Calculating Exposure


Today’s Question: I just purchased a 10-stop neutral density (ND) filter. Could you advise on how to calculate exposure times?

Tim’s Quick Answer: The process of calculating exposures when using a neutral density (ND) filter can actually be very simple. Start with exposure settings that are proper for the scene without the ND filter, using the Manual exposure mode. Then add the ND filter and adjust the shutter speed by the number of stops indicated for the density of your ND filter.

More Detail: In essence, I recommend starting off as though you weren’t using an ND filter at all. Get the camera setup without the ND filter attached to the lens. This makes it much easier, among other things, to compose your photo.

You can use whatever method you want to determine the appropriate exposure settings, but you’ll want to end up in the Manual exposure mode. If you’re not comfortable using Manual mode to establish the initial exposure, you can start with a semi-automatic exposure mode (such as Aperture Priority) to determine appropriate exposure settings. Then switch to Manual exposure mode and dial in the exact same settings.

It is worth noting that under most circumstances I will start off with the ISO at the minimum value and the aperture at the smallest opening (largest f-number) when capturing a long exposure photo that employs an ND filter. However, part of the benefit of using an ND filter is that you can get a relatively long exposure even with an aperture that isn’t stopped down all the way. This enables you to, for example, create a long exposure that also has a narrow depth of field.

Once you have determined your exposure settings and have everything else configured (such as composition and focus), you can add the ND filter. It is worth noting that the Xume Adapters ( I’ve talked about in the past make this step much easier, since the ND filter can then snap into place using magnets.

With the ND filter attached, the final step is to adjust the shutter speed based on the number of stops of light being blocked by the ND filter. In this case, for example, the question relates to a 10-stop filter.

Generally speaking, your camera will adjust shutter speeds using one of the dials on the camera, in either half-stop or third-stop increments. In other words, you will turn the dial either two or three “clicks” for each stop. Make sure you’re turning in the direction to extend the exposure time, not shorten it. Then turn by 20 “clicks” if your camera is set to half-stop increments and 30 “clicks” for third-stop increments.

I am in the habit of counting out stops of light based on sets of clicks. I generally have my camera set to adjust exposure in half-stop increments, so for each two clicks I will count off a stop. To me this is a little easier than counting out twenty clicks, for example.

Overall the process here is very simple. Setup your camera based on no ND filter being used, then add the filter and adjust the shutter speed by the number of stops needed for that filter.