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Recently long exposure photography has been quite popular, mainly in the landscape photography arena. One of the reasons (among the others) is that through a long exposure it is possible to visualize a scene with much more softness and harmony in respect to a standard exposure.
Thanks to the rapid evolution of digital cameras, we are now able to take really long exposures without getting so much digital noise, due to sensor overheating. Moreover, the improvements in neutral density filter quality allow us to take pictures with almost no color cast, and no decrease in sharpness.
If you love long exposure photography and you would like to push the exposure time to the edge, this technique of long exposure photo stacking is for you.
Here is the quick explanation of the Long Exposure Stacking technique: by taking several pictures (usually at least 30 seconds each for the purpose of this technique) you can blend them to get a result the equivalent of a long exposure photo with a total exposure time equal (or almost equal) to the sum of the single exposure frames.
This technique is mostly useful, and recommended for landscape photography, but some of the benefits may also be used in other types of photography, where you need maximum quality with several minutes of exposure. The step by step guide will be focused on landscape photography.
Before going over the explanation of the technique, there are at least a couple of required skills you should have (apart from the basics for taking pictures with your camera):
Let’s get on with the fun part: the implementation of Long Exposure Stacking!
The photographic gear needed for this technique is luckily the same gear needed for taking long exposure photos. So if you are already familiar with long exposure photography, you are set. Otherwise here is a short list of all the tools:
For further details about taking long exposure photography, you can check the related resources listed at the bottom of the article.
Never hit the shutter without having properly planned the photo. Of course there are a lot of variables involved, but planning can really play the most important part in a good photo. Do not underestimate the power of accurately planning a landscape photo – because luck can help you once, but planning your photography consistently, will increase the probability that you’ll be ready when the right conditions are present.
Once you have planned your photo and you are on location, you should then decide the two most important things in any photo (at least this kind of photo): composition and exposure (supposing you are taking landscape photography, depth of field should not be a variable in most cases). Composition is up to your taste, while exposure is a complex matter and depends upon different factors.
First thing every landscape photographer usually does before taking any picture, is metering the light to decide the exposure time. The final exposure time depends on several factors, such as strength of the wind, the movement of the clouds, water flow, and so on. The question at this point is, “How long shall I expose for?”.
If the light is changing suddenly, or there is a strong wind which can shake your camera/tripod during exposure, you may decide to avoid really long exposures (to avoid wasting shots or missing the right light). This is where this technique comes to your rescue.
Suppose you would like to have a total of eight minutes of exposure, instead of taking one single long exposure file you can split the total exposure time into four consecutive shots of two minutes each (for example). You can freely decide the final length of each exposure and the total number of shots, just keep in mind that you should keep the gap of time between the different photos to a minimum, if you are planning to do stacking. Needless to say that you should be careful in avoiding moving the tripod and the camera between each shot of the sequence. Once you have your consecutive shots, in the next step you will understand what to do in post-processing.
Before you start shooting: make sure you focused correctly and your tripod is stable (avoid sand or uneven floors where possible).
After the shot: make sure your histogram is fine and your picture is properly exposed, or exposed to the right.
Arco Della Pace by Luca Libralato: 6 x 130 second exposures, for a total of 780 seconds (13 minutes)
You should now be at home, or in your favourite cafe/pub with your laptop, and hopefully a coffee/beer. You should have downloaded all of your shots and selected the ones you would like to stack.
The stacking process will be shown for Adobe Photoshop (almost any version supports the feature) but you can use any software of your choice, which allows the use of layers. In Photoshop there are two different methods that can be used to stack files, with the goal to merge them into a simulated longer exposure:
With the first method the procedure is semi automated, since you just need to go to Files > Scripts > Load Files into Stack
All you have to do is then select the single long exposure, flag the boxes to Automatic Align Source Images and Create Smart Object.
Once you have a new document with the Smart Object selected, all you have to do is go to menu Layer > Smart Objects > Stack Mode > Mean. This will weigh all images embedded into the Smart Object, rendering them as a new image which similar to what you would have obtained with a single long exposure of a total time equal to the sum of the single exposure.
You can check the result on the image below.
By using the second method (which I trust can be used with every software which allows layers) the blending process is manual. The rule is that you have to fine tune the opacity of each layer by decreasing it after each shot.
The mathematic rule is as follows: each shot should have an opacity equal to 1 divided by its position in the stack. Here is a table which shows the rule:
This process will exactly replicate the first method (the result will be identical to an automatic Stack in Mean mode).
At this point, all you have to do is follow your typical post-processing workflow:
Here is the final result of the example
This technique is very useful sometimes because of its flexibility. Moreover you should consider the benefit in increasing the final quality of your file because of the stacking. It has been proven that by stacking images you can benefit by reduction in random digital noise equal to the square root of the total photos used. So for example if you use four photos, you will have in static part of the photo a reduction by half of the total random digital noise, which is a great achievement especially for underexposed portions. This technique is used a lot (even if with some different variations and implications) in astrophotography or in other fields when increasing the exposure time of a single shot is not suitable.
Don’t hesitate to ask questions in the comments box below if you need help or further explanations.