Photo Basics – Filters

So far in the series, we’ve looked at the three components that make up exposure:  the shutter; the aperture; and the ISO setting.  I call this the Exposure Triumvirate.  We’ve also looked at what information a histogram conveys and how reading a histogram needs to be done in context with the image.  And we’ve looked at exposure meters, what they do and how the can be fooled.

In this instalment, we’ll take a look at filters.  We’ll look at some of the different types available, discuss what they do and how they can be used.  Now, there are literally hundreds of different filters available on the market.  Some are special effects filters that, for example, create starbursts out of specular highlights.  Some are filters that create a vignette in the image.  We’re not going to delve into these.  Similarly we’re not going to discuss filters that are used in conjunction with flash or other studio lights.  We’re going to discuss the major types of photographic filters that are (or were) some of the more commonly used and that most photographers will have in their kit bag if they have filters at all.  I say ‘were’ because some of the filters we’ll talk about are colour contrast filters that are used primarily with film; although they can be used with digital as well.

I’ve been asked the question, and I’ve seen it asked on various internet photography boards, ‘What filters should I buy?’  Some people will jump in and say well you need a polarizer and several ND filters and a handful of GND filters and ……. Huh?  What?  Why?  The best answer to the question ‘What filters should I buy?’, in my opinion, is ‘What type of photography do you do?’  Why is that the best answer?  Because filters are made to fill specific needs.  Can they sometimes be used for filling other needs?  Sure.  But, for example, someone who does only portrait photography in studio likely has little need for a GND (Graduated Neutral Density) filter.  So the advise that photographer received on that internet board wasn’t overly useful.

In this article, we’ll look at the following types of filters:  Polarizers; Neutral Density; Split Neutral Density; and Graduated Neutral Density.  We’ll also talk about some filters that aren’t overly relevant for digital photography but still are for film photography such as the colour contrast filters and what I’ll refer to as white balance filters (colour compensation filters).

The Polarizer Filter

The polarizer filter is probably the most common filter you’ll find in photographers’ kit bags.  This is particularly true for outdoor/landscape/nature photographers.  It’s the only filter, that I’m aware of, that can’t be replicated digitally.  This is part of the reason it’s still so popular and important in the digital age.  Explaining what it is and what it does is a bit difficult.

Light travels in waves, not in straight lines.  Some of these waves oscillate vertically.  Some horizontally and some at angles.  Light coming from all these different angles causes reflections as it bounces off different surfaces.  These different angles of reflected light have the effect of causing glare and a reduction in contrast of our photos.  The polarizer filter helps by reducing this glare.  It does this by filtering out light in all but one angle.  A polarizer is made up of two rings, one which rotates and the other is fixed.  The outer, rotating ring has a polarizing material bonded to it.  This polarizing material only lets in light from one angle.  You can control what angle through the rotation of the ring.  The graphic below will help illustrate.

Polarizer (click for larger version)

Now, this is a very simplified graphic but it illustrates the point.  The dark grey is the polarizing material.  The white stripes represent the angle that light waves can pass through the filter.  As you can see, by rotating the filter, light from different angles is allowed to pass while all other light is blocked.  By rotating the filter we can block the light that’s causing the glare in our images and in doing so increase contrast, clarity and enhance colours.

When talking about a polarizer many people will say it deepens blue skies or that it’s great in the fall because it deepens rich fall colours.  Well, yes it does do that but not directly.  Reflections and glare come not just from surfaces around us like water or glass.  Light also reflects off dust particles water in the air.  The polarizer works on these types of reflections as well.  When the filter deepens a blue sky, what’s happening is that the reflections off dust and water in the air are being reduced which means less glare and increased contrast.  Same with foliage.

The only surface that won’t be affected by a polarizer – that is where the polarizer won’t work – is on bare metal.

Polarizers come in two flavours – Linear and Circular.  This doesn’t have anything to do with the shape of the filter.  It has to do with how the filter allows light to pass.  Linear polarizers are an older type and were used before modern autofocus and metering systems were put into cameras.  If you have a camera with an autofocus system, you need a circular polarizer.  There is a reason for this that I’m not going to go into in this article but if you want more on the reason why, Bob Atkins has a good article about it on his site.

It’s probably pretty obvious from what these filters do but they do cause a reduction in light that hits the sensor or film.  Typically it’s about 1 1/2 stops.  If you’re using your in camera meter, this will be taken into account automatically.  If you’re using a handheld meter, you’ll need to remember to add about 1 1/2 stops of exposure to the meter reading.

