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I receive a lot of questions about usage of optical filters on DSLRs. Many people switch to DSLR from their compact digital camera these days, and generally they don't have much idea about what they are for. So I decided to write a list of the most essential filters that a DSLR owner should have so that they can take the full potential of their camera and also increase their level creativity and flexibility.
These are UV Filters, Polarizing Filters and Neutral Density or ND Filters.
1- UV Filters
This is one of the most recommended filters of all because the main puspose of using this filter on your DSLR is to protect the front-element of your lens against scratches, dust, dirt, water and so on (Read more about protecting your lens and keeping it clean). In film days,it was designed to block the UV light falling on film, digital image sensors are not that much affected by the Ultraviolet light.
So you may ask why not any other filter but UV filter. The answer briefly is that the UV filter doesn't effect the amount or nature of light passing through it so it doesn't make any change in the final image. But in reality, any extra optical element is a image potential quality degrader. If the filter is a high-quality one, you will probably not see the its influence unless you investigate really hard or pixel peep your images. So I would always recommend you to get the highest quality ( and the most expensive in most cases) UV filter you can if you care the image quality and planning to keep it on your lens most of the time.
One side note for the ones who have very little idea about the lenses and filters: Lenses have different filter sizes so you need a matching filter for every lens. And the other fact is that filter gets more expensive as this filter size increases.
2- Polarizing Filters
Polarizing filters is probably the most useful filter on DSLR cameras. I will not get in to technical details as there are articles that can do this much better than I can. Very briefly, certain wavelenghts are allowed to pass through the filter and others are blocked.
Mechanically, Polarizing filter is an adjustable filter with an inner ring and outer ring. Inner ring screws on to your lens and the outer ring can be rotated for adjusting the effectiveness of the filter.
Polarizing filter is used for absorbing glare and reducing-eliminating the reflections. It also increase the vibrancy of the sprecific colors. So lets list few areas where Polarizing filter has the biggest effects:
Reflections: You can block the light entering the lens from a reflective surface by adjusting the angle of the front element of the polarizing filter. Water and windows are good examples of where you can make the reflections disappear on your photos. As you rotate the front element, you can observe how your your filter treats this light coming fromr eflective surfaces. One common example of use to photograph your friend sitting inside a car thorugh the windshield. Polarizing filter will eliminate the reflections on the windshield so that you friend will appear on the photo. Polarizers works best when the light to come be reflected is coming with an angle. The light coming directly to the sensor plane will not be effected by polarizing filter. So in the example above, you will need to position your camera approximately 35° to the windshield.
But keep in mind that polarizing filter can not block out the reflections on metallic surfaces
Sky: Other source of reflections are the water and dust particles in the air. These reflections make your photos less colorful and vibrant. When a polarizing filter used, the color of the sky will turn from light blue to dark blue whick will look more appealing. You can also let the clouds almost disappear or make them better more visible by adjusting the front element, and what you can get is a higher contrast and dramatic scene which makes it ideal especially for landscape photographers. Remember that the polarizing filter is most effective for the sky when the sun is at a 90° angle to your camera.
Polarizing filter also males the foliage looking greener and more vibrant.
One thing you have to know about the polarizing filteris that it reduces the light passing through the lens normally about 1.5-2 stops. But depending on the brand or type of the polarizer, this can change from 1 stop to as much as 3 - 3.5 stops.
While some of the effects (colors) gained by Polarizing filters can be replicated on post processing with digital image editing softwares, effects like eliminating reflections are impossible to replicate and can only be achieved during shooting.
3- Neutral Density Filter
Even though not as popular as the first two filters, ND filters are essential for certain types of situations. Neutral Density Filter is a grey filter which reduces the amount of light passing through the your cameras lens and falling on the image sensor or film. This comes in handy when you want to take long exposure photos in bright sunny days or well lit places.
For example, when you want to photograph a waterfall on a very bright day, at a slow shutter speed to create a motion blur effect. In order to do this, you will need shutter speed on the order of tenths of a second, sometimes even longer. There will probably be so much light that even at min ISO level and a minimum aperture of your lens, the calculated shutter speed will still be too fast. In this situation, by attaching an appropriate neutral density filter you can reduce the amount of light couple of stops, thus allowing a slow shutter speed and more pleasing effect.
Another place of use is when you want to shoot with very large apertures to get a nice bokeh (blurry background) under bright light.
For example when you want to shoot a portrait of your son under bright sun with your Canon 85mm f1.2 L at f1.2 for perfect bokeh and lets say what your camera calculates is a shutter speed of 1/6000 sec.. And unfortunately your Canon Digital Rebel Xti / 450D goes only up to 1/4000 sec.. Then what you will get is an overexposed photo. What you can do is to use an appropriate neutral density filter to reduce the light passsing through and resulting shutter speeds down to 1/4000 sec.
ND filters are quantified by their optical density or equivalently their f-Stop (i.e. 0.3 ND, 0.6 ND, 0:9 ND or 1-stop, 2-stop, 3-stop)
One type of ND filters is the Graduated ND Filter. Graduated Neutral Density filtrers are also know as Split Neutral Density Filters or simply Graduated Filters.
A graduated ND filter is similar to an ND filter except that the intensity changes across the surface of the filter. This is useful when one region of the image is bright and the rest is not, as an example: