NOTE: This tutorial is outdated, please check the most recent and more elaborate Long exposure guide here.
How to calculate the exposure time?
Choosing the right exposure time when using ND filters can be quite confusing.
First of all, do a light metering without the ND filter. Most DSLR’s have a built-in light metering function. Just set your camera to aperture priority (AV for Canon, A for Nikon), select the aperture you want to use and press your shooter about half way in. The proper exposure time will appear on your LCD. If you camera can’t do this, use a light meter.
Remember the shutter speed on the LCD and go down the number of stops that your ND filter filters. Let’s say your camera calculates an exposure time of 1/500s without your ND8 filter. What shutter speed do you need to use with the ND8 on your camera? Easy. Remember, ND8 is equal to 3 F-stops. So, just go down 3 full stops of shutter speed. 1/500s: 1/250s – 1/125s – 1/60s. Now you put on your ND filter, set your camera to manual with the aperture you’ve chosen earlier and a shutter speed of 1/60s.
Now, calculating the exposure time can be easy if you just need to go down 2 or 3 stops. But going down 10 stops might be difficult without a calculator… So, for your ease (and mine), I created this chart seen below (click to view it large).
I think it might use a little explanation. The first line resembles the shutter speed given by your camera without a ND filter (the line that starts with 0, 0, 0, 1/8000s, 1/4000s, 1/2000s etc). That is your reference point. Imagine you’d want to use a 6 stops ND filter. First you meter the light with your camera without the filter. Your camera might say you’ll need a shutter speed of 1/500s. What shutter speed do you need to use with the ND filter? Go to the top row (row’s are horizontal, column’s are vertical), this is your reference point. At this row, go to 1/500s. Go down in that column until you see a 6 on your far left in the F-stop reduction column. I highlighted this example in purple. You’ll see that you are going to need a shutter speed of 1/8s.
So once again to be perfectly clear: if the camera meters the light at 1/500s without the 6 stops ND filter, than you’ll need to use a shutter speed of 1/8s with the ND filter.
One more example: the camera meters the light at 1/30s without the ND filter. You want to use a 10 stops ND filter. What shutter speed do you need with the ND filter? If you follow the chart correct, you’ll see that you’ll need to use a shutter speed of 30 seconds.
A quick note about the chart
In theory this chart should be perfect. However it has its flaws…
The darker the ND filter, the less reliable this chart gets. For example, if my camera calculates an exposure of 1/30s without my 10 stops ND filter, I would need to use an exposure of 30 seconds with the 10 stops filter. This is true in theory, but doesn’t always quite work out right in real life. I recommend going 2 or even 3 stops further, so using 2 or 4 minutes of exposure time.
For the lighter ND filters (until about 6 stops) this chart holds up pretty nicely.
And a tip: always take notes while shooting. Make it as detailed as possible. What type of light (hard sunlight, moon shine, strobe, cloudy), what time of day, the subject, what did the sky looked like, what shutter speed did your camera calculate, what shutter speed did you end up using, etc, etc. These notes will prove their use in the future.
In short: when using heavy ND filters, go down 2 or 3 stops further than the chart says. When using light ND filters, the chart is mostly correct.
An overview of ND filters
There are many ND filter manufacturers, some produce only light ND filters, some produce a whole range of them. The filters come in different price ranges and in different sizes. For your and my comfort I created this chart that has all of this information on just one page (click to view larger)
A work in progress
“The ultimate filter guide for your long exposures” may sound a bit pretentious. And it is. I want to cover everything related to long exposure photography. However, this guide can only be really ultimate if all of your questions have been answered. Although I strongly believe that I’ve covered a large area, I can’t imagine every single question of you has been answered.
If you have any questions, comments, remarks or critiques, let me know. I will add your questions or comments to this guide. Only with your help, this guide can become really ultimate.