Photo tip of the week: What ND filter?

Neutral density filters are basically dark glass or resin which allow a photographer to lengthen the shutter speed in order to achieve an effect that is not reproducible in post processing alone. The duration of the shutter speed will depend on how dark the filter is.

The degree of ‘light blocking’ is often referred to with regard to the ‘stops of light’ it blocks. An alternate terminology uses exponential values of the number two to reflect this principle. For instance an ND filter blocking 2 stops of light can be called ND4 (2^2), and a denser filter blocking 10 stops of light can be called ND1024 (2^10). With the terminology out of the way, let’s move on to some questions you should ask yourself to help achieve the best when using these filters.

1 What else do I need?

Before you embark upon long exposure photography, there are several pieces of equipment that are vital for achieving high quality results. The first is a firm and steady tripod. If you are running exposures upwards of several minutes, any slight movement may render the image unusable and if you are shooting in golden hour, much of the ‘peak’ light may have been wasted during that exposure!

Secondly, a cable release or remote is essential to achieve exposures of greater than 30 seconds. Some firmware changes such as ‘magic lantern’ and some camera applications (such as for the Sony A7r series) offer an alternative to a cable release, but I prefer the former.

The filters themselves come in two main forms. The first is the circular type which screws on directly to the front of the lens. The advantage of this setup is that there is no opportunity for light to ‘leak’ between the filter and the lens itself. The main disadvantage is the inability to use these filters in conjunction with other GND filters if that is your shooting style.
Some circular polarisers will allow another screw-on ND filter to be attached, but this tends to lead to vignetting; hence polarising options are also limited with the ‘screw-on’ setup. The second (and my preferred) type is the square glass filter which fits into a filter holder.

There are many brands which supply kits for this setup but I am currently using the Nisi brand as its holder contains a circular polariser within the filter holder itself. Most filter holders allow two further slots for graduated neutral density filters (GNDs) to be placed in addition to the ND filter itself. The main disadvantage of this setup is the potential for ‘light leak’. This becomes more of an issue for the denser ND filters as reflected light finds its way between the holder, filter and lens. Thankfully most ND filters upwards of 3 stops come with a foam padding which minimises this phenomenon.

It is important to place the ND filter in the filter slot closest to the lens , as otherwise there are even more gaps for light to leak behind the filter. Light may also leak into the camera itself from behind, particularly when shooting opposite the light source. The most common source of leak in this scenario is the optical view finder. Simply standing between the light and the camera may well suffice to prevent this from occurring but a more foolproof approach would be to use a cover for the viewfinder. This might be a dedicated cover situated on canon camera straps, a small strip of duct tape, or even a dab of blue-tac!

2 How dense should I go?

If you are new to the world of ND filters, I would make the following rough recommendations for your first purchase. A two or three stop ND filter is good for smoothing out dynamic water while retaining some texture. This suits waterfall or seascape shooting. A 6 stop ND filter is what I would generally use during golden hour in order to create exposures of 1-2 minutes for smooth water or clouds . A 10 stop filter (or denser) would be what I would use during the day outside of golden hour as the bright ambient light requires much more ‘light blocking’. Be aware that certain brands of ND filters tend to have a colour ‘cast’ which becomes more evident at longer exposures. This can usually be corrected with a white balance adjustment in any RAW file converter. I have found Lee filters relatively cast free up to 6 stops and Nisi filters generally cast free right up to 15 stops.

3 How is my camera set up?

If you take long exposures frequently, turn off long exposure noise reduction. If this is accidentally left on, you will be waiting for your camera to process the image for the same duration as the shutter speed after the image has been taken. You can use post processing noise reduction to achieve the same effect.
Generally, you will want your ISO to remain low. This is camera dependent but I would generally not take very long exposures above ISO 400. This may seem conservative , but the combination of long exposure and high ISO noise can be quite damaging to an image. If your camera has the ability to expand its native ISO, activate this. For instance, the canon EOS 5D MKIII has a native low end ISO of 100 but can be lowered further to ISO 50. This one stop reduction in ISO allows you a 100% increase in shutter speed for your given setup .

