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Filters for Digital Cameras

by Bob Atkins, 2006


With film cameras filters have traditionally been used to modify both the spectral content (color) and intensity of light, as well as generating special effects like soft focus. Digital cameras operate somewhat differently with respect to color though. Color modification can easily be done "in camera", and that's what you're doing when you set white balance. So while for film warming and cooling filters, or filters which convert fluorescent light to look like daylight, may be required, digital can achieve the same effects by internal manipulation of the digital data.

So the first type of filter you really don't need for digital is a color modification filter. These would be warming filters like the Hoya 81 series or the Tiffen 812, cooling filters like the 82 series and fluorescent-daylight conversion filters.

So which filters do you need for digital? Well, there are probably 6 main types:

  • A polarizing filter
  • A UV filter
  • A Neutral Density filter
  • A graduated Neutral Density filter
  • Filters for Infrared Effects
  • Special Effect Filters

Polarizing filters

You can't digitally simulate the effect of a polarizer. It can darken blue skies, increase contrast of partly reflecting subjects (like leaves and flowers), and reduce reflections from non metallic surfaces like glass and water. I'd put a polarizing filter #1 on my list of "must have" digital filters.

Polarizing Filters

Note that all DSLRs will require a circular polarizer. Digicams generally don't care and are quite happy with linear polarizers. Their effect on the image is exactly the same, but for various technical reasons involving the partly reflecting reflex mirror in DSLRs they need a circular polarizer to avoid metering errors (and possible potential problems with AF).

UV filters

UV filters block ultraviolet light, which can cause a bluish cast in images shot where there's a lot of UV (e.g. high altitudes). However most people use UV filters to protect the front element of their lens as much as to block UV. I'm not going to take sides in the debate over whether of not it's a good idea to use a UV filter all the time. Certainly using a low quality filter can degrade image quality, but using a good filter shouldn't really have any noticeable adverse effects on the image.

Neutral Density Filters

Netural Density FiltersNeutral density filters simply absorb light of all wavelengths. A 0.6D filter absorbs 2 stops of light and a 0.9D filter absorbs three stops of light. The major application is to allow the use of slower shutter speeds or wider apertures than would otherwise possible. For example in bright sunlight at ISO 100 (the lowest speed on most DSLRs, though some will go to ISO 50 and others won't go below ISO 200), the shutter speed at f16 (the smallest aperture you should probably use on an APS-C DSLR to avoid diffraction blurring) is 1/100s.

If you want a "flowing water" effect when photographing a waterfall, you won't get it at 1/100s. With a 0.9D (3-stop) ND filter you can reduce the shutter speed to 1/12s, which will start to give you that "silky" look for the water. At the other end of the range, if you wanted to shoot at f1.4 in bright sunlight to minimize your depth of field, even at ISO 100 you'd have to use a shutter speed of 1/12500s, which is faster than most DSLRs allow. By adding an ND filter you could drop that to 1/4000s or 1/2000s which most DLSRs do have.

Note that a polarizer will act as about a 2 stop ND filter, so if you have a polarizer, get a 3 or 4 stop ND so you have more options.


Graduated Neutral Density Filters

Grad ND filters are used when the dynamic range of a scene exceeds the capabilities of the camera (film or digital) to record detail in both the brightest and darkest part of the scene. The go from clear on one side to a neutral .6D (2 stop) or 0.9D (3 stop) neutral filter on the other. A typical use is to record detail in a bright sky (or mountains) while simultaneously recording detail in a foreground in shadow. You have to hope that the dark/light transition is a straight line though, since that's what a grad ND filter is designed to cope with.

Graduated Neutral Density Filters

Grad ND filters are available with "soft" and "hard" transitions between the two halves and they are available in a number of sizes with a variety of different holders. The filters are typically much larger than the lens diameter to allow then to be positioned with the transition at any angle and any position in the frame. Typical sizes might be 84mm x 120mm (84mm is the width of a standard Cokin filter holder).

