Long Exposure Photography
Keeping the camera’s shutter open for extended periods
Long Exposure Photography was the theme of this autumn’s online photography talk, which was held live on 25th September. As usual, the talk was recorded, and you can watch it below. I hope you’ll enjoy it, along with this article which summarises much of what was said in the talk.
Watch the Long Exposure Photography talk here
What is long exposure photography for?
There are two main reasons for using long exposure photography; ie shooting with the shutter open for a long period (say, from 0.5 seconds upwards to minutes or even hours). These are:
a) When there simply isn’t enough light to obtain a well-exposed image using the ‘standard’ fast shutter speeds, such as 1/30, 1/60, 1/125 or 1/250 second.
b) When you want to blur movement within the image frame; for example flowing water or traffic.
Examples of the first of course include such situations as:
- A very dull, grey day, particularly if you’re shooting under a canopy of trees;
- At dawn or sunrise, while the sun is still below the horizon or is just rising above it;
- At dusk or sunset, after the sun has dropped below the horizon or is just setting;
- At night, when there is of course very little light, even under moonlight.
In all these situations, the simplest way to shoot is simply to put the camera on a tripod and use a long exposure time to deliver sufficient light to the sensor. The less light there is, the longer the shutter will need to be open. You can of course help things along by having the lens aperture wide open, but you can only take that so far. Moreover as the lens aperture gets wider so the depth of field (ie the amount of the image that is sharply in focus) decreases. You could also put the ISO up, making the sensor more sensitive to whatever light there is, but then your image quality deteriorates with graininess increasing.
Using long exposure photography to create motion blur
Of course, when light levels are low – as described above – it is easy to use a long exposure to blur anything that’s moving during the exposure. In fact, you don’t have any choice but to use a long exposure in order to get a well-exposed image. So anything that’s moving during the exposure will be blurred.
But what do you do when you want to blur movement during a bright sunny day (or even a bright cloudy one)? In these conditions there is just too much light for the camera’s exposure meter to allow for a slow exposure. This will be so no matter how much you close the lens aperture down to restrict the amount of light passing through, or how low you set the ISO to decrease the sensor’s sensitivity.
So what’s the solution? The answer lies in the use of filters, specifically neutral density (ND) filters. These are evenly darkened across the whole filter to decrease the amount of light passing into the lens and from there to the sensor. They come in a whole range of grades, decreasing the amount of light by as little as just two-fold up to over 1000-fold. The ‘neutral’ part of the name is there because they manage their light-reducing ability without changing the colour balance of the light. You hope.
Needless to say, a tripod is again essential. It may be a very bright day but if you’re going to use a slow shutter speed to blur movement in the image then the camera must be rock steady. Hand-holding the camera will only result in the entire image being blurred, not just the moving part!
About neutral density filters
ND filters come in two forms: circular and square. The circular filters screw onto the front of your lens, which can be quite convenient. However, it also means that if you have several lenses all with different lens-front thread sizes, you will need a different set of filters for each lens. This can be partially overcome by using a variable filter, one in which you can change the filter’s dark density by rotating a ring on the filter. You may then only need one or two filters for each lens rather than a whole battery.
The square filters fit into the same kind of holder as is used for ND graduated filters (used mainly for darkening overly bright skies). With square ND you need to have only one set of filters for all your lenses. Just a number of adapter rings of the right thread sizes will be needed to fit the filter holder in place in front of the lenses.
Stopper ND filters
The square filters are the system of choice when using ‘stopper’ filters. These include the ‘six-stopper’ and the 10-stopper, extremely dense filters that reduce the amount of light passing through by six stops (or 64-fold) and 10 stops (an incredible 1024-fold) respectively.
With these stopper filters it’s essential that no light leaks around their edges and into the lens, so they come with a strip of felt fixed along the filter’s back. This must fit snugly up against the frame of the filter holder for them to be effective.
Once a stopper filter is in place it is almost impossible to see any of your image view through the viewfinder or LCD screen, so it’s important to set up your image composition and focussing first, and then very very gently slide the filter into place without disturbing any zoom or focus settings.
The camera’s exposure meter usually works quite well even through a dense stopper filter, though I have found that sometimes it tends to under-expose the images. If that happens, then simply over-expose and reshoot.
Whether you choose to use one of these dense stopper filters or any of the many less dense types will depend on your own personal taste, how much blur you want to instil and just how dark or bright the conditions are. The brighter the conditions, the denser the filter you’ll need to get a slow enough shutter to create good motion blur. The key, as always, is to practise and experiment.
And finally….
That really is pretty much it for long exposure photography. It is actually reasonably straightforward. In poor light all you need is the camera on a tripod. In bright light you need that tripod plus a set of ND filters. And then you’re good to go.
Enjoy your photography!
