Joel Tjintjelaar

It’s no secret that I love long exposures. However, I already declared and explained my love for long exposure photography in another article. This article isn’t about why I make long exposures, it’s about the filters I use to make the long exposure.

— Filters

Since I’ve received so many questions about filters, and more specifically ND filters, I thought it was time to write a guide about this subject and hopefully answer all of your questions.

Picture taken by Martin, http://www.flickr.com/photos/mbiskoping/298129486/

Shooting manual

First of all, a quick note before you start using ND filters.
Almost every single shot taken with an ND filter will be shot in manual mode. The problem with most of the ND filters is that they are simply too dark for the camera to calculate a proper exposure. For example, if I put my 10 stops ND filter on my camera, set it to ISO 100, f/22 and take a shot at night in aperture priority mode, it would automatically set the exposure to 1 second. Which is just ridiculous, even without the ND filter 1 second is just way too short.
Even if you’d use lighter ND filters, these problems still occur. Besides, you buy and use an ND filter to achieve certain specific effects. These effects can only be achieved if you tell your camera exactly what it needs to do.

ND filters, what are they?

The most important filter to make long exposures, is the ND filter. ND is short for Neutral Density, sometimes referred to as “grey filter” or “dark glass.”
A perfect ND filter should filter the light equally. Basically it means that an ND filter shouldn’t have any effect on the colors. If you shoot a certain scene with or without an ND filter, then the colors should be exactly the same. Unfortunately this doesn’t always happen, because some ND filters are just better than others. I’ll discuss this topic in more detail later in this guide.
Now, let’s say you’ve bought a B+W 106 ND filter. This filter reduces the light by 6 f-stops. But what does that even mean? Actually, it isn’t that hard. Say you’re out shooting landscapes and you need a shutter speed of 1/250s to expose the landscape correctly. But, hey, you want a long exposure. So, put on your B+W 106 ND filter. What shutter speed should you use now? As I said, the B+W 106 ND reduces the light with 6 full f-stops, so go down 6 full stops with your shutter speed:

1 stop down: 1/125s
2 stops down: 1/60s
3 stops down: 1/30s
4 stops down: 1/15s
5 stops down: 1/8s
6 stops down: 1/4s

A picture that normally takes 1/250s to capture now takes 1/4s.
So basically an ND filter is a device that filters the light by a certain factor. Because the light is being filtered, it takes longer for your sensor to catch all the light and by doing so you’re creating a longer exposure.

Typical confusion

There is much confusion surrounding ND filters. Manufacturer A sells 0.3, 0.6 and 0.9 ND filters, while manufacturer B sells ND2, ND4 and ND8 filters and finally manufacturer C sells 2, 4, 8 and 10 stops ND filters.
To make this a bit more understandable I created this chart.

— Chart 1

First of all, we have the F-stop reduction. If someone said that they have a 10 stops ND, they’d mean they have a ND filter that reduces the light by 10 f-stops. If you follow the chart, you’ll see that a 10 stops is equivalent to a 3.0 optical density or a filter factor of 1024. By the way, manufacturers aren’t that precise when describing the filter factor of their ND filters, especially when the filter factor gets really huge. Look, a manufacturer could say that their filter has a filter factor of one million, forty-eight thousand and five hundred seventy-six or they could just say that it has a filter factor of over a million.

Secondly, optical density. It’s actually a scientific term, which means “absorbance”. But the scientific explanation isn’t really what we’re looking for, are we now? What does it mean if a manufacturer sells an ND 0.9? Just look at the chart. ND 0.9 equals a 3 f-stops reduction or a filter factor of 8.

Lastly, filter factor. Hoya is one of the manufacturers that uses this term for their filters. They sell an ND4, ND8 and more. The “4” and “8” indicate the filter factor. Look again at the chart and you’ll see that ND4 is equivalent to 0.6 Optical Density or 2 stops.

When to use ND filters

By now you should know what a ND filter is and how it works. Still the question remains, when do you use it?
There are 3 main reasons why you should consider buying an ND filter.

1. To create long exposures, where you normally can’t. For example: a 60s daylight exposure is only possible with the use of an ND filter. Long exposures in daylight can create some very interesting effects.
Example:

— Square VIII – /ï by Joel Tjintjelaar, on Flickr

2. Sometimes the sun can be so bright that shooting with a wide aperture will only result in overexposed images. An ND filter can help you out.

Screw-in vs filter holder system.

There are many manufacturers selling ND filters, but when you break it down, there are only two types.
1. The screw-in filter.
2. The Filter Holder System.

The screw-in filter is like any typical filter. You simply screw it on to your lens.
The filter holder system is something different. You place a piece of glass in a holder that is attached to the lens.
Both systems have their advantages and disadvantages.

Advantages screw-in

- Easy to pack. They are small and hardly take any room in your camera bag.
- Many manufacturers make screw-in filters, so they’re easy to find.
- Stack and combine filters. Try using a ND filter with a polarizer.
- Really dark ND filters like the 10 stop or higher are only available in screw-in format.

Disadvantages screw-in

- Usually more expensive than the Filter Holder System.
- Not a “one size fits all”. If you have a 58mm lens and a 77mm lens, you’ll need two ND filters or a step up/down ring.
- Some lenses have a rotating front element. So, when mounting the ND filter you can accidently change the focus points and your focal length.

Advantages Filter Holder System

- Relatively cheaper.
- You can use one filter for multiple lenses. If you buy the right holder, you can fit a filter on lenses with different diameters.

Disadvantages Filter Holder System

- Depending on your system and how many filters you use, it can take up quite some room in your camera bag.
- The filters used in the Filter Holder System are easier to scratch or break. Not really a con if you’re just careful with them.

GND filters

The Gradual Neutral Density (GND) filter isn’t a typical filter for long exposure photography, but it can certainly improve your images.

A typical problem when shooting landscapes is that you either overexpose the sky and/or underexpose the foreground. You can solve this problem with HDR, use one or more strobes or simply try to compensate by choosing an exposure somewhere in the middle of under- and overexposing.

There is however a much easier solution for this problem: a GND filter. It’s a filter with a gradient on it. The upper half of the filter is usually darker than the bottom half. This way it becomes so much easier to shoot a landscape that is properly exposed: both sky and foreground look good.

Although I don’t normally use the GND filter for long exposures, it certainly is very useful. Besides, it appears to me that most long exposure photographers use a GND.

More on Long exposure photography in part 2, so be sure to check it out next week

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