You can also get round grad ND filters which screw into a lens like a normal filter, but unless you always want your dark/light transition running across the exact center of the frame, they may not be ideal.

Graduated Neutral Density Filters

The above shows the effect of an ND grad. On the left is the "straight" shot. On the right is the effect of an ND grad with the transition across the horizon and the ND part over the upper half of the image.

Note that you can duplicate the effects of ND filters by making multiple exposures and combining them in software. However that means taking multiple images and if you have a moving subject that could lead to problems. For landscape work though (which is where an ND grad is most often used), the digital stitching of multiple images taken with different exposures may be just as effective (even maybe more effective), though it probably requires more effort than just taking the shot using a filter.

InfraRed Filters

Just about all digital cameras use a filter in front of the sensor which allows visible light to pass but blocks almost all (but not quite all) of the infrared. This is needed because typical sensors are very sensitive to infrared and if it was allowed to reach the sensor it could mess up both exposure and color balance. Since you can't see in IR, you don't need (or want) IR response for general purpose photography. 

InfraRed Filters

However if you want the special IR effects (black sky and white foliage) you can use a filter such as the Hoya R72, which blocks all of the visible light while allowing IR to pass though it. Though the camera's IR blocking filter will remove most of this IR light, some will get through, and since the visible light is now blocked, a long exposure will record an image in IR light. In most cases the image will need a long exposure at wide aperture and high ISO settings and so the image will be noisy. There are companies who will remove the IR blocking filters from the camera to enable much better IR response, but this isn't cheap (typically around $500) and will void any warranty on the camera as well as making it tricky to use for normal photography.

Special Effect Filters

The effects of most of these can actually be duplicated digitally. You can do very good soft filter effects for example. Maybe there are a few effects like "starbursts" or multiple images that might be easier to do with a filter than in software, but personally, if I really wanted such effects, I'd probably go the software route since the effects are more controllable.

Why filters (and other optical elements) need to be anti-reflection coated

Below is a diagram of what happens when a ray of light passes through a filter:

MultiCoated Filters

 

Light comes in as a ray [1] and hits the filter surface at point [a]. Most of the light is refracted to point [b], but a small amount is reflected as ray [3]. This is light reflected from the filter surface and it does no harm except to make the ray passing though the lens slightly dimmer.

At point [b] a small amount of the light is reflected back to point [c] but most is refracted and emerges from the filter as ray [2]. This is the main light ray, the one that we want and it's where most of the light goes.

Most of the light reflected back to point [c] is refracted and emerges from the front of the filter as ray [4] and goes off into space,  again doing no harm, but a small amount is reflected back to point [d].

So far, no problems, but it's here where we see the first hint of trouble. Some of the light reflected from [c] to [d] gets reflected back again, but most of it emerges from the filter along ray [5] and heads off towards the camera where it isn't wanted.

The process repeats (see the green rays) and so you get a number in increasingly faint rays heading back into the camera. All we want is ray [2], the other rays just cause problems. The same thing happens at every glass/air interface in the lens, so you can see that by the time the light has passed though all the elements in the lens (usually between about 6 and 20), there are all sorts of unwanted rays bouncing around inside the lens. With digital sensors, there's also light which refects off the sensor and may eventually end up coming out of the front of the lens, as well as  bouncing around and ending up headed back to the sensor again!

By using anti-reflection coatings on all air/glass surfaces the intensity of all these extra rays can be greatly diminished. The main effect of these extra rays is to reduce image contrast by causing flare. There are so many reflections that the light essentially gets "scrambled" and so doesn't form an image. It's very low level light, but it can be enough to noticeably drop contrast.

With no anti reflection coating at all, about 4% of the light gets reflected at each glass/air interface. This means that ray [2] has about 92% of the original intensity of ray [1] and that ray [5] has about 0.15% of the intensity of ray [2]. Not much, but enough to cause problems. If the lens elements were not coated, you'd lose 1/2 the light after passing though 8 elements!

