As a general rule, most of the pictures you take during the day are shot with a short exposure. And why not? There's plenty of light during the day, and a long exposure might create unwanted motion blur and maybe even put you at risk for that dreaded camera shake. With the right equipment, though, daytime long exposure photography can be a fun and interesting way to get some really unique and surreal-looking shots of otherwise ordinary scenes. But capturing a good photo with a long exposure can be tricky even at night time, so it's worth understanding some of the secrets behind this very versatile technique.
Why shoot long exposures during the day?
Daytime long exposures kind of go against everything we know as photographers. Daytime, we generally understand, requires short exposures, which will freeze the action and allow for shorter depth of field. So what kinds of images can you get with a five minute exposure in broad daylight?
In short, long exposures take moving objects and substances and make them misty and ethereal. Water becomes more like a low-lying fog, and clouds lose their definition. Slow-moving people, animals and traffic take on an almost animated quality which is otherwise difficult to achieve in a still photo, and fast-moving objects become almost invisible. You can use a long exposure to turn almost any scene into something that looks less like a photo and more like the creation of someone's imagination.
You will need different exposure lengths depending on what you want to achieve in your final image. A simple motion blur that still keeps objects fairly visible may only require a 5 to 15 second exposure. To get that misty look in water or to make clouds lose their definition, you will need a much slower shutter speed of 1 minute or more. At longer exposures (5 to 10 minutes or more), moving objects will vanish entirely. You can use long exposures such as this to create the illusion of empty streets and sidewalks in tourist-infested areas.
But you can't do any of these things if you don't have the right equipment. Fortunately you don't need much, so don't worry about having to double the size of your camera bag any time you are chasing long exposures during daylight hours. A few basic tools are all you need.
First, a neutral density filter. A dark one.
When shooting long-exposure photographs during the day, light is your enemy. To avoid overexposing your pictures you'll need a neutral density (ND) filter, and not just any neutral density filter. You will need one with at least six stops of light reduction, preferably nine or 10 (a 10 stop ND filter will be labelled as having a 3.0 optical density with a filter factor of 1000x). And this is the one piece of equipment you can't skimp on--a 3 stop ND filter won't do the trick. Depending on what you want your final image to look like, you will need one that will allow you to do a 1 to 5 minute exposure in broad daylight. If your filter isn't strong enough, you might as well go back to taking long exposures the old-fashioned way: at night.
A tripod is not optional
Obviously, you can't use a long exposure if you don't have a tripod. Depending on your lens, camera shake starts to become noticeable at around 1/30 sec, which is at least 1,800 times faster than the shortest exposure you'll be using to create these types of images.
DSLRs (even the more advanced ones) don't go past 30 second shutter speeds in the automatic modes, so you'll need to use your camera's "bulb" setting in manual mode to do exposures longer than that. Depending on your camera, you may also need a remote release for anything over that 30 second margin, since on some cameras the bulb setting requires that you hold down the shutter button for the length of the exposure (other cameras just require you to press once to open the shutter, and again to close it).
How are your math skills? You'll need to use them.
Daytime long exposure requires a little bit of math and a lot of experimentation. Your through-the-lens metering system isn't going to work well through a very dark neutral density filter, so you'll have to determine your exposure by metering without the filter (using the same ISO and aperture you plan to use for the shot), and then adjusting the shutter speed by the number of stops your filter blocks. For example, if your camera metered the scene for 1/15 of a second, you will need to count down to the correct shutter speed from there. If your ND filter blocks one stop, your shutter speed will be 1/8 of a second--or the next slowest setting. If your filter blocks two stops, your shutter speed will be 1/4 of a second, or two settings slower than the original 1/15. For a 10 stop filter, you will need to go all the way down to 1 minute.
Tip: there's at least one iPhone app out there that will help you calculate the exposure time (it's called "LongTime Exposure") and there's also one for Android. To use this app or another one like it, you just find your ND filter and match it to the exposure time you metered for without your filter attached. The app will then return the correct shutter speed for your filter.
As a general rule, you will need to shoot using your camera's lowest ISO setting, with the aperture as small as your equipment will allow. This lets in the smallest amount of light possible, which is what you want when using a long exposure during the day. Shooting in RAW will give you photos that are easy to manipulate, which is something you may need to do with these kinds of images--noise, color cast (more on that below) and hot pixels can all be problems with long exposure photographs shot during the day.
Some common problems with daytime long exposures
Very dark ND filters (beyond six stops) may create magenta casts in color photos. This is because a neutral density filter only blocks light across the visible spectrum - infrared light is still allowed through. Many photographers dislike this pinkish-purplish cast, but it can be difficult to correct even in post processing so you may either have to live with it or simply change your images to black and white. Or you may discover that you don't mind a strange tint in your already strange finished photo.
You may also have trouble with vignetting, which is the darkening of your photo's corners. This happens because the ND filter itself may block some of the light coming from the scene. It is more likely to occur with filters that have a thick mount, or when you "stack" filters to achieve extra stops of light reduction. You can reduce vignetting in post processing, though it may be easier to simply crop it out.
I know I'm always telling you to experiment, but that's especially true for daytime long exposures. The truth is you don't really know exactly what your image is going to look like until you see it on your computer screen. But the good news is that you're very likely to be wowed by some of the images you create this way--and you may even find yourself becoming a little bit addicted to the technique.
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