When you are using an ND filter, there are several ways to calculate the exposure time required for a correctly exposed image. For each ‘stop’ of light that the ND filter blocks, you need to double the exposure time in order to have a correctly exposed image. For instance, if a scene requires a 1 second exposure without an ND filter on, you will need an 8 second exposure while using a 3 stop ND filter (1X2X2X2). Unfortunately, the denser the filter, the more difficult it becomes to calculate the exposure time. There is one trick you can use that is independent of using an app or your mental arithmetic skills. Try this out:
- Compose your image and obtain the correct focus in manual focus mode.
- Make sure you are using the optimum aperture for the scene and do not change it for the rest of the steps.
- If you have live view on your camera, turn this on.
- Insert your ND filter (of any strength)
- Turn the ISO to 6400
- Obtain the correctly metered exposure at these settings and note how many seconds this is.
- Turn the ISO to 100 and turn your camera to bulb exposure mode
- Convert your previous exposure measured in seconds (at iso6400) to minutes and this will be your required exposure at iso100. For example, if at iso6400, the scene required a 3 second exposure, it will require a 3 minute exposure at iso100.
The reason this method works is that the conversion from iso6400 to iso100 is a difference of 6 stops of light. As is the difference between 1 minute (64 seconds to be precise) and 1 second.
How to Take Long Exposure Shots with NiSi Filters
by Dylan Toh