For a single layer antireflection coating, reflection can be reduced from 4% to 2.5%. Multilayer coatings reduce reflection even more. A typical multicoated surface might only have 1.5% reflection and the best coatings, like the 12 layer coatings on the Hoya Pro 1 series filters, lower reflection losses to around 0.15% per surface.

Which Filters

So assuming you want a polarizer, UV or ND filter, which one should you buy? Actually it's more of which one shouldn't you buy. You shouldn't buy an uncoated filter or a monocoated filter if you can avoid it for the reasons described above and you shouldn't buy the cheapest filter you can find since it's important that the glass be flat and the sides be parallel - and that's not something you're likely to find in the cheapest filters. I've been happy with Tiffen filters, though they are sometimes hard to find in multicoated versions.. I've also been happy with Hoya filters. Hoya is probably the major manufacturer of multicoated and supermulticoated filters at reasonable prices. Their filters are very good and their Super Multicoated Pro 1 series have the lowest reflectivity coatings of just about any filter on the market. B+W and Heliopan also make good filters, though they tend to be more expensive then equivalent Hoya filters.

Glass ND grads are available in a number of sizes and from a number of  suppliers, for example Singh-Ray (84mm x 120mm) and Tiffen (3"x3" and 3"x 4")   Hi-Tech and Cokin make "optical plastic" filters which are considerably cheaper, but which are more susceptible to scratching. The Cokin filters are "graduated grey" and may not be fully neutral. The square or rectangular ND grads attach to lenses using a custom adapter (the Cokin adapters are inexpensive and quite popular).

IR filters are available which transmit different wavelengths of IR. Probably the most useful for DSLR work is the Hoya R72 which blocks just about all visible light but transmits near IR light above 720nm which will (with some attenuation) pass through the camera's IR blocking filter. There are stronger IR filters like the 87C, but these work better with IR film than DSLRs since they only transmit longer wavelength IR which is totally blocked by the digital camera's built in IR blocking filter.


© 2006 Bob Atkins (www.bobatkins.com)

Article created 2006

Readers' Comments


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Mike Dodd , May 25, 2006; 08:31 A.M.

Can you give any advice on use of polarising filters with digital besides the information already mentioned as a Hoya high quality circular polariser on a canon 20D sometimes gives a nasty brown muddy appearance to the image. Whereas on other occasions it works well to intensify the colours. This kind of effect was much rarer on film.

I suspect this may be something to do with the exact angle of the sun and general quality of light, if its a fairly low angle sun and you are not quite using the 90degree angle then the image takes on a muddy appearance rather than the nice higher intensity orange glow. I am also talking about thousands of image not just one or two occasions.

Mathieu Landry , May 29, 2006; 12:40 P.M.

It should be noted that in the free software 'Picasa' a Graduated tint function is available and works quite well, thus negating the need for such a hardware filter too.

Brett Elliott , May 29, 2006; 06:50 P.M.

I don't agree that software can take the place of a filter. The camera only has so much dynamic range. If (for instance) the sky is 3 stops brighter than the ground, if you correctly expose the sky, then the ground will be pretty dark. Sure, you can use software to lighten it, but when you "turn up" the ground with the software, you're also turning up the noise.

David Henderson , May 29, 2006; 08:40 P.M.

Bob; I believe, and Singh Ray have confirmed, that their nd grad filters are made from CR-39 optical resin- essentially the same material as used by brands such as HiTech, Lee and Cokin. Also the same material as is commonly used for spectacle "glass". Singh Ray did initially manufacture from glass, but some years ago changed to optical resin in response to breakage issues.

Curt Weinhold , June 01, 2006; 09:05 P.M.

I have been using liner or circular PF on Nikons ever since the 8008. No difference in autoexposure or auto focusing that I can detect. Currently use a linear PF on a D70 & D2x. No problem!

Max Zomborszki , June 03, 2006; 04:28 A.M.

