Advanced If you shoot landscapes - especially scenes containing water, such as waterfalls, oceans and lakes, you probably already know something about using a neutral density (ND) filter. The neutral density filter is the go-to tool for any photographer who wants to take a long exposure during daylight hours. All those stunning images of misty oceans and rivers that you've admired were probably taken with ND filters. But if you don't have a set of ND filters there is another trick you can employ to capture similar images - and it's less expensive and more flexible than a set of ND filters, too.
Before I explain this trick to you, it's important for you to understand how one type of ND filter in particular works. The graduated neutral density filter is a tool that was most likely invented by a photographer who was starting to get irritated by all the blown-out skies in his otherwise beautiful landscape photos. Because the sky in almost any daylight scene is brighter than the rest of the scene, a graduated neutral density filter - which is darker at the top and transparent at the bottom - will block some of the light coming from the sky while leaving the rest of the scene alone, resulting in a better exposure.
Like standard ND filters, graduated ND filters are available in different stops, and just so you'll have more stuff you need to buy, they also come in "hard" and "soft" versions, which basically means that the transition from dark to transparent is either abrupt or gradual. Now, since you're a photographer, you're used to having to buy expensive stuff. But a full set of high quality graduated neutral density filters might cost as much as (or more than) your DSLR, and for a lot of photographers (especially hobbyists) that's a hard pill to swallow.
This was landscape photographer Tony Brackley-Prower's beef with graduated neutral density filters when he pioneered a technique now widely known as "the magic cloth technique".
No, you don't need to infiltrate Madam Lestrange's vault to find this "magic" cloth, because you can really use just about anything to achieve the desired effect, from your smart phone to your sock - though if you're shooting a snowy scene you probably don't really want to use your sock. But the technique really is that simple, and if you don't have a dedicated "magic cloth" with you, just about anything will do in a pinch.
How to use the magic cloth
The magic cloth technique is exclusively used with long exposures of at least two seconds, though you will find it easier to execute with exposures that are a lot longer than that. The basic concept of the magic cloth technique is very simple. Just cover the front of your lens with your magic cloth or whatever object is standing in as your magic cloth, and then - using your camera's timer or a cable release - open the shutter and slowly raise the cloth. Fast movement will simulate a hard ND filter, while slow movement will simulate a soft one. Sounds like a no-brainer, right? Except that there's obviously a lot more to that, since the length of time you allow the cloth to block your lens will determine how much light is going to reach the sensor. The more time you hold it there, the more light you block. If you ever printed film images in a darkroom, this will seem familiar to you - it is very similar to the dodging and burning techniques film photographers once used to get the correct contrast in a print.
To get started with this technique, Tony Brackley-Prower suggests setting your exposure compensation at +2, which will help with your foreground exposure. Use as small an aperture as your lens will allow (f/16 or f/22, ideally) and make sure your ISO is also very low: no more than 100. You will most likely also need to purchase at least one standard (non-graduated) neutral density filter, preferably one that blocks at least 8 stops of light. Combining this technique with an ND filter will help you achieve those longer exposures that you need in order to have success with this technique.
So besides expense, why is this better than a graduated ND filter?
The coolest thing about the magic cloth technique is that it is dynamic, which means you aren't limited by where that transition in your graduated ND happens to be. You can apply the technique to the top or bottom of the frame, or even divide the frame into multiple parts and block each part for a different amount of time. You can create movement in the clouds by applying the magic cloth to the sky in short intervals rather than exposing it all at once. If an annoying tourist walks into your scene and starts poking around in a tide pool, you can cover your lens with the magic cloth and wait until he goes away. And if you're shooting at night and you want to capture both the foreground and the night sky without any star trails, you can use this technique to get the right exposure for both parts of your scene.
OK so what's the catch?
There's a reason why most photographers just bite the bullet and buy the set of filters - because the magic cloth technique, though very cool, requires a lot of patience and - worse - a lot of trial and error. Obviously any technique that relies on manual manipulation (vs. that little computer in your camera that can do everything in exactly the same way every single time) is going to be highly error-prone, which is why we photographers are always thanking our lucky stars that we no longer shoot expensive film. Since you'll need to shoot many frames and spend a lot of time in the field when experimenting with the magic cloth technique, you should probably only try it when you aren't out hiking with a bunch of people who'd really just like to get back to the campsite already. This technique can create magic, but it's going to take a lot of practice.
Like everything else, the magic cloth technique is one that you'll master over time, and with enough experimentation you'll get a pretty good feel for how the light will need to be manipulated in any one scene. Just don't forget that you're likely to have more failed images than good ones - and remember that if you keep working at it eventually the good ones will be those jaw-dropping, wow-inducing images that make other photographers green.
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