As important as your choice of cameras and lenses is, sometimes it’s a simple filter can make all the difference to your photos – and Neutral Density (ND) filters are the most important you can buy. Why? They simply let you get shots that you otherwise couldn’t. That’s partially true of other filter types, like polarisers, but the impact of NDs is arguably more profound, widespread, and useful.
Unfiltered, the maximum exposure time is 1/15sec. With a five-stop ND it lengthens to 2secs.
ND filters are all about cutting down the amount of light entering the camera and that means you gain greater control over shutter speed and aperture. Essentially, less light entering the camera means slower shutter speeds and wider apertures are available where you’d otherwise over-expose the image. Typically, this means the ability to shoot landscapes and urban scenes with motion blurred subjects, or work with very wide apertures in bright light, which is useful for portraits, sports and wildlife.
Adding a strong ND filter in daylight means you can blur moving subjects, like crowds, with a slow shutter speed…
… or use wider apertures in bright light, so you gain more control over depth-of-field.
NDs are rated in strength, and this tells you how much light they will block out – and therefore what change in exposure you’ll get when one is added to the lens. The strength of the filter can be written in a few different ways, but it always equates to exposure stops. Most of the time, filters are rated in full stops – one, two, three stops, and so on, all the way up to 10, 15 or above. Crucially, an ND filter will also cut out light without wildly changing the colours in the image – it’s this that sets them apart from cheap alternatives like welders’ glass or gels.
NDs come in different strengths, so you can control the amount of light you cut out.
There are two main ways that an ND filter’s strength is rated – Optical Density or Filter Factor – and which is used just depends on the manufacturer. Optical Density is written as ND 0.3 for a one-stop filter, ND 0.6 for a two-stop filter, ND 0.9 for a three-stop filter, and so on, with a 10 stop filter written as ND 3.0. For the same one-, two- and three-stop strengths, Filter Factor would be written as ND2, ND4, ND8 with a 10 stop filter written as ND1024 (or sometimes ND1000 for marketing purposes). As you can see, with Filter Factor the number doubles for each additional stop of light it cuts out, while with Optical Density it simply rises by 0.3.
You can choose to use square filters that slot into a holder and this way several filters can be stacked together, as well as using the holder on different lenses.
ND filters can be bought in either screw-on or square form, with the former attaching directly to the lens, and the latter slotting into a holder that sits around the front element. However, you may also be limited to which kind of filter you can use by the lens you’re shooting with. For instance, because they have a large and convex front element, some very wide-angle primes and wide-angle zooms don’t support screw-in filters and need suitable filter holders instead. Some lenses will have the ability to use smaller, slot in filters that sit within the light path or over the rear element. If you have a lens that accepts screw-on filters make sure to check the filter size which is written on the lens.
Screw-on filters have their advantages, including size, speed of operation and portability.
There are advantages and disadvantages to both square and screw-on filters. For instance using a holder allows you to use the same filter with different lenses, so long as you have a suitable adapter. But holders can be unwieldy. For this reason they’re more likely to be used by those working slowly like landscapers. Screw-on filters are easier and quicker to mount, and move around with, but stacking them is less easy and they only fit particular lenses unless you use stepping rings, so they’re more likely to be used in portraiture or sports and wildlife.
Some lenses, like ultra-wide-angle zooms, require specialised filter holders.
When choosing a filter, or filters, the other question is how much light do you need to cut out. This depends on the brightness of the conditions you’re working in. Taking landscapes as an example, if you want to reduce shutter speed to get motion blurred clouds or water, you’ll need a much stronger filter in the middle of the day than you’d need at dawn or dusk – more available light means a stronger ND is required to achieve a certain shutter speed. So, while a three stop (ND8 or ND 0.9) filter would take you from 1/8sec to 1sec, if your shutter speed is 1/200sec, it will only drop to 1/25sec, so you might need something stronger. Conversely, if you’re working in lower light levels, say starting with a shutter speed of 1/2sec, you’d only need a one stop (ND2 or ND 0.3) filter to get the same 1sec exposure. A three stop filter might be too much as it’d take you to 4secs.
If you’re working at dawn or dusk you won’t need as strong a filter to extend the shutter speed into the seconds.
For reasons of versatility, it’s therefore good to invest a set of filters – a mix of strengths gives you the ability to modify your choice of filter based on the conditions. But that’s not all. You can also stack separate filters to increase the light-stopping effect. The effect of stacking is simply additive, so if you have one, two and three stop NDs, and use them all, you’ll have approximately the same effect as a six-stop filter.
Stack too many screw-on filters and you’ll eventually see hard vignetting in the corners of the frame.
The downside of stacking is that, no matter how good the quality of the filters, increasing the amount of glass the light has to pass through can affect image sharpness and cause flare. What’s more, depending on whether you’re using a filter holder or screw-on types, you may be limited in terms of how many filters you can stack – either the holder will only allow a certain number, or if you’re staking screw in filters their edge will eventually begin to show as vignetting. So from that point of view, again, investing in a range of filter strengths, for instance a 1, 2, 3, 5 and 10 stop set is a good idea.