How to calculate exposure times with neutral density filters

March 8, 2017 -

So you have got yourself one or more of those magical, mystical pieces of glass we photographers refer to as neutral density filters. You head out to capture some images of that smooth silky smooth water or moving clouds that ND filters are known for, set your camera up, slide in or screw on your filters and start clicking away. But the image that appears on the back of your screen is either underexposed, or at the other end of the scale, overexposed. You start playing around with and randomly adjusting the camera settings adjusting each of those responsible for the exposure triangle (aperture, shutter speed, ISO) hoping that you can figure it out and eventually come away with an image you are happy with.

 

For many of us, the above scenario is exactly what happens when we first start using neutral density filters such as the 6 stop (also known as ND64) or 10 stop (also known as ND1000). The thing to remember here though is the name of the filter itself gives us the information that is required to make the exposure times calculations that are needed. There shouldn’t need to be any guess work put into these calculations. A “stop” of exposure is a physical calculation. To be exact, a stop of exposure in either direction is either double (an increase in a stop) or half of the amount of light (a decrease in a stop) allowed to enter the camera. Typically on any camera, this is a single turn of a dial in either direction for aperture, shutter speed or ISO.

 

The single camera setting you should be focussing at least to begin with is the shutter speed. So after taking the base exposure without any filters, the setting you would be adjusting is the shutter speed. For most cameras, if the shutter speed needs to go over 30 seconds you will need to use “bulb” mode and an external remote. The important thing to remember in the beginning if this is the case is to ensure the other exposure settings on the camera are identical to the base exposure. Once you get used to calculating a “stop” of exposure it will be relatively easy to calculate variations based on the aperture and ISO, but for now lets focus only on the shutter speed in this article.

 

When using neutral density filters such as the already mentioned 6 stop or 10 stop, the exposure setting that you should be focussing on to begin with is the shutter speed, as mentioned above. So lets use the following scenario and calculations through the charts below to work out the shutter speeds using ND filters from this example base exposure setting (the calculations the camera has performed without any filters attached to the lens) –

 

Here is the base exposure without an ND filter we will use in this example –

Mode – Aperture Priority
Shutter Speed – 1/120 secs
ISO – 100
Aperture – f/8
Focal length – 16mm

 

Here is an ND calculator chart for 6 stop, 10 stop and 15 stop ND filters. The top line is the base exposure time without filters on. The next three lines down are the relevant exposure times required compared to the base exposure.

 

Download this chart here.

 

Here is the 6 stop (ND64) calculation variation based on the above settings –

Mode – Aperture Priority
Shutter Speed – 1/2 secs
ISO – 100
Aperture – f/8
Focal length – 16mm

 

Here is the 10 stop (ND1000) calculation based on the above settings – 

Mode – Aperture Priority
Shutter Speed – 8 secs
ISO – 100
Aperture – f/8
Focal length – 16mm

 

Here is the 15 stop (ND32000) calculation based on the above settings – 

Mode – Bulb (as the exposure will be above 30 secs)
Shutter Speed – 4 mins
ISO – 100
Aperture – f/8
Focal length – 16mm

 

To use the chart above, the top line is the base exposure time. The next three lines down refer to either the 6 stop, 10 stop or 15 stop exposure times that are required based on the initial base exposure (the calculation without filters on). If anyone has an example image taken with a 15 stop of anything above 24 hours, please send it through. We would love to see an example 😉

 

As is mentioned above, these examples focus on using the shutter speed setting only. For beginners, this is the best place to start. Once you get a basic understanding of how the shutter speed is affected by ND filters, the same calculations of either halving or doubling exposure time can be applied by changing the ISO or aperture. There are also other factors to consider such as dropping light, so it can be important to take those variations into consideration for longer exposures of multiple minutes. But to begin with, focus on the shutter speed and you should be able to walk away with images that are perfectly exposed every time.

 

Here are a couple of example images using ND filters –

Cape Le Grande captured by Kieran Stone with a 6 stop ND filter – Canon 5DMKIII, Canon 16-35mm f/4, ISO50, f/8, 0.6secs, 6 stop ND + CPL

 

Mortimer Bay Fence captured by Tassiegrammer with a 10 stop ND filter – Canon 5DMKIII, Canon 24-70mm f/2.8L II, ISO100, f/18, 264secs, 10 stop ND

 

Recommended mobile apps to calculate ND exposure time – 

Photopills – Available for iPhone

ND Calc – Available for Android

NC Calculator Free – Available for Android

Neutrally+ – Available for iPhone

 

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