Neutral Density Filters

The following are some of the practical aspects of choosing and using Neutral Density (ND) filters.

What are ND filters?

• Dark glass (or resin) which allow a photographer to lengthen the shutter speed.
• Screw on (circular) vs Square (slot in) – see below in accessories
• Light blocking terminology:

Stops blocked	1 stop	2 stop	3 stop	10 stop
Exponential version	ND2	ND4	ND8	ND1024
0.3 version	ND0.3	ND0.6	ND0.9	ND3.0
Shutter speeds
No filter = 1 sec	2sec	4sec	8sec	1024sec

Accessory checklist:

• Tripod: must be firm and steady
• Cable release : essential to achieve exposures of greater than 30 seconds. Some firmware changes such as ‘magic lantern’ and some camera applications (such as for the sony A7r series) offer an alternative to a cable release but I prefer the former.
• Filter holder and square filter set up. Allows 2(Lee default) to 3(Nisi default) filters to be stacked in front of the camera. Lee requires an additional slot for a 105mm CPL, Nisi system has a CPL attached to the default sized adaptor ring itself. Insert ND filter on the closest filter slot with padding firm against edges of the holder to prevent light leak.
• Screw on filters need no filter kit. These minimise light leak but are unable to ‘stack’ with other filters. Can be used with CPL but could lead to vignetting.
• Optical view finder cover (to prevent more light leak). Can be on the camera strap itself (Canon) or can be anything invented ! eg. Blue-tac, duct tape.

 Density recommendations (approximate)

• 2-3 stop : for waterfalls, seascapes in bright light : target shutter 0.5-5 seconds

1 second foreground using a 3 stop ND filter 3 minute sky using a 10 stop ND filter

• 6 stop : for fast moving clouds, seascapes in low light: target shutter 5-60-seconds

6 stop ND filter for 1 minute exposure at dawn , fast moving cloud

• 10 stop: for elimination of water textures, slowish clouds: target shutter 1-5 minutes

10 minute exposure using a 15 stop ND filter after dawn with slow moving cloud

• 15 stop: same target as 10 stop except in brighter conditions: target shutter 1-20 minutes
• Vari-ND: limited by screw on setup and some brands have bad cross polarisation artefact.

 Camera setup :

• Turn off long exposure noise reduction. (noise reduce in post processing)
• Turn off autofocus . Leaving it on may result in the camera hunting in the dark for and lead to an out of focus image
• Keep ISO low. This is within camera limitations. I like to keep ISO <400. (if going much higher, could you just achieve the same results with camera settings alone or less dense ND filter?)
• Activate expanded ISO (eg. ISO50 for Canon users)
• Adjust aperture according to given scene. At your lens’s sweet spot at f8, the exposure may not be long enough after minimising ISO and placing your most dense ND filter.

Shot at F11 with fading light. I worked for a 2 minute exposure after experimenting prior which meant using iso200 with a 3 stop ND filter.

 Composition tips:

• Usual rules of composition apply
• ND filters allow simplification of ‘chaotic’ elements eg random water motion, unattractive clouds. Use these to emphasise areas of detail within your image
• Compose without ND filter on to achieve your desired composition and to find your focal point manually.
• Don’t let the ND filter dictate the shutter speed. Use ND filters to achieve the shutter speed you desire for the scene.

Short exposure version of a scene (which allows many more options in changing light both with regard to shutter speed and compositions – this is a vertorama)

Biting the bullet and going for a long exposure can be hit and miss. More hits arrive with more practice!

Calculating exposures

• For every stop of light blocked, exposure duration doubles. (See first table)
• Use apps if you don’t mind bringing devices into the field
• Alternatively, use this approach

This shot at Mannum Falls was taken with 10 stop ND filter – viewing through the viewfinder would be pitch black, hence composition using live view is a good work around.

How long do I want?

• See the above section in ‘density recommendations’
• Start with that filter and adjust ISO/aperture accordingly to achieve the exact shutter that you need for the scene.
• During your test exposures without the ND filters, take some dedicated shots with shorter shutter. You  might like them more!

Varying appearances of water and sky according to shutter speed and density of ND filter used. All images used the same settings in LR.

The 10 stop version was the pick of the bunch for me, but if I change my mind, I could use any of the previous images.

With or without filters?

• Think about combining filters to achieve a single exposure result
  • Using an additional GND for the sky
  • Avoid stacking ND filters as the filter in the 2nd slot will almost certainly let light leak in
• Think about taking an image for dark areas of the scene without the ND filter on as the ND filter will require a very long exposure for deeply shadowed areas. Blend them in post processing.

This 10 stop ND filter shot has some elements of shadowed areas blended from a shot with the ND filter. This was done in the field to save the time required to achieve a proper exposure with the 10 stop ND on.

Ultimately, repetition leads to reflexes and if the routine is a reflex, more thought can go into the art of achieving a result instead of the mathematical and technical processes of using filters. Enjoy!

Disclosure:  I am a Nisi brand ambassador who tries to write neutrally and objectively. If you are interested in purchasing filters after reading this article, feel free to email us dm@everlookphotography and I will be able to give  a discount code for Australian customers.

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