There are also some specialty polarizer filters such as warming polarizers or gold tone polarizers.  For me, personally, these have less appeal and the effect of these ‘enhancements’ can be replicated in the digital darkroom.

A couple things to keep in mind when using a polarizer.  Polarizers are at the greatest effect when you are at 90 degree angles to the sun.  Because of that, using wide angle lenses with a polarizer can cause uneven polarization which is often evident in skies due to the large field of view of the wide angle lens.  The image below illustrates the effect.

Uneven Polarization (click for larger version)

The last thing to be aware of is that many polarizers are relatively thick due to the rotating ring.  This thickness can cause vignetting with wider angle lenses.  Thin ring polarizers are available but they are more expensive (surprise, surprise).  Keep in mind also that if your shutter speed goes too low, a tripod will be in order to keep images sharp.

Neutral Density Filters

Neutral density filters are pretty simple.  They do one thing and do it quite well.  They block light.  Neutral density (ND) filters block some of the light that comes through the lens onto the film or sensor.  They’re called ‘neutral’ because the grey colour of these filters doesn’t cause a colour cast in the photos, hence they are neutral from a colour balance and colour temperature perspective.

ND filters come in various strengths.  The most common are 1, 2 and 3 stop.  These can be notated in a couple different ways.  Some makers will label them .3, .6, .9, for example.  Others will label them 2X, 4X, 8X, for example.  The 2X, etc. makes intuitive sense since each stop of light is double or half the amount of light.  2X is one stop.  4X is two stops and 8X is 3 stops.  The other labeling system is less intuitive but it works the same way.  .3 is one stop, .6 is 2 stops and .9 is 3 stops.  The system can continue as well.  1.2 would be 4 stops, for example.

Why would we want to use ND filters?  The main reason to use them is to control shutter speeds.  If, for creative reasons, we want to use a shutter speed that’s slower than what we can achieve with a normal exposure, we can put an ND filter on and get that slower shutter speed.  This will most commonly be the case when we want to show movement in an image.  The classic example is flowing water.  Many people like that soft, silky look to flowing water.  That requires slower shutter speeds to achieve.  Depending on the speed the water is flowing, it may require several seconds.  We may not be able to get that slow shutter speed without a filter.  By using an ND filter, we can slow down our shutter speed to get the effect we want.  Another example is moving clouds.  By slowing down the shutter speed with an ND filter we allow the clouds to move through the frame and get that soft, wispy look.  You can just start to see the effect in the image below.

Cloud Movement (click for larger version)

Slower shutter speeds would have enhanced the effect.

A currently popular version of the ND filter is a 10 stop version.  This REALLY slows down the shutter speed.  Think about that.  If you have a normal shutter speed of 1 second, a 10 stop change will give you a 1024 second shutter speed.  That’s over 17 minutes! Want wispy clouds and silky water?  That’ll give it to you in spades.  Now, that is a bit extreme and while it does produce an interesting look, it’s probably not something we want in every image.  That said, Marcin Stawiarz has made it his signature look in combination with black & white and produces some stunning imagery.

ND filters can be stacked to increase the effect.  The effect is additive.  If you stack a 1 and a 2 stop ND filter, you get 3 stops of light blocking capability.  If you were to stack a 2 stop and a 3 stop you’d get 5 stops of light loss – not 6.

Another use for ND filters is to control the aperture and depth of field.  If we want to create a shallower DoF than conditions will allow, we can use ND filters to block light and allow us to open the aperture and reduce our depth of field.

There are also what are called variable ND filters.  With these, you can dial in the amount of light blocking you want by rotating a ring on the filter.  These can be useful as they could mean carrying fewer filters and would eliminate the need to stack filters in some situations.  These variable ND filters only come in the screw on (discussed below) type; however.

As with the polarizer, if you’re using your in camera meter, the effect of the filters will be compensated for automatically in the exposure settings.  With a handheld meter, you need to make the compensation yourself.  Depending on the strength of the filter(s) being used, there may not be enough light coming through the lens for the autofocus or metering systems to work properly.  In this case, you’ll need to focus and meter without the filter, note the exposure, switch to full manual mode and dial in the appropriate amount of exposure compensation yourself.  This can happen when you get more than about 4 or maybe 5 stops of filter effect.  The same caveat regarding shutter speeds applies here as it does with the polarizer.

Polarizers, because they cause a loss of light through the lens, can also be used as neutral density filters from the standpoint of exposure.  Keep in mind the other aspects of the polarizer which you may or may not want in your image.  The best option is to use the right filter for the job at hand but sometimes we can make substitutions in a pinch.

