One Filter Ring to Rule Them All

Benefits of Filters

Filters are attached to lenses to achieve effects such as changing color cast, etc.  Thanks to digital imaging, many filters can now be simulated in postprocessing (Photoshop, Paint Shop, etc.).  However, filters are still relevant.  Some people use UV filters to shield their lenses from physical damage after all. :)  Seriously, there are some filter effects that are still difficult or impossible to replicate in postprocessing:

1. circular polarizer: cuts reflection and glare.  The blue of the sky can be made to look deeper blue.  You may be able to avoid some reflections in glass, depending on the angle of the glass relative to the camera (think of all the bus tour photos you can take! :) ).  You may have seen those postcards with tantalizingly clear waters for snorkeling.  Part of that clarity is from blocking reflections.

2. neutral density filter: reduces the exposure value.  You've seen those waterfall shots with flowing water.  You're savvy enough to know those were done with long exposures.  What you might not have known (unless you've tried it) is that those long exposures are sometimes impossible without a neutral density filter, especially for newer DSLRs with a base ISO of 200.  In addition, an ND filter can also be used to get a wide aperture (for shallow depth of field) in bright conditions, and can give your flash a higher output than an equivalent exposure at a higher shutter speed that requires high speed sync.

3. graduated neutral density filter: reduces the exposure value for part of the scene.  This is one way to expand the dynamic range of your image.  You can adjust the graduation to cover just the brighter parts of the scene you're capturing (e.g. clouds, windows).

4. split field - increases the depth of field.

5. infrared: blocks visible light, allows photography of infrared wavelength.  Infrared is a photographic genre in itself.  Sorry I don't have samples.  But here are links.

In short, filters are useful when taking shots outdoors.

Cokin filter holder system
Are you getting ready to buy filters?  Well, don't get one just yet.  The typical round screw-in filter is screwed onto the lens.  Each of your lenses has a certain filter size, such as 67mm, 72mm, 77mm, etc.  You buy filters for each filter size you have.  Although filters can be stacked one on top of the other, the filter rings of the stacked filters become cumulatively thick enough that they can cause vignetting.

Unlike typical filters, the Cokin system uses a filter holder and square or rectangular filters instead.  An adapter is screwed onto your lens, then the filter holder is in turn attached to the adapter. Filters are then slid into the filter holder.

Advantages of the Cokin system:
1. Adjustable positioning. Cokin filter holders can be freely rotated and the filters can be moved up or down.  This makes the filters usable even if the lens rotates during focus.  For graduated filters, this allows precise control of where the graduation begins.  For vignette or selective focus type filters, this allows the precise positioning of the vignette or area of selective focus.

2. Speed - it's faster to slide filters on and off a Cokin filter holder than screwing filters on and off lenses.

3. Filters can be shared between lenses. Just buy a filter holder and adapter ring for each lens, or just adapter rings.  Filters of the same series can be shared between lenses.

4. Stackable - a Cokin holder can hold up to 3 filters (1 of which can be a rotatable filter) at the same time. Cokin holders can also be paired together to allow stacking of up to 6 filters, though vignetting can become an issue for anything less than a telephoto lens.

5. Less susceptible to vignetting - because the filter holder is generally much wider than the lens, there is less chance for vignetting.  Cokin recommends these filter holders for these focal lengths:
A series: lens up to 62mm filter size, for 35mm focal length equivalent of 35mm or longer.
P series: lens up to 82mm filter size, for 35mm focal length equivalent of 28mm or longer.
P series wide angle (BPW700): offers 20% more angle of vision, for 35mm focal length equivalent of 20mm or longer. Uses same filters as P series, but accommodates only 1 filter at a time.
Z series: lens up to 96mm filter size, for 35mm focal length equivalent of 20mm or longer.
X series: lens up to 118mm filter size, for 35mm focal length equivalent of 15mm or longer.


6. Handholdable.  It is possible to do without a holder and simply handhold filters in front of the lenses.  This can help further avoid vignetting.

7. Cost - Cokin type filters usually cost less than equivalent round screw-in filters of similar quality because of economies of scale and because the Cokin filters are simpler to manufacture.

8. Less likely to damage your lens.  High quality round screw-in filters are often made of glass.  The problem is that if the glass is shattered, the fragments can damage your lens.  Cokin filters (and some clones) are made of CR-39 optical plastic, which I'm supposing is less likely to shatter and I'm also supposing shattered CR-39 fragments, being less dense than glass, are less likely to scratch your lens.

Disadvantages:
1. Size.  It adds to the bulkiness of your lens.
2. You can't use your lens cap.  You can, however, buy a lens cap specifically for a Cokin holder.  The lens cap doesn't cover the lens completely, and it may allow dust to reach the lens, but at least your lens will be protected from bumps.
3. You can't use your lens hood. You can, however, buy a lens hood specifically for a Cokin holder.  The Cokin lens hood isn't a perfect design because it is uniform in thickness (not petal shaped), so there's a high possibility of vignetting for wide angle lenses.

Cokin isn't the only filter holder maker.  Lee and Singh-Ray also produce filter holder systems but their filters are more expensive than Cokin.

Cost:
So how much damage will this do to your budget?  The Z and X series are expensive but the A and P series are affordable.  An original Cokin P series holder with an adapter costs less than $20.  On ebay and other places, there are copies of the Cokin P series holder and filters at a fraction of that.  For less than $35, I bought these from Fotodiox:
- Cokin P holder clone (Tian Ya brand) with aluminum 72mm adapter
- circular polarizer
- ND8 (3 stop reduction)
- ND4 (2 stop reduction)
- graduated ND
- a case for each filter
- lens cap

I chose Fotodiox because other Cokin clones are made of unspecified plastic, whereas Fotodiox's are specifically made of CR-39 plastic (same material as some eyeglasses), just like some of Cokin's filters.  In addition, Fotodiox ships from USA, so it allows instant gratification :D  (I had to wait for my D300 remote from Hong Kong for 3 weeks...)

Fotodiox also sells the ND filters as a set (ND2, ND4, ND8).  If you ask them, they can substitute the ND2 for a graduated ND, which is what I requested.

Fotodiox has spectacular customer service.  They sent me the ND2 instead of the graduated ND by mistake but when I followed up, they offered to send the graduated ND without the need to return the ND2.  My ND4 and ND8 were also chipped, and they sent replacements without questions.

(Note: I'm not affiliated with Fotodiox.)

I will update this post with "real world" shots using these filters.

Update: Here are test shots of the Fotodiox generic filters.