When to use filters

A typical shot of flowing water blurred by a slow shutter speed. An ND8 filter, which reduces the light intensity by three f-stops, enabled a two-second exposure at f/11 to optimise the blurred effect.

Digital sensors are much less affected by UV radiation than film and haze penetration is less of a problem because most cameras include some form of built-in compensation that is at least as good as a regular haze filter. The colour correction provided by skylight filters is handled seamlessly by all auto white balance systems. So the traditional reasons for using these filters no longer apply.

In fact, there are valid reasons to avoid filters when shooting with digital cameras. If your camera has an anti-aliasing filter in front of the sensor (and most do) it presents a flat and highly reflective surface that causes light to bounce back into the lens. When this scattered light hits a flat piece of glass (such as a filter), it reflects back to the sensor causing veiling flare. Contrast and colour saturation will be significantly reduced.

Filters still have their uses, but there are situations where they should be avoided and, when used, you must be discriminating in your choice of filter. Even the highest quality filters can degrade image quality by increasing veiling flare and reducing image contrast.

It may not be by much. In fact, you may not be able to see it unless you enlarge the image and/or examine it very closely and have a comparison image taken with the same lens without a filter. But, if you have invested in a top-quality lens, you want to be able to achieve the best performance it is capable of ““ with no compromises.

In this feature we will look at situations where filters can provide genuine benefits.

UV, skylight and polarising filters have been popular since the days of shooting on film. Note the slightly warm colour cast of the skylight filter and the increased density of the polariser, compared with the transparent UV filter.

1. Filters as Protection

Fitting a protective filter (UV or skylight) is wise when you’re shooting in places where there is salt spray, blowing sand or dust, all of which can etch the surface of the lens. A filter is also recommended when photographing small children and pets, particularly if there’s a likelihood of the subject touching the lens.

When shopping for a protective filter, try to avoid cheap filters. Even the best filters can degrade image quality; cheap filters will degrade image quality.

Keep your filter spotlessly clean for optimal performance, particularly when photographing backlit subjects. Even a trace of grease or dust will increase veiling flare and reduce image contrast.

Look for filters with neutral colours. A filter that isn ‘t neutral doesn ‘t simply alter the white balance, it actually absorbs a specific waveband of colour. This changes colour relationships and intensity in uneven ways, resulting in a less subtle image.

Most skylight filters are slightly warm and UV filters can also apply slight colour casts. Even when you try to remove them with white balance adjustments, the end result is unlikely to restore the original colour relationships.

While some attenuation of blue might be acceptable when you need to cut atmospheric haze, even a slight yellow cast will attenuate deeper blues in the scene. This can mean losing some of the deep blue colour you might want to capture. Colour casts will also cause a slight loss of light, so you sacrifice a little bit of shutter speed.

If colour reproduction is important and you want to achieve consistent colour across your lenses, don’t mix and match brands of filters. Stick with one brand as any colour casts are most likely to be the same for all filters of the same type. It’s worth paying extra for filters with multi-resistant coating because they are genuinely more resistant to dust and scratches and will usually do a better job and last longer than uncoated filters.

2. Filters for Control

Three types of filters are used for exposure control: polarisers, neutral density (ND) filters and graduated filters. Special effects filters are also available but in most cases it is easier to use the digital filters associated with the camera’s image processor, or apply effects when images are edited. We’ll cover polarising filters in the Insider feature in the next issue, so we’ll concentrate upon ND and graduated filters here.

ND filters reduce the amount of light reaching the sensor without affecting the colour of the image. Their main use is for exposure control by providing greater flexibility to select wide lens apertures and/or slow shutter speeds in bright ambient lighting.

This illustration simulates the effects of the most popular neutral density filters, showing how up to four f-stops stop of darkening can affect brightness levels without altering image colours or tonal relationships.

ND filters are available in a wide range of densities, providing a reduction in light from one f-stop to 13 f-stops, the latter being rated as ‘extreme’. The table to the right shows the density ratings of typical ND filters, along with the most widely accepted notation and the light ‘transmittance’ percentage.

The most common application for ND filters is enabling slow enough shutter speeds to be used to capture motion blur, particularly with water. The intensity of the filter depends on how fast the water is moving and the brightness of the ambient light.

You can usually get by with a three-stop ND filter when photographing waterfalls or cascades in normal daylight because a couple of seconds’ exposure is enough to create an attractive blur. With seascapes, longer exposures will be required, typically 20 seconds or more, since the sea and clouds may not be moving as quickly. Allow for reduced light intensities on cloudy days and when the sun is close to the horizon.

No filter was needed to capture blurred spray in this shot taken just on sunrise. A 14mm lens was used on an M4/3 camera for a two-second exposure at f/8. Timing was critical to the success of this shot.