Hard Split & Soft Graduated Neutral Density Filters

I’ve grouped these into the same category because they essentially perform the same function in a slightly different way.

These filters do the same thing as ND filters in that they block light from entering the lens and hitting the sensor or film.  The difference is that the light blocking properties are only on part of the filter.

With split (also called hard) ND filters, there is a definitive line on the filter at about the halfway point.  One side of the line is clear (no light blocking) and the other side of the line is grey (light blocking).  The graduated (also called soft) ND filters, there isn’t the definitive line.  The light blocking area of the filter is faded into the clear area over a small distance.  It’s a gradual transition.  The graphics below will help illustrate.

Soft Graduated ND Filter (click for larger version)

Hard Split ND Filter (click for larger version)

You can see that even with the Hard version, there isn’t an absolutely clean dividing line across the filter.  But despite the softness of the edge, which helps to make it so there isn’t a line across your image, there isn’t the transition from grey to clear like there is with the soft edge ND filter.

These types of ND filters are particularly useful in situations where you have a wide variance in light intensity from highlights to shadows.  Think of a bright sky and darker foreground in a landscape.  The split/graduated ND filter can be used to block light in the sky portion of the shot while keeping all the light in the foreground area and effectively narrow the dynamic range of the scene and allow the entire scene to be rendered within the range that can be captured by the sensor or film.  With some types of these filters, you can also angle the filter to apply the light blocking to one side or the other or to one corner or another.  The images below show the effect with and without a soft grad ND filter.

Image Without Filter (click for larger version)

Image With Filter (click for larger version)

Because of the trees on the right, I angled the filter so that it was applied more from the upper left corner rather than from the stop straight down.  The sky and water on the left side have been affected where the water and tree area in the lower right corner haven’t.

Like regular ND filters, they come in different strengths.  It’s not uncommon for the soft type to be stacked with the transition areas overlapping.  Because the transition is gradual, they can be overlapped without causing a hard line in the image.  Overlapping the hard type is more difficult, of course, as you’re likely to see the line in the overlapped area.

These filters do have some limitations.  The hard split version really only works well with a scene where there’s a flat horizon line.  They don’t work well, for example, in a scene with hills.  This is where the soft grad ND can work better.  Because the transition is gradual, it can be used in instances where there isn’t a sharply defined horizon with the graduated transition being placed over the uneven horizon.  Even this has limits; however, and if you’ve got a very hilly or mountainous horizon, even the softer version may not work for you.

With the maturation of HDR and other sophisticated image blending methods, some people say there’s no longer a need for these types of ND filters.  While I agree that HDR or exposure blending is one approach to the problem of too wide a dynamic range, there’s still something to be said for getting it in camera.  And if a split/grad ND filter can help do that, then why not use it?  Spend more time shooting and less time on the computer.

Further down in the article, we’ll talk about screw on vs. square filters but they do make a difference when it comes to these split type ND filters.

Some people also will claim that they can replicate a split ND filter in the digital darkroom.  Lightroom had a built in graduated filter.  Hard and soft splits can be created in Photoshop as well.  While this is all true, saying that these replace using a filter in the field is a mistake.  If you shoot a scene with a wider dynamic range than your sensor or film can capture, you’re going to have overexposed highlights.  You can apply all the digital split filters you want, you’re never going to recover those overexposed highlights.  There’s no information there to recover.  These digital versions can be used in the editing and image optimization process for an already good image but can’t be used to rescue a mistake.  Get it right at the time of shooting as much as possible.  Don’t rely on the computer to make up for poor technique or laziness.  As with the other filters discussed above, keep in mind the need to use a tripod if shutter speeds go too low.

White Balance and Colour Contrast Filters

What I’m referring to as White Balance filters are those that add a colour cast or change a colour cast in an image.  The reason I refer to them as White Balance filters is because the effect of these can be replicated with great precision on RAW images in a RAW image converter such as Adobe Camera RAW, Lightroom, Nikon’s Capture NX Pro and others.  In my earlier (Yet Another) The Power of RAW article, I explained how this works and why it doesn’t work so well for JPEG images.  So if you’re shooting RAW, you really don’t need these types of filters.  If you’re shooting JPEG you can still make some adjustments, just not with the same effectiveness.  With the ease of changing white balance on digital cameras (as opposed to changing film in a film camera) and with how good Auto White Balance is in most situations, even if you’re shooting JPEG, these white balance filters aren’t really necessary in digital photography.