I have been using liner or circular PF on Nikons ever since the 8008. No difference in autoexposure or auto focusing that I can detect. Currently use a linear PF on a D70 & D2x. No problem!
The problem might arise at a certain polarizing angle, at others there will be no problems. Think of the partly reflecting mirror as a polarizer itself. Put a linear polarizer in front and you can get the cross-out effect at certain angles. Then the automatic system (usually auto focus) that is supposed to get the light from the semi-reflecting mirror gets less and might cease to function properly. The passive AF-system in use in most moderns (D)SLRs needs a certain amount of light to work properly.

Of course the manufacturers will not recommend you using a filter that will cause the camera automatics to work unreliably. You might get it to work most of the time, but how much that it worth if you miss that one shot you really want? When buying new polarisers the price difference between circulars and linears are usually not that big. I prefer my auto-focus to work reliably. YMMV.

Mark U , June 27, 2006; 07:47 P.M.

Filters can also be useful in some other specialized situations. A proper solar filter (ND 5 - reducing the light by a factor of 100,000) is essential for solar eclipse or sunspot photography. There are also other specialized astrophotography filters, such as those that pass the near infrared hydrogen alpha emission line.

Colour filters can still be a handy way to produce images with a desired warm or cool colour balance - set the white balance before mounting the filter. They can also be useful for handling subjects that have particularly intense colours that blow out one particular colour channel: select a complementary filter to reduce exposure in that channel, and then compensate for it in post processing. Doing this avoids underexposing the other channels to avoid blown highlights in the "intense" channel, and thus risking additional image noise.

Don Lorenz , July 04, 2006; 09:42 P.M.

Thanks for the great article. I have been putting together a filter collection. Right now i am putting a 58mm and 52mm set together.

Axel Cordes , July 27, 2006; 06:21 P.M.

You did not mention the "rings" - As I dont want to cary too much filters I?m using the adapter rings. Any hint on that?

kris baum , August 29, 2006; 12:06 A.M.

Wait a minute!!!

you've just eliminated spectral/colour filters because we can colour correct on camera or in photoshop with digital.

Any modification to an image in photoshop degrades its quality. The more extreme the modification, the more the image is degraded.

does anyone know the optimum colour temperature to take photos with a CCD/digital camera? Ie. the combination of CCD cells at that colour temperature produces the highest amount of contrast???

i would then suggest that a filter to change the light temperature would be required to get as close to this native colour temperature of the digital camera.

its like a ND filter. You can easily modify a sky by darkening it in photoshop. But the level of detail isnt as great if you either took two exposures 1 for foreground, 1 for sky or you used an ND filter.

Jason Kulas , March 30, 2007; 11:55 A.M.

If the picture has great dynamic range you might manage to simulate the effect of having used a graduated ND filter in software if you separately mask the brighter vs. darker part of the image (& then it won't even matter if your transition is a straight line or not!). The software DxO Optics Pro claims to be able to do this automatically for you (I haven't tried it yet, though there's a free demo). I'd guess it's just seeking out areas of darkest & lightest and processing them differently. Here are their claims & examples of lighting fixes: DxO lighting

Naeim Tavakkoli , December 18, 2007; 03:17 P.M.

Would you please give me an advice of using polarizer filters in sunny days with lots of snow everywhere. I?ve recently bought a D80 Nikon and started photography more seriously than before but it is winter time and I live in Canada! Taking pictures with a wide open aperture in most of the cases doesn?t end to a satisfactory result which is because of lots of reflected light I guess.

SGT SostanD , June 05, 2008; 11:14 P.M.

You Should use for Snow and bright light a Hoya ND2 or ND4 ND 8 all help with the bright light in winter in canada

Image Attachment: P4061040.JPG

Larry B , May 20, 2011; 04:17 P.M.

Are UV filters not built in to most digital cameras these days, making UV filters unnecessary for exclusion of UV light? The article above suggests that one may see a difference in an image if a UV filter is used, and with a film camera, that is true, but I think not true for almost all digital cameras.

Either a UV or clear filter may be desired for lens protection, but that is a different benefit than improving/changing image appearance.


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