I recently took a trip to El Capitan State Beach, remembering it fondly from going there as a kid, at least a decade ago. Hoping to be treated to some visually entertaining weather, there were next to no clouds, and none of the morning coastal fog I had expected. Nonetheless, the tides and weather cooperated long enough to allow me to test out an idea I had; using multiple shorter exposures to simulate the effects of a single long one taken with a Neutral Density filter.
For most lenses, it’s probably just worth one’s while to shell out for a decent ND filter, but for higher thread diameter filters, it gets prohibitively expensive for anyone who isn’t getting a lot out of their photos whether it’s money or serious enjoyment. Since a lot of landscape (and seascape) photography is done using very wide lenses, it’s not at all uncommon that they require a 77mm or 82mm filter. I was using two wide lenses on this trip, Tamron 11-18mm and 20-40mm which both require a 77mm filter.
The least expensive static ND filter sold under a recognized brand I was able to find was by Tiffen and would cost about $30, ranging up to a more preferable Hoya or B+W filter costing about $70. This is free! Even if you already have satisfactory ND filters, I can still see this being useful when you forget them at home or don’t have the time to get one onto your lens.
Variable ND filters are another option, and particularly preferable, since more than one normal profile filter causes visible vignetting at the wide end on some ultra wide angle lenses. Likewise, that rules out the option of combining two polarizing filters as described here. At some point in the future I’ll give this a try, but for now it’ll have to reside on my To-Do list.
In my experimental case, I tried several variations on scenes, ranging from 1/5 second exposures all the way up to 4 seconds. In either case, it is of course necessary to use a tripod, and when shooting as close to the incoming waves as I was, that leads to its own problems which I’ll address later on.
So now we get down to exactly how to accomplish this effect. Since I know Photoshop, I’ll explain how to use it to do this. I’m sure you can do it with other graphics programs and the execution is likely pretty similar to what I’ll show.
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Taking the Frames:
You’ll want to set up with a tripod to do this or suffer camera movement on stationary objects. Compose your scene and select appropriate exposure settings in manual mode to obtain a properly exposed scene with as long of a shutter speed as possible. Try not to set an aperture lower than F/16 if possible as this can lead to image sharpness degradation as a result of diffraction.
As with normal long-exposure photography, a longer exposure lends a smoother appearance to moving subjects and is hence ideal. I probably wouldn’t bother doing this unless I could obtain a shutter speed 1/5 second or slower.
If you’re shooting a creek/waterfall/river, it doesn’t really matter when you take the photo as the movement of the water is generally quite constant. For waves at the beach it’s more challenging as there are times when the water is hardly moving at all. You have to anticipate the movement and take your photo when the water is moving as much as possible. On top of that, the water will turn the sand soft and your tripod will sink in during the exposure. Small piles of rocks beneath the legs help with that, but don’t alleviate it entirely.
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Combining the Frames:
Before we begin, if you’re doing more than 5 frames, I recommend you download VerdantVista.com’s Image Averaging Photoshop Script. This will make setting layer opacity vastly easier when you’re working with more than just a few images. It works very quickly and is simple to install and use.
First things first, you’ll want to get your RAW files prepped and decide if you want to work in a 16 or 8 bit workspace. If you’re working with JPEGs you can feel free to skip this step. When you open the files from within Photoshop, the Adobe Camera Raw (ACR) window will pop up, allowing you to apply revertible changes to your images. At the bottom of the window, you’ll see a hyperlink which says something like “Adobe RGB (1998); 16 bit: 4912 by 3264 (16.0mp); 240ppi”
Click that and it will open up a Workflow Options dialogue box which looks like this:
Right now, the only option we’re concerned with is ‘Depth.’ Your setting here will indicate which workspace Photoshop will use for the following steps. 16 bits will allow you more freedom in adjustment without creating as much banding or noise. Feel free to adjust the RAWs you plan to use in advance.
Once you’re finished with your images, click “Done”
From within Photoshop go to the Scripts menu and select “Load Files into Stack…”
In the browse option, select which photos you’d like to use. They can be any format that Photoshop is capable of handling. If you are worried that there was movement between each of the frames, check the “Attempt to Automatically Align Source Images” box. This process will take a long time if you’re using a lot of frames.
Once it’s done its thing, you should have a stack of layers, each one one of your frames. If it did have to do alignment, you will need to crop the edges of the image a little bit.
At this point, if you’ve installed the Image Averaging script, return to the scripts menu and run the script. It’ll work for any number of images you have loaded into the layers stack. It’ll prompt you to merge all layers once it’s finished.
If you didn’t load the script, you can set your layer opacity manually in the Layers pane. The bottommost layer needs to be at 100% opacity, the next one up 50%, the next up 33%, and so on. The opacity of each layer is dependent on the number of layers in the stack below and including itself, 1/X where X is the layer’s number from the bottom of the stack. If it is the 2nd layer from the bottom, it is 1/2 or 50%. If it’s the 4th layer from the bottom, it is 1/4 or 25%. If it’s the 20th layer, then 1/20 or 5%. This seems strange at first as 50% + 50% = 100%, but in the context of opacity it makes sense with a little thought.
You’re done with averaging at that point, and you can do whatever you need to to get your file ready to save. If you need to modify the image directly, you may merge all the layers and do so.
I’ve been trying this technique; I always thought that you had to use a median stack. I suppose that there could be special situations where you should or shouldn’t use this technique. But I have tried using this technique too, and the results seem to differ from that of a single long exposure because there’s a delay between frames, which is why I still have my ND filter. I’ve tried doing a welding glass filter, but haven’t found a way to get rid of the green tint so I opt for a Haida 10 stop.
With a sufficiently large number of frames, particularly of a somewhat chaotic system like moving water, it does a good job approximating a longer exposure. There are certainly many cases where it would not work nearly as well though. Median seems to work differently where it is taking the middlemost value for a given pixel in a stack of images rather than the average value for that pixel. A classic use case being removing tourists from scenes. If you attempted that with averaging, you would instead have ghosts of people rather than a properly empty piece of scenery. In other cases, like noise-reduction, the outcome should be approximately the same.
This technique is mostly for people without ND filters or who forgot theirs. Cheap alternatives might be sunglasses, welding filter, or even stacked polarizers, but all of those have some drawbacks, like the green tint you mention. In a perfect world we could have all the tools we need all the time, but it doesn’t hurt to have alternatives for when the world reminds us that it is imperfect.