As you may be aware I shoot a lot of landscape and architectural photographs. For landscapes I'd say I use filters in 95% of my shots to balance exposures and/or control shutter speeds. With architectural (especially exteriors) its roughly the same. To be honest both forms of photography are very similar in setup.... from timing to equipment how one sets out to take photos of a beautiful seascape or a modern piece of architecture are very alike. What I will try to explain below is, what filters are what, what they do to your images and advise on whats best for your camera system. I'll speak almost exclusively about Neutral Density filters and just tip on polarizers towards the end.
I'll start by explaining all the terminology. Ok so there are basically a few different types of filters. Neutral density filters themselves come in 3 different variants (that I am aware of). A plain full neutral density filter is one where there is no gradation across the width of the filter. So from top to bottom of the resin/glass it will be a neutral grey. They come in either square or rectangular slide format or circular screw in types. Why do they call it neutral you might ask....? Theoretically they are a neutral grey and shouldn't cast any colours across the area of the image they are used upon. In reality most ND filters cast some amount of colour but with some being much more 'neutral' than others. I've tried a few different types starting with Cokin that give a purple/pink tone to B+W which give a brown/orange and finally to Lee (where I remain) that give as neutral a colour balance I have seen so far with maybe a very very slight blue cast on heavier grads. Of course all casts are removable in post processing if shooting RAW but it can get tricky when using graduated filters where they only effect part of the image. These graduated filters are exactly that... graduated from grey to completely clear. They come in a few different types.... hard graduated, soft graduated and reverse grads. Hard graduated filters fade from dark to clear quite quickly. Upon measuring they fade in around 8mm. Soft graduated filters on the other hand fade much slower across the frame. Upon measurement they fade in around 30mm. The other type of filter is a reverse ND filter (or stripe filter) which has a dark central portion. All these filters are available in various formats, colours, materials and intensities. This intensity is general measured in F-stops but is also sometimes measured as optical density or percentage transmission. Have a look at the table below (from Wikipedia) for various densities and what they equate to (Lee write the optical density on their filters) ....
|ND||FRACTION||OPTICAL DENSITY||F-STOP||% TRANSMITTANCE|
One question that I am always being asked by fellow photographers is 'What type of filters should I buy...??? Hard or soft graduated'. I always answer this question based on my own personal experience in using the Lee system. I have both hard and soft and soft ND filters and have used them for both landscape and architectural photography. Personally I have found that on a 35mm equivalent digital sensor (5d or 5d2) that the hard grads work best. I find that the graduation from grey to clear to be about enough when used on both wide angle and short telephoto lens. In terms of usage I would say I use the hard grads about 85% of the time. Grads in general would have less of an effect on a cropped sensor as technically the graduation would be happening across a greater area of the frame. If using soft grads on a cropped sensor on a wide angle lens the effect of the graduation would be so gentle that one might nit see the effect of its use on the image. As a basic test I measured the size of the front element glass on the front of my Canon 17-40L, a fairly typical and popular wide angle lens on both full frame and cropped Canons. It measures approximately 55mm.Considering tolerances of the sensor etc I'd say around 50mm of that is used. If using a soft grad on a full frame 35mm camera you'd be left with around 10mm above and below the graduation. In use I have found this to be just about useable on full frame but the effects when using anything under a 2 stop grad are hard to see on the finished images. On a cropped sensor the effect of the soft grad would be even less pronounced. So on that basis I would always recommend Hard Graduated filters for 35mm equivalent DSLR cameras (full frame or cropped sensor). Lee themselves give similar advice....
Now I will list a few reasons why one should use neutral density filters......
- To purposely slow your shutter speed for a specific effect eg. blurring water, motion blur or to make moving items (like cars or people) disappear
- To decrease your depth of field on very bright days. If you want to use an aperture of f1.8 but the shutter speed ends up being over the maximum of your camera you can lower that shutter speed by reducing the amount of light coming into your camera with an ND filter.
- You might use a grad filter to balance the exposure in a scene where there is a big dynamic range across the frame.
There may be a few others but the ones above are the reasons that I use ND filters anyway...!!!
Of course there are Polarizing filters as well and while technically they are not ND filters they do reduce the amount of light coming into your lens by between 1.5 and 2.5 stops. So the effects listed above would also be achieved. Polarizers can also enhance colours in blue skys and vegetation by reducing the amount of glare and reflection. This reduction in reflection can be used to great effect when shooting in shallow waters where there are nice rock formations just below the surface. Also useful for reducing reflection in glass...! If shooting skies ensure you shoot at the best angle - for nice blue skies move around so that the sun is at 90 degrees to the subject - anything more or less and the saturation is reduced and the effect wasted. Be careful using them on extremely wide angle lens on blue sky days as some parts of the sky will look overly saturated and false. To be honest I generally use polarizers for reducing reflection and haze as the saturation effect I can almost mimic in post processing. Vignetting can also be an issue on wide angle lens so try to use slim versions. Circular screw in versions are generally easier to use and easily available therefore more popular.
Ok shit I am babbling again.... This was supposed to be a short post..... I hope my advice (ramblings...????) help someone when choosing or using filters. I may expand this post in time to speak more about the usage of ND filters but for the moment I'll leave it as is...! I'll finish by giving you a few links for further info and advice.....