows the holder to be attached. These rings don’t need glass, so they are much cheaper.

For these three reasons, if someone is going to shoot landscape or seascapes predominantly, I would recommend a square filter system. In particular, I would always recommend the Kase 100mm square system.

When should I use a round system?

I hope you see the logic to why I recommend a square system for landscape photography. The obvious question then is why do we need to have round filter systems? The answer is that you need the right filter system to do the job you are looking to do. There are two circumstances when I could justify owning a set of circular filters.

The first is if you use a DSLR or mirrorless camera to shoot video. There is a standard in videography called the 180-degree rule. Without going into too much detail, to capture the way the human eye experiences motion in real life, the shutter speed is set to twice the frame rate. We generally shoot video at 24 frames per second, so the correct shutter speed must be 1/48th of a second. The closest shutter speed we have to that is 1/50th of a second, so when shooting video, we need to set this as the shutter speed. Let’s consider that we want to shoot with a wide-open aperture to get a shallow depth of field, and it’s a bright, sunny day. Shooting at 1/50th would seriously over-expose the picture, so we need to reduce the amount of light hitting the sensor. This is where we would use an ND filter. We could use a square filter system, but it is much more portable to use a round filter.

Another use of round filters is when shooting stills with flash photography. Again, imagine the same bright and sunny conditions and the need to shoot with a wide aperture. Some older flash systems have a maximum sync speed around 1/160th – 1/200th of a second. The sync speed is the fastest shutter speed you can use and allow the flash to operate correctly. Again, if the photograph is over-exposed at the sync speed, an ND filter is used to balance the exposure. Once more, the round systems are much more portable for this use than a square system with a holder. Kase, also offer a round filter starter system using the same great glass used in the square system.

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About the author

As well as running Edinburgh Photography Workshop, Rich Dyson is a professional photographer. His photographs are regularly used in newspapers such as The Times, Guardian and Daily Telegraph. He also had two solo exhibitions as well as being featured in a members sponsored exhibition in the Scottish Parliament. You can see and buy his photography at richdysonphotography.com

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