There’s a lot of confusion about photography filters out there – and one source of that confusion is their varying shapes. Why are some photography filters round, and others square or even rectangular? There’s a very good reason for each of those shapes.
Round Filters are the obvious choice because they can screw onto your round lenses. It’s a convenient shape that makes sense in many cases – but not always. UV or Haze filters fit neatly over your existing lens, and some photographers use them to protect their expensive glass. Some photographers used to carry a collection of round colored filters, too – but those are no longer necessary, since their effect is easy to replicate in post processing.
Circular Polarizer (CP or CPol) filters are also round. That’s because they need to rotate smoothly in order to work properly. A circular polarizer filter cuts through glare to show better color and detail – and it reduces the amount of light coming through the lens by about two stops at full effect. We often use our Benro CPol filters when we’re photographing wet leaves or rocks, and when we’re shooting water and want to see beneath the surface.
I know I’ll get called out in the comments if I don’t mention Variable Neutral Density Filters. 😉 These handy little filters are made up of two Polarizer filters stacked on top of one another. They allow you to reduce the amount of light coming through your lens up to about 10 stops – depending on how you adjust the rings. Like a CPol, a VariND filter is always round, since it needs to rotate in order to work properly.
You can purchase round ND and GND filters as well… but there’s good reason to consider square and rectangular filters instead. Here’s why.
Rectangular Filters are rectangular for a reason – they are designed to slide up and down in their holders. The most common of these is a Graduated Neutral Density (GND or ND Grad) Filter. You’ll need an adapter and holder to use a rectangular filter, but the added flexibility makes it more than worth the cost.
A GND filter is designed to reduce the amount of light coming through part of the lens. These filters are great for landscape photographers because they allow us to photograph sky and foreground with a single exposure, even if the range of light is pretty broad. A 2-stop GND will reduce the amount of light coming through half the lens by 2 full stops – so I can cover the sky in my composition with the dark part of the filter. The landscape in the foreground will remain unaffected.
Once I’ve set up my composition, I attach my adapter and holder, and slide my GND filter into place. I can slide my GND filter up and down until the transition between the dark and light part of the filter aligns with the horizon in my photo. If you are using a round GND filter, you can rotate the filter to adjust for an uneven horizon, but you can’t slide it up and down – so you are limited to a centered horizon in your composition. That’s a limitation I’m not willing to settle for.
I carry five Benro GND filters when I’m on location. Three “soft” filters, which provide a gradual transition between their dark and light halves – and two “hard” filters, which provide a less-gradual transition. The filters reduce the light coming through part of the filter by differing amounts – between 2 and 6 stops. I can stack the filters in my Benro holder if necessary.
So, round and rectangular filters make perfect sense… but why would you need a square filter? I carry three Benro Neutral Density filters with me wherever I’m shooting… and you guessed it… mine are square.
There’s no need to rotate or adjust an ND filter. It’s designed to reduce the amount of light coming through your lens. I love to use an ND filter when I’m shooting waves on a beach. I can use a long shutter speed to blur the motion of the waves for a silky effect – and ND filters come in handy when we’re shooting waterfalls too.
The square shape means it fits into the same holder I use for my rectangular GND filters, so I can stack filters to my heart’s content. A round GND filter would work just as well, but it’s much more convenient to slide filters in and out of the holder as necessary, rather than removing it to attach or remove round filters, and then replace it again. My Benro filter holder even lets me adjust my Circular Polarizer filter without removing the whole setup.
When you are working with filters, keep in mind that vignetting can be a real problem. You don’t want to be able to see your filters through the lens. They should be wide enough that their edges rest outside the viewing area of your widest lens. Check the corners of your compositions to be sure you are using the right size filters and holders. I use a step-up adapter ring so I can use larger filters (100mm ND and GND), which remain out of sight even when I’m shooting at 16mm.
Filters are great additions to your kit, and can make a real difference to your photography. Take the time to understand how they’re used, and experiment with them until you feel comfortable using them. I won’t travel without mine!