Long exposure is an exciting and popular genre of photography – and for good reason. The variety of images possible and the ability to reinterpret an everyday scene by capturing longer periods of time can lead to some amazing photographs. Using high quality neutral density (ND) filters, such as the Hoya PROND range, to extend shutter speeds, can result in some spectacular images that wouldn’t normally be possible using traditional methods of photography.
As a long exposure photographer, I often receive questions on how best to use ND filters in the field. It is not quite as simple to affix the filter and take the shot. There are various stages of planning and methodology required to achieve great results. I set out below my personal workflow and whilst this is naturally not the only way to do things, through my experience I have found that this works best for me.
All photography requires a little planning and forethought. What is the shot we are actually trying to achieve and what is it that we are trying to convey to the viewer. Long exposure photography is no different. In fact it takes a little more thought as condensing periods of time into a single image, you have to visualise in advance where any movement will occur and how this will affect the final image.
Take extra care to ensure that you consider the borders of your image when taking a long exposure photograph. Bear in mind that the periods of movement can bring things into (or remove from) your frame, such as tree branches that may detract from the overall image etc.
Set up and Focus
When you have thought about your composition and the desired photograph, it is necessary to set up the camera on a sturdy tripod and frame the image. It is at this stage that I recommend taking a test shot to determine correct exposure, depth of field and focus. Bear in mind that you should try to use the lowest ISO possible to keep noise to a minimum and select the appropriate aperture for the required depth of field. To read more about aperture and depth of field, you can read my earlier article here.
Once you are happy with the results of these test shots, turn off autofocus, as the camera will struggle to refocus when the filters are attached. If your camera or lens has any image stabilization or vibration reduction modes, then these should be disengaged as well, as they may introduce a slight blur where the camera is on a tripod.
What ND filter should you choose?
The next step is to attach the ND filter. Hoya PROND filters come in a range of different strengths of light reduction. This is measured in ‘stops’ of light. A common question here is what strength filter should you use? Well this depends on a mixture of things but ultimately what you are trying achieve in the final photograph. If you are seeking to enable the use of a slightly longer shutter speed to capture the essence of movement in the image, then consider using the 2-stop (ND4) filter. If you want to capture much more movement, such as fast moving clouds and water then a 6 stop (ND64) filter may be more useful. To be able to extend shutter speeds extensively and capture ethereal images then consider the 9 stop (ND500), 10 stop (ND1000) or the 16 stop (ND100000). These can dramatically extend shutter times.
Calculating the correct exposure
How to determine the correct shutter speed to compensate for the chosen ND filter often confuses some photographers as it requires a calculation. Quite simply, consider the earlier test shot that you took and make note of the shutter speed. The aperture should not change as this will change the depth of field and the overall look of the image (as well as the exposure).
Then you need to consider that for each reduction of a stop of light, the shutter speed needs to be halved. So if you are using a 6 stop filter then you need to halve the shutter speed 6 times. This can be seen more easily as a written example:
If the test shot is 1/250th of a second and a 6 stop filter is attached, then the final shutter speed will be 1/4 of a second. The original shutter speed is simply halved 6 times: 1/250 – 1/125 – 1/60 – 1/30 – 1/15 – 1/8 – 1/4.
Taking the shot
Ensuring that the autofocus is off (as above), put the camera to manual mode and dial in the settings – the original aperture and ISO from the test shot and the newly calculated shutter speed for the appropriate ND filter.
It is important to cover the viewfinder to prevent any stray light entering into the camera. Most cameras either have a switch or a rubber eyepiece cover to do this. Then using a remote release, trigger the shutter.
Depending on the selected ND filter, there can be times where the shutter speed will extend beyond 30 seconds. As a 30 second exposure is the maximum amount of time that is allowed in manual mode, then it will necessary to switch to bulb mode and use a remote trigger to extend the shutter time beyond 30 seconds.
Review the Image
The final stage here is to review the image captured. Check the histogram to ensure original test shot, as they should both be similar. You can read more about histograms in my earlier article here.
If your image is under or overexposed, then you can simply reshoot the image with a longer or shorter shutter time to compensate.
Neutral density filters open up a whole new array of opportunities to shoot scenes in a unique and creative way. A little forethought is required to anticipate how the final image will end up. The longer shutter-times will capture extra movement which can lead to dynamic yet ethereal images. I hope that my accompanying images provide you with some examples of what can be achieved. You can see more of my images on my website at antonyz.com/portfolio
I hope that you find this article useful. For more detailed information on ND filters and all other aspects of long exposure photography, please do check out my new book – Mastering Long Exposure Photography – A Definitive Guide, published by Ammonite Press and available at all good bookstores and online. https://www.ammonitepress.com/photography/photography-techniques/mastering-long-exposure/
Naturally please also feel free to contact me if you have any questions or comments relating to this topic or any other photography question. Please do pass this post onto any photographer friends that you think may find this useful.