Practical advices about ND filters PART I

Staff Blog

Practical advices about ND filters PART I

Oct 15, 2016

• How it works?
• Types and varieties of ND filters.
• Difference by density.
• Difference by construction.
• Typical situations.

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If you are passionate about photography and not afraid to experiment, this article is for you! You probably already use UV or polarizing filters. UV filters protect your lens from physical damage, as well as from UV light, which can negatively affect the images.
Polarizing filters are more interesting, because increase contrast and eliminate reflections from the water and glass surface. But now we will talk about much more interesting things - about neutral density filters (ND filters).

ND filters are next popular after the polarized filters. Unfortunately, they are paid much less attention, although it is difficult to overestimate the creative potential they provide the photographer.

Here we will try in a simple but more detailed way to tell about these filters - their varieties and situations in which they can be used.

In the 1st part we will give you general information about ND filters. How they work, what the types are and in what situations they can be used. In the 2nd part we will tell in detail about how to use filters in photography. In the 3nd and 4th parts - the same for time-lapse and videography. And finally in 5th part we will tell about recommendations on how to choose ND filters.

How it works
The main purpose of ND filters is reducing the incoming light in the same proportion across the entire spectrum. In simple terms, the filter reduces the amount of light entering the camera. These filters give possibility to shoot at any time with any exposure settings in over lit conditions.

It should be noted that the over light problem can be solved by changing the aperture. However, first of all this way is limited (there is also risk of diffraction), secondly, it will increase the grip and negatively affect the composition. Thirdly, in terms of extra long exposures this method can not be used.

Let's look at the types and varieties of ND filters, and then discuss the typical situations in which they can be used.

Please, note that shooting with an ND filter is often associated with long exposures, so you will need a good tripod.

Types and varieties of ND filters
ND filters are included in the product lines of almost all filter manufacturers. In terms of quality there might be serious differences, but in classification of ND filters all of them are identical. In this review we will use as an example Kenko REALPRO ND filters and Kenko Variable NDX filters, therefore, we will use classification and markings according to this manufacturer.

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So ND filters can be classified by density level (the amount of decreased light) and by construction.

Difference by density
ND filters are classified by density. In other words by the degree of reduced light (fading).

The ND letters in the name of the filter will definitely tell you that it is neutral density filter. After the letters there is numeric designation of the degree of light to be reduced. Actually, if you look at ND filters you will notice that filters with different density level differ from each other (one darker than other).

Variable ND filters are limited by the practical density range between ND2-ND400, due to the technological limitations (we will explain it below). The density of such filters is changed by rotating one of the frame towards another.

Kenko REALPRO ND filters with fixed ND level present a wide range of models from ND4 to ND1000, which covers almost all the photographic situations. So the only thing you need is to read the mark correctly and choose the appropriate model.

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The most light filter with fixed ND level is Kenko REALPRO ND4 which transmits forth part of the light that is 25%. But in photography it is not accepted to measure the light in that way so we usually use exposure value (EV) or f-stops. For example, ND4 filter will lower the light by 2 f-stops.

On the camera screen the exposure state is shown by the digital scale, where over- and underexposure is indicated with "+" and "-" numbers. For example, overexposure on 2 f- stops shown as "+2EV" or simply “+2".

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Here’s a handy table for easier understanding. It could be used for choosing the ND filter of any manufacturer.

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Also check wonderful application «Long Exp» by Ilya Genkin. It is a set of different calculators for long exposure. For example, there is a "Stacking ND Filters" section where one can calculate the exposure for multiple ND filters. Also, it has a table above and many other useful calculators. Unfortunately, this app is available only for iOS in both free and paid version, but we would recommend to purchase the paid one, because it is a really useful app.

For the amateur photographer, all these marks and f-stops may seem somewhat confusing. So let's look at a simple example.

Let's say we have a very sunny weather, camera with the maximum shutter speed 1/4000 sec., minimum sensitivity ISO200 and fast portrait lens with aperture of f/1.3.

We need to shoot at maximum aperture to get a beautiful background blur. In that case, the maximum possible exposure of the picture will be: 1/4000 sec. ISO200, f/1.3. Even with such settings because of the bright weather the photo is overexposed on 2 f-stops (camera menu shows +2EV).

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The only thing we can do with the camera is to close the aperture on 2 f-stops up to f/2.8. In this case the exposure will be correct, but you will lose depth of field, which is critical in portrait photography.

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So here ND filters will come in handy. Returning to the top table, we can easily define that ND4 filter is just we need, because it is reducing the light on 2 f-stops. Using the filter we will get correctly exposed shot with desired depth of field.

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This very simple example shows the principle of operation with ND filters and hope it will help you to select the most suitable model. But the reality is somewhat different. Therefore, if you shoot in RAW-files, a slight overexposure is not so terrible. On the other hand, if we are talking about the creative potential, there may be situations when you need to reduce the light up to 1000 and more times (i.e. 10 or more EV steps). Later we will consider all the most typical situations of using such filters.

Difference by construction
Also ND filters are divided into fixed and variable. Fixed one consists of a glass with a specified density level. Kenko REALPRO ND are fixed ND filters. Variable filters allow to dynamically control the density level from ND2 to ND400. In fact it is 2 combined linear polarizing filters and the density can be changed by rotating the upper frame. Kenko presents two models of variable ND filters - Variable NDX and PL FADER.

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The disadvantage of fixed ND filters is that for each shooting situation you need to use certain filter with desired density or combination of several filters. However, this approach is more correct, because guarantees higher quality in terms of resolution, color and allows to use extra-long exposures. Other disadvantages are high price and low quality of budget models (low resolution and color shift).

A great advantage of variable ND filters is that they allow to dynamically change the level of density. You don't need to change or attach filters with different density. Just simply rotate the upper frame to achieve the desired effect within the operating range of the filter.

Typical situations
Before we proceed to the practical side, it must be said that the scope of ND filters is very extensive.
First and foremost, ND filters are used in classic photography. For example, in portraits, for more depth of field or in landscape photography for very long exposures.
Also, ND filters are often used in time-lapse. In this technique hundreds of serial shots are combined into the video with accelerated effect.
In the professional videography it is also difficult to manage without ND filters. Next, we will consider the most typical situations.

To be continued....