ND filters can also be used to eliminate human subjects from streetscapes and architectural photographs. Exposures of two minutes or more will be required to make people ‘vanish’ and it will only happen if they move through the scene.

Another use for ND filters is to enable wide lens apertures to be used to obtain a shallow depth of field on bright, sunny days. In such situations, an ND filter can provide the only way to blur out distracting backgrounds in portrait shots.

ND filters can also be used creatively to extend exposure times in order to record blur patterns produced by moving the camera with respect to the subject. In bright sunlight, the optimal exposure is 1/2 to one second at f/8 or f/11. Mounting the camera on a tripod restricts camera movements to up/down or side-to-side, while hand-holding provides greater flexibility. With practice you will learn to pre-visualise the effects of different types of movements.

Creative use of an ND filter to expend exposure times. This shot was taken with the camera hand-held while the shutter remained open for 3.2 seconds. The blurring is due to camera shake, which is mainly vertical in nature.

The main issue associated with ND filters is that add-on filters not only reduce the light reaching the sensor, with DSLR cameras they also reduce the light passing through the optical viewfinder. Cameras with EVFs will automatically compensate by brightening the ‘finder’s screen. Shooting in live view mode can achieve a similar effect and both types of screen usually support manual brightness adjustment.

Some cameras come with dial-in ND filters that reduce recordable light without affecting the ‘finder or screen image. However, the extent of adjustment is usually restricted to a maximum of three or four f-stops.

Graduated filters have variable light transmission with one end being optically clear (and transmitting 100% of the light), while the opposite end is of neutral density or has a colour that changes the hue in the areas it covers. The transition between these zones can be abrupt (‘hard edge’) or gradual (‘soft edge’).

This illustration shows the differences between soft edge and hard edge graduated neutral density filters.

Hard edge filters are used when there is an abrupt change in brightness, such as you would see in the zone between a dark area of land or sea and a bright sky. Soft edge filters are used when the light and dark portions are not distinctly separated and when horizons aren’t totally straight.

ND graduates are mainly used to bring an overly-bright part of a scene within the dynamic range the image sensor can record, thereby preventing highlight clipping. Coloured graduates introduce colour, while also reducing the light level in the filtered part of the scene.

The top picture was taken without a filter, while a graduated ND filter with a soft edge was used for the lower photo to enable an exposure that would bring out detail in the foreground without over-brightening the sky.

Like polarisers, graduates are angle-critical and they must be correctly aligned to cover the area you want to darken. Although they darken only part of the scene, this can be enough to affect the camera’s metering system. Evaluative multipattern metering usually yields the best results.

Use the brightness histogram to set exposure levels, ensuring the filter reduces the light levels enough without blowing out highlights or losing shadow information. If the histogram doesn’t reach the right hand end of the graph, the filter is probably too dark to deliver good results.

Like regular ND filters, ND graduates come in different densities, the most popular being ND2, ND4 and ND8. Most photographers tend to buy a set of graduates to handle different lighting conditions.

Without a graduated ND filter, the sky in this scene would have been too bright to show much detail in the clouds (which are the main subject of the picture). The bottom quarter of the frame should be cropped off before the image was printed to improve the composition of the picture.

Practical Matters

Filters are normally made from resin, polyester or glass. Polyester is the cheapest option and has the lowest optical quality. Glass filters are usually the most expensive and many include advanced coatings to minimise flare and ghosting as well as darkened rims to further reduce reflections.

Circular, screw-mounted filters are usually easier to adjust and more compact to carry around because filters with the same diameter are stackable. But, if you have two lenses with different filter thread sizes, you may need to buy two filters (which aren’t stackable), whereas with a system type of filter you would only require another adaptor ring for the filter holder. It’s best to buy a filter to match the largest lens you own and get step-down rings for fitting it to smaller lenses.

Stacking filters is not recommended for actual shooting even though a polariser can be attached on top of a UV protective filter. The first problem is that the depth of the filter rims will probably cause a small amount of vignetting, particularly with wide angle lenses. The second is that each filter you add increases the number of glass-to-air surfaces and, with it, the potential to degrade image quality.

To minimise the risk of vignetting, choose screw-in filters with slim rims and select a larger filter holder than the minimum required, particularly for wide angle lenses. And minimise the number of filters, leaving them off unless they provide a genuine benefit.

Square, slot-in filters require an adapter and holder, which means more gear to carry. Initially they are trickier to set up, but they’re more adaptable. It’s easy to stack ND filters for increased exposure times or combine different types of filters, such as ND graduates with a polariser.

 

This article is an excerpt from  Photo Review magazine Issue 60  

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