Aperture considerations are not specific for long exposure photography, though you may need to ‘stop down’ to f22 if you need to lengthen an exposure beyond what your specific ND filter can achieve at a lens’s sweet spot, which is usually f8-f11. Turn live view exposure simulation on. Its utility will be evident in an example given below. Lastly, use manual focus and find your focal point while taking an image without the ND filter on. If you accidentally leave autofocus on, the camera may well hunt unsuccessfully for a focal point when the ND filter is on with a strong possibility that the resulting image will be out of focus. It is important to shoot in RAW to maximise your control over white balance corrections and selective noise reduction as well.

4 How do I see the image?

By their nature, ND filters will prevent you using the optical view finder effectively because they are blocking light. Exposure simulation on live view may overcome this , but not for very dense filters or in low ambient light. It is therefore important to have a base image with your desired composition set and to take note of the shutter speed without the ND filter in order to calculate your final shutter speed with the ND filter in place.

I also find that with certain scenes, images with short and long shutter speeds have different appeals and you may end up choosing to use the shorter exposure after reviewing the end results. It is helpful to arrive at a scene with plenty of time to shoot (well before sunset for instance) such that you can work out if a long exposure suits the scene from a few test images. Creating a long exposure image works best when you are committed to a preconceived end result rather than falling upon a nice image purely by chance.

5 How long do I need?

The basic principle of exposure prolongation is that for every stop of light blocked, the exposure duration doubles. With less dense filters, this is not a difficult calculation to make. For instance a 1 second shutter speed without an ND filter in place will require an 4 second shutter speed in order to achieve the same overall exposure with a 2 stop ND filter in place. The math becomes far more difficult for 10 or 15 stop filters. If you don’t mind bringing phones or other devices into the field, application such as ‘photopills’ can calculate the recommended shutter speed for you based on the shutter speed for an image taken without the filter in place. If your shutter speed is going to extend beyond 30 seconds (requiring bulb mode), the following steps could help you calculate the exposure without an external device and is very quick. After you have taken some test images to work out composition and have a rough idea that your exposure will extend past 30 seconds, follow these steps.

6 How long do I want?

The above tips so far have made the assumption that the ND filter is controlling the shutter speed. What I find more useful is to work towards a specific shutter speed for a given scene. This means not only choosing the right strength of ND filter for the scene, but adjusting ISO and aperture strategically as well. For instance, with slow moving cloud, a very long exposure of greater than several minutes may be required in order to achieve a sense of motion in the sky. In a day time situation, this may not only require an ND filter , but an adjustment of ISO to as low as your camera will allow as well as adjusting the aperture as small as you need to. On the other hand, with fast moving cloud, the same very long exposure may result in complete blurring of the sky when in fact, a slightly shorter exposure such as 30 seconds to 1 minute may suffice. In this scenario, you may have a less dense ND filter but alternatively, you can increase your camera’s ISO and open up the aperture taking care to make sure that the required depth of field for the scene is maintained.

7 With or without you?

Combining images taken with and without filters is something I frequently do in order to achieve cleaner images and in order to save time in the field. As exposure times lengthen, the possibility of long exposure noise increases significantly, particular in shadow areas of the image. For this reason, I make sure that my test series of images are also properly exposed for the dynamic range of the given scene. I often use the static elements of this image to blend in with the moving elements of the long exposure taken with the ND filter. Also, taking the images for a dark foreground without an ND filter on will save significant time as the shutter speed required for some darker aspects of a scene may well take several minutes to achieve. Whether you choose to use GNDs in combination with ND filters all depends on how much you emphasise achieving a desired image in the one exposure. Generally speaking, I try to do this but there are situations when GNDs are the inferior option to blending images in terms of quality.

Overall, the above tips need to be put into practice in the field over and over again. You will find that by doing so, choosing the right filter for the approximate right shutter speed in the right light becomes intuitive rather than a tiresome mathematical exercise. There is also the temptation to shoot every image as a long exposure which may limit your choices at a given scene. Having a good understanding of the potential variations ND filters provide can allow you to expand your possibilities rather than contract them. ❂