A Stop is a measurement of light. If you add 1 Stop of light, you've doubled the amount of light entering the camera. If you lose 1 Stop of light, you now have half the amount of light entering the camera. On most cameras, there are also 1/3 Stop intervals. That means you will see two 1/3 Stop options between each Full Stop. Pull out your camera and set it to Manual Mode. Use the main scroll wheel to change your Shutter Speed. The Full Stops will be 1/30s, 1/60s, 1/125s, 1/250s, etc... (remember, since Stops double the amount of light, the length of the Shutter Speed doubles each time) Follow along with the charts below and use your camera to scroll through the Stops.
These charts show the 1 Stop increments for each main camera setting - Aperture, ISO, and Shutter Speed. Each Stop either doubles the amount of light, or halves it.
As you travel up the Aperture Chart, from left to right, you are halving the amount of light. f/1.4 is wide open, and receives a lot of light! f/22 is a narrow pinhole and only lets in a small amount of light. Remember, these are the Full Stop increments. So f/8 gets half the amount of light of f/5.6. Depending on your lens, you may not be able to access all of these Stops. Your lens may only open up as wide as f/4 for example. There are also two 1/3 Stop increments between each value below, you will likely see these in-camera.
As you travel up the ISO Chart, from left to right, you are doubling the amount of light. (The opposite of the Aperture Chart)
So ISO 400 is twice as bright ISO 200. Again, you will also see two 1/3 stops in between each value below as you increase your ISO in camera.
This is the most important chart for Long Exposures, since we need complete control over the Shutter Speed. As you travel up the chart, from left to right, you are doubling the amount of light (as well as the Shutter Speed). So 1/2s is twice as long as 1/4s and therefore gets double the amount of light. Since this chart is so long, it probably isn't displaying correctly in your browser. Download the JPEG linked at the top of the page for a better view.
|1/8000s||1/4000s||1/2000s||1/1000s||1/500s||1/250s||1/125s||1/60s||1/30s||1/15s||1/8s||1/4s||1/2s||1 sec||2 sec||4 sec||8 sec||15 sec||30 sec||1 min||2 min||4 min||8 min||16 min||32 min|
Now that we have a clear view of the Stops, we can use this to determine the Long Exposure Shutter Speed.
Remember, the Little Stopper by Lee Filters is a 6 Stop ND filter and the Big Stopper is a 10 Stop ND Filter. That means the Little Stopper slows down the Shutter Speed by 6 Full Stops on the Shutter Speed Chart.
Example 1: Let's say we're using a 10 Stop ND filter. That means we are slowing the Shutter Speed by 10 full stops of light. The current shutter speed, without any filters, is 1/500s. We can look at the Shutter Speed chart and simply count over 10 columns. 10 columns over from 1/500s is 2 seconds. So we can add our ND filter and change the Shutter Speed to 2 seconds.
Example 2: This time we're using a 6 Stop ND filter. Current shutter speed, without any filters, is 1/4000s. Counting 6 columns over from 1/4000s gives us 1/60s.
Example 3: We purchased a 10 Stop ND and a 6 Stop ND, for a total of 16 Stops. The Shutter Speed is 1/250s without filters. Counting over 16 columns gives us 4 minutes. We have to set the camera to Bulb Mode and manually time the exposure. We stop the image at precisely 4 minutes, but the image is still really dark! What happened?!
Unfortunately, each filter is not exactly the correct density. The 6 Stop is actually 6 and 1/3 Stop; the 10 Stop is actually 10 and 2/3 Stops. That gives us a total of 17 Stops. With this information, we can now change the Shutter Speed to 8 minutes to get a properly exposed photograph. We also need to remember that any photos taken with the 6 Stop now need to be a 1/3 Stop longer than we initially thought. Read this Blog Post for more information.
Example 4: It's starting to get dark out, the current shutter speed is 2 sec. We have a 10 stop ND filter with us. That would mean the shutter speed would have to be at least 32 minutes! That's just not gonna work. Thankfully, we can always adjust the ISO or Aperture (or both!). Lets say we only want the Shutter Speed to be 4 min. That means we have to add 3 stops of light (3 columns between 4 min and 32 min). So, we can either increase the ISO from 100 to 800 or the Aperture from f/8 to f/2.8. (or any combination of ISO and Aperture, as long as 3 Stops of light are added)
Example 5: We are at the beach photographing the waves. We want to capture the motion of the waves, so we need a Shutter Speed of ~1/2 sec. Our current settings are f/5.6, ISO 100, and 1/1000s. We have a Polarizer and a 6 Stop ND. The Polarizer will cause us to lose roughly 1 Stop of light. Therefore, we effectively have a 7 Stop filter. Counting over 7 columns from 1/1000s gives us 1/8s. Still not slow enough. We still need to lose 2 stops of light. We can change the Aperture from f/5.6 to f/11 and now we can shoot a properly exposed photo at 1/2s.
Example 6: This time we're gonna go for it. Let's stack the Big Stopper and Little Stopper for a total of 16 Stops. It's a bright, sunny day out. The Aperture is f/8. Current Shutter Speed, without filters is 1/125s. After we add the filters, we need to count over 16 columns! That gives us a Shutter Speed of 8 minutes. You will need to set the camera to Bulb Mode and use a remote shutter. Start a timer on your smartphone and begin the exposure at the same time. I also turn on Long Exposure Noise Reduction to prevent Hot Pixels from forming. Keep in mind, now the 8 minute long photo will actually take 16 minutes total. After the normal photo finishes, the camera will take another 8 minute long Dark Frame. (This only happens when Long Exposure Noise Reduction is turned ON)
Lee Filters recently created a free application that makes Long Exposure Photography super easy! You can download it on the Google Play Store and the App Store. This is how I calculate my long exposures.
This app provides Shutter Speeds for the Little Stopper (6 Stop ND), Big Stopper (10 Stop ND), and Super Stopper (15 stop ND). If you already have any of these filters, or a filter of the same ND strength, you're all set! It even includes a built in timer!
If you have a different strength ND though, it's crucial you understand Stops. For example, if you have 16 and 1/3 Stops, you can click the 15 Stop option to find your Shutter Speed, you'll just need to look down 1 and 1/3 stops on the right side of the screen. Once you start using the app this will make more sense. As long as you understand that adding 1 Stop doubles the Shutter Speed, and that there are two 1/3 Stop increments between each Full Stop, you should be good.
Don't forget, most cameras need to have Long Exposure Noise Reduction ON in order to remove Hot Pixels. This will double the exposure time. After the camera takes an 8 minute long exposure, it will automatically take another 8 minute long exposure (without any light reaching the sensor) and then create the final image (minus Hot Pixels). Therefore, if you are shooting a really long exposure, 32 minutes, it will take over an hour to get 1 photo!!
If you don't want to wait all that extra time, you can try to fix Hot Pixels in Photoshop. You will need to use the Dust and Scratches Filter, this is the best method I've found for removing Hot Pixels in post-production.