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What is an ND-filter?

Definition and use of a Neutral Density filter.

An ND-filter or a neutral density filter is like a pair of sunglasses that you put in front of your lens. The purpose it to reduce the amount of light that hits the sensor, just like you want to protect your eyes from the strong light on a sunny day.

A variable ND-filter sitting on my beloved Nikkor 16-35 mm lens.

The point with an ND filter is to allow you to keep the shutter open for longer time without clipping the highlights in your picture or in other words simply avoid that you over expos your picture.

Why would you then want to keep the shutter open for a long time? Take a look at the picture below. It is taken over several seconds. The stones of course do not move, while the waves of the sea are reduced to pure silk. Moving parts become blurred, stationary items stay sharp. That is one effect you can get with an ND filter. You can also shoot waterfalls, harbors, etc. with this technique and get similar results.

Long exposure. The stones stay put, the waves of the water are reduced to pure silk…

Another application is simply that there is so much light that even when you shoot at the fastest shutter speed your camera offers and base ISO, you still get too much light. Then an ND filter – like sunglasses – comes in handy to reduce the amount of light.

You can also use the long shutter opening to move the camera instead, and this is what is done in intentional camera movement where you move the camera to create a blurry effect.

ND filters come in variable and fixed ones. I prefer the variable, because I can then turn the ring to reduced effect (ND8), so I can actually see what I am shooting and then once focused, return the ND filter strength to full throttle (ND2000) and get the most out of the filter. If you have a filter with a fixed value, you typically find you have to take it off the lens in order to focus, and the re-mount to take the picture. The variable filter saves you a bit of work.

The light reduction your ND filter yields is typically measured in ND values – if you look at the picture of my ND filter above, you can see it ranges from ND8 to ND2000. That is a reduction of 3 stops to 11 stops. That is a lot! You may be able to get by with less than that.

ND filters are not cheap, and you want to get a good one. The “neutral” in ND is to be taken very literally – you don’t want the ND filter to change the color or toning of the light. It should really stay neutral. Cheap filters unfortunately often has this un-wanted side effect. Make sure you get a filter with good reviews – it is quite expensive, yes, but you will not enjoy a filter that alters the colors. Believe me – been there, seen it, done that. Not to be repeated.

Further reading

What is aperture? And why important?

What is shutter speed?

What is lens flare?

Lens flare is bright areas of light in your image, that you do not find in reality! It is caused by the way light hits the front side of the glass in the lens and is scattered inside the lens.

Here below I have taken a shot of me holding my iPhone up in the dark and taking a picture of it. You see leftmost a hectogon shaped green light (the same shape as the aperture blades) followed by several colored bright spots. This is flare. None of these were there when I studied my iPhone with the naked eye:

A bright light source (here from an iPhone – more typical the sun) in stark contrasts often brings out lens flare.

If the light comes into the lens from a very steep angle, then the result can be a milky white area – sometimes covering the entire frame. And the contrast in that area is significantly reduced – things appear to be washed out.

Here I hold the light from the iPhone so it comes into the lens from almost 90 degrees and from the left. If you’re thinking: 2001: A space odyssey, then you know what creative tools Stanley Kubrick used.

If you want a good test if your lens has flare, go out at night, find a tall lamppost and point your lens towards it. Move the light from the lamppost in and out of the frame to provoke flare. You will probably be able to get some flare effects, especially if you leave the lens hood at home and choose a zoom lens. Zoom lenses have more complicated constructions than primes and tend to produce flare more easily.

Some love flare. In my experience, videographers love flare. When you watch TV or go to the movies, you will quickly spot flare used as a creative tool. It certainly underlines the strength of the sun on a sunny day. Also some photographers love flare, they even add it to the image in post processing!

Others do not love flare so much. If you want realism in your photos, the flare is to be avoided, as it is a thing created by your lens. It sort of gives away that a lens was part of producing the image.

Some lens constructions are better at reducing flare than others. Modern coating of the lens glass greatly reduces flare, but cannot eliminate it fully. Zoom lenses have more complicated constructions and are hence more prone to flare – primes less so. And finally, you can use a lens hood to reduce the flare. I sometimes even hold the hand against the sun to protect my lens from getting the light that produces flare.

In my opinion there is no right and wrong when it comes to flare. It is a creative tool. The problem comes when it is there and you don’t want it or the other way around. But hopefully this article has given you some ideas what to do in these cases.

Thank you for reading this far! Questions and comments are more than welcome!