This week I'll be continuing last week's theme of long exposure photography, covering several methods that can be used to achieve the effect.
read part 1
There are two main ways to approach long exposure: in-camera and in post.
The most straightforward way of achieving a long exposure is by using a slow shutter speed in-camera, but there are a few obstacles. Lowest ISO you can usually count on is 100 on digital cameras and 50 on film, and if you're shooting in the daylight, that may not be enough, unless you're shooting a very fast-moving object.
If you're using a digital camera, you should also be aware that the sensor tends to heat up considerably during long exposures, producing large amounts of digital noise - overnight exposures are impossible with digital cameras for this very reason. If you're using film, you can make your exposure as long as you want, but you need to be careful measuring the light and using your best judgment to select an appropriate exposure time.
If you are pursuing a long exposure during shooting, here are some of the things you can do to achieve a longer shutter speed:
- use an ND filter. Neutral density filters go as dark as 10 stops, and you can combine several filters to go even darker if necessary.
- limit the amount of ambient light. If shooting in bright daylight it can be almost impossible to blur motion, especially if you don't have filters to use. To maximise your exposure time shoot at dusk, in the shade, and aim for overcast weather.
- use the smallest aperture possible. Many lenses on 35mm SLRs don't go beyond ff22-32, but if you have access to a large-format camera you may get an aperture over f64. A pinhole camera will have an even higher number - depending on the diameter of the hole it can be as small as f200. If you're using photographic paper to make your negatives, that can also help to extend your exposure, as most papers fall in the range of ISO 3-25.
There are several ways you can achieve long exposure in post, but the main principle is the same - you combine several exposures into a single frame to create the effect. This can be done in two ways - automatically with a software or manually in Photoshop, but whichever method you choose, the taking images stage is the same. You need to use a tripod in order to preserve the framing, and you need to keep the exposure as consistent as possible. Even though stacking programs can still work with differently exposed images, manual stacking gives best results if the exposure in all the images is exactly the same.
The number of frames varies - depending on your subject matter and desired effect, you can stack from just a few to dozens and even hundreds. In low light conditions, at night for instance, it's advisable to keep your shutter speed low to avoid excessive noise, opting instead for more frames. In daylight with fast moving objects, like waterfalls, you can stitch together just a few frames exposed for longer.
There are several programs out there that do the stacking automatically. This method is good for combining a large number of exposures and is most commonly used for astrophotography to achieve very long star trails. Some programs don't work with RAW files and require conversion, but other than that all you have to do is upload the images and wait. With a large number of frames, it's common to leave it running overnight.
Some other software options:
Manual stacking can be time consuming and it takes a lot of processing power, but it does give you more flexibility and control over the final image.
To do this, you need to load your images as layers in a single file. To do it from Bridge, select the images you want to stack and go Tools > Photoshop > Load files into Photoshop layers.
If you're worried that your framing may have shifted during exposure you can use the automatic alignment option in Photoshop. To do this, go to the File menu, select Scripts, then Load Files Into Stack. Select the files you want and tick the Attempt to Automatically Align Source Images box.
Once your images are loaded, select all layers and turn them into a smart object (Layer > Smart Objects > Convert to Smart Object).
All that's left to do is to choose the stacking mode in the Smart Objects menu. There are quite a few options, but I found that for this purpose you get the best results with Maximum, Mean and Median stacking.
Photoshop files made with this method tend to be extremely large, so I would recommend rasterising the stack layer once you're happy with the way it looks for further editing.
There are other ways to stack images manually by using different blending modes or changing layer opacity, but I'm not very familiar with these methods (see resources below).
Whichever method you choose to use, long exposure is a great creative tool that can achieve a wide range of interesting results. I hope this brief tutorial was helpful, but if you want to explore the technique further, I''ll leave some more resources below.
A great astrophotography and stacking tutorial.
A detailed guide to in-camera long exposure.
Digital star trails tutorial.
A guide to star trail stacking through blending modes.
Stacking through opacity change tutorial.