For those who are still shooting film, these filters can be quite useful.  They break down into a couple of groups.  There are the colour correction filters and the effects filters.

Colour correction filters are ones which correct for the built in colour cast of a film.  If you’ve shot normal, daylight film (e.g., Kodak Portra or Fuji Sensia) indoors with incandscent lighting, you’ll have noticed a strong yellow colour cast to your images.  Similarly, if you shoot daylight film in a shadowed area, you’ll notice a cool, blue cast to the images.  This is because film is manufactured with a built in white balance.  Daylight film is balanced for ‘normal’ daylight conditions of around a colour temperature of about 5000K, give or take.  There are also tungsten balanced and fluorescent balanced films.  These have a built in white balance of about 3200K and 6500K respectively.  If you’re using the right film under the right lighting conditions, there’s no problem, your images will come out with the proper colour.  Sometimes we can’t do that though.  Sometimes we get stuck shooting with daylight film indoors under either incandescent or fluorescent light.  This is where the colour correction filters come in.  These are coloured filters, in varying colour temperatures, that ‘correct’ the white balance of the film to work under the specific lighting conditions.  A blue, 80A filter will help correct for the strong yellow colour of incandescent light, for example.

The effects or colour enhancement filters do just the opposite.  Rather than removing a colour cast from an image, these filters are used to impart a colour cast to the image.  A warming filter, for example, adds a yellowish or warm cast to the shot.  A cooling filter, on the other hand, adds a blue hue to an image.  Unlike colour correction filters which are rated for specific colour temperatures, the colour enhancement filters come in different strengths but aren’t rated for specific temperatures.

As with the polarizer and neutral density filters, these coloured filters cause a loss of light.  The amount of light loss will differ depending on the filter.  Automated, in camera meters will take the filter into account.  With hand held meters, you’ll need to adjust the metered exposure for the filter factor.  Due to the loss of light, you may need a tripod to give you sharp images if the shutter speed goes too low.

Colour contrast filters are used largely in black and white photography.  We discussed the effects of these filters in the Seeing in Black & White series, which are all linked in the table on the Tutorials page of the site.  Again, because of the ease of converting to black & white in the digital darkroom, these aren’t used as much any longer with digital cameras.

There are times when you can still make use of them; however.  If you’re shooting in monochrome mode and capturing JPEGs, the effects of these colour contrast filters will be seen in the image.  If you’re shooting in RAW with monochrome mode turned on, you can see the effect of the filter in some OEM RAW conversion software as the monochrome setting will be picked up by the OEM software.  This is not the case with third party software.  The caveats in the Seeing in Black & White series about the quality of digital in camera b&w photography still hold.  The better options are to capture in colour and convert after the fact.

Screw on vs Square Filters

The way filters attach to your lens is of two main types.  Screw on filters have threads that screw onto the threads on the front of your lens.  Square (or rectangular) filters are used with a filter holder and adapter rings.  The adapter rings screw onto the lens and the filter holder attaches to the ring.  Filters are then placed into slots on the holder.  Filters of this type are sometimes referred to as system filters.  Lee is one brand of square filters.  Cokin is another popular maker of square filters and filter systems.  Cokin systems come in 4 different sizes which allows the user to buy the size that works best for their lenses and not have to spend more money than needed.  At one point I used Cokin but now use the Lee system.  There are other manufacturers of filter systems as well and a number of third party filter makers have filters that will fit a variety of systems.  There are countless makers of screw on filters, with Tiffen being one of the most well known.

Whether you’re going to use screw on filters or a filter system, one thing is true – you get what you pay for.  Inexpensive filters will be of lower quality and degrade your image more than more expensive, higher quality filters.  If you’re going to spend several thousand dollars on lenses, it doesn’t make much sense to put a cheap $15 filter in front of it.  Buy the best you can afford.

There are pros and cons to both types of filters.  Screw on filters are made to suit specific sizes of lenses so you can buy specifically what you need for your lenses.  If you’ve got lenses that take filters of different sizes you may end up having to buy more than one of a specific filter.  You can also use what are called stepping rings to enable you to use, say, a 52mm filter on a lens that takes 48mm filters.  Using a larger filter on a smaller lens with a stepping ring can work.  Using a smaller filter on a larger lens with a stepping ring isn’t advised as you’ll often introduce vignetting (darkening or cutting off of the corners of the image).  Screw on filters are typically less expensive because they’re smaller than the system filters.

System filters are, frankly, more flexible than screw on versions.  You buy one system (adapter rings, holder, filters) and use this on all of your lenses.  Adapter rings can be bought in sizes to fit a wide variety of lenses and the holder is made to fit on any of the system’s adapter rings.  This means you only buy one set of filters too.  Filter systems are a larger investment, typically, than screw on filters.  The filters are larger which makes them more expensive.  Particularly when you get into polarizer filters, the cost increases dramatically as you increase the size.  Lee’s 105mm polarizer filter, for example, is about $300.  I use a less expensive, but still good quality, Sigma 105mm polarizer which sells for about $170.

With filter systems, you also lose the ability to the standard lens hood.  You can buy system hoods but, again, these are quite expensive.  With screw on filters you can often use the standard lens hood.

When using split filters, filter systems have big advantages.  With split screw on filters, you’re stuck with where the manufacturer places the line (typically about the middle).  With square (or rectangular) filter systems you can move the filter up or down as needed to place it where you want in the scene.  You can also rotate the holder to easily place the line on an angle if needed.

Some of the currently popular ND filters such as the 10 stop and variable ND filters are only available as screw on types.

Most screw on filters are glass.  The quality of the glass may differ but they are glass.  System filters come in a few different types.  They can be glass or a few different kinds of plastic.  The glass versions are heavier and more expensive.  The plastic versions are lighter, less expensive but also more prone to scratching so additional care needs to be taken with them.  Optically, a high quality plastic filter won’t really be different from a high quality glass filter.

A Note on Stacking Filters

Stacking filters can be done.  It should be done though with the knowledge that it can have the effect of reducing overall image quality.  Higher quality filters will degrade image quality less – much less than cheaper, lower quality filters.  So while it can be done, it should really only be done when the proper filter isn’t on hand (i.e., stacking a 1 stop and 2 stop ND filter because a 3 stop isn’t in the kit bag).

The Skylight/UV Filter Debate

I’ll close this article with a few words on the concept of using a skylight or UV or haze filter on a lens at all times.  Many people advocate this.  They do so because they feel it protects the front element of the lens from damage.  And it does.  It’s cheaper to replace a filter than a lens.  But is is necessary?  In my view, in 99.9999999999999999………% of cases no.

Any time you put something in front of your lens, another piece of glass or plastic, you’re going to degrade the image quality.  How much depends on the quality of the filter.  Most of the skylight/UV/haze filters you see on the front of lenses are cheap with lower quality glass that’s going to degrade image quality a fair bit.  Inexpensive filters of this type will also often not be coated or have lower quality coatings.  The coatings used on filters are the same as coatings used on lenses.  The purpose is to prevent reflection off glass surfaces which can introduce flare, glare and reduce image quality and contrast.  What’s the sense of putting.  Does it make sense to pay for a more expensive one then?  In my view, in 99.999999999999999999………….% of cases no.

With proper shooting technique, you can avoid the problems that people refer to most often when they talk about using a filter to protect a lens.  Use a lens hood.  Put the lens cap back on when you’re not shooting.  If you’re going to be carrying the camera any distance between shots, put it back in the bag.

Now, if you’re regularly shooting in desert wind storms or other places where airborne shrapnel is flying around, by all means, go ahead and protect the front of your lens with a filter.  Otherwise, it’s not likely necessary.

Another reason people will state for keeping one of these on their lenses at all times is that it reduces atmospheric haze and makes images clearer and sharper.  Well no, it really doesn’t.  At least not that really matters in most cases and not in shooting situations most of us will find ourselves in most of the time.  Where a UV filter can come in handy is in higher elevations.  In the mountains, UV is more prominent and can add a blue tinge to images (UV is on the same end of the spectrum as blue).  In these cases a warming filter or white balance adjustment can do the trick.  In addition, while film was sensitive to UV radiation, digital sensors are basically not (they are sensitive to infrared but digital cameras are made with IR blocking filters), so the need for these filters to block UV is negated with digital cameras.

Many camera stores will try to push these filters on customers when selling lenses.  They’re not doing it because they want you to protect your lens.  They’d love it if you broke the front element of your lens and had to replace it.  They’re doing it because these types of accessories have high profit margins.  It’s a bottom line issue for the camera stores.

In the end, whether you do or don’t use one of these on your lenses for protection is up to you.  I know many people who do.  I know many people who don’t.  You’ll need to decide for yourself.

So that’s the end of the article.  As always, I hope you’ve found it helpful (if you’ve made it this far) and as always feedback is welcomed.

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