One of the great things about photography, is the ability to influence time. Just by changing the exposure time, you can freeze motion, or blur all the moment into a soft blur. And a special part of this is long exposure photography with the use of an ND filter. And that’s what I will be looking at in this post.
What is a ND filter
A ND filter is very dark piece of glass. The purpose of it, is to block a certain amount of light to enter the cameras lens. There are different types of filters with different strengths. They can range from filters that block 50% of light up to filters that block 99.9% of light.
All the filters are named based on how much stops of light they block. So you have a ND1 filter, that blocks 50% of light. That means you will have to use an exposure that is twice as long, so it’s one stop of light blocked. The ND2 filter, will block 75% of light and you will have to use a 4 times longer exposure , and so on until ND10 (ore even more). Sometimes the filters are not named based on the stops, but on how much they multiply a 1s shot (at least I think that it :)). So you have a ND1, ND2, ND4, ND8, ND16 until ND512, ND1000 (should be 1024) and similar. But when you buy a new ND filter, always check how many stops it blocks, as that’s the important information.
There are different qualities of ND filters, and the two most important parameters are, if they introduce a color tint and if they create additional chromatic aberrations. The priciest ones out there create almost none in both cases. The cheap ones on the other hand, will usually create a strong green or purple tint, which needs to be removed in post-processing.
How to take a long exposure photo
Using lighter ND filters, like the ND2, ND4, ND8 is quite simple. You just put the filter on the lens and take the photo. They make for a longer exposure, but the difference is not so big, and you can usually fit the shot under the 30s that every camera allows. It get’s a little more complicated by the darker ND filter, especially by the 9-stop and higher. With a filter like that on your camera, you will not see anything through the viewfinder, and mostly not even through the live view (depends on the camera). So what to do then?
The process is simple, but of course a tripod and a shutter release remote here are mandatory. The steps are:
1. Frame the shot without the filter
2. Focus, either manually, or use auto-focus and turn the auto-focus off after that.
3. Go to the Manual (M) mode
4. Set your ISO and aperture that you want to use
5. Meter the exposure by half pressing the shutter button (remember it :))
6. Switch the camera to Bulb (B) mode, make sure the ISO and aperture are set the same
7. Put on the filter.
8. Calculate the needed exposure time
9. Take the shot by holding the shutter button (preferably on the remote) pressed for the required amount of time.
Let’s look at step 8 now, how to calculate the exposure time. You can either do it your self, by multiplying the exposure time you metered before you put on the ND filter. Fore each stop of the ND filter, you multiply it by 2. So if you have a 1s exposure and a ND9 filter, you mulltiply it as 1*2*2*2*2*2*2*2*2*2 so 9 times, which gives you 512 seconds. Of course that is quite simple by 1s exposures, but what if you exposure was 1/400s? That makes it a little harder to calculate. But since everyone caries a phone these days, you can help yourself with any of the available apps that do this for you. You just need to download one of the available ND calculators, and enter the starting exposure and the ND filter stops and it will give you the result right away. Here is one of those apps for Android and one for Windows phone, there are many more, of course also for iOS.
Also, regarding the remote, you should use one that allows to lock the shutter button, so you don’t have to hold it for the whole time. By 30s this is still manageable, but by a 5 minute one, you will no longer feel your fingers after you are done :)
Making it easier with Promote remote or Magic lantern
If you have a Promote remote, or other programmable remote, you can make it easier for yourself, as you can just program the desired time through the remote. You then don’t have to check the time on the camera, as it will do that for you.
With the Magic lantern firmware, you can do it even easier, as you can set up the exposure time even directly in the camera. It also boosts the brightnes in live view, so making it easier to focus even with the filter on. And it also removes the 30s limit for HDR brakceting, so you can do a HDR exposure series even with the filter. If you don’t know how to take a long exposure photo with Magic lantern, check my video on it. You can find it at the end of the How to take multiple exposures video on my Videos page.
That’s all for this post. Feel free to ask any questions, and btw. the two photos included were of course taken with a 9 stop Hoan ND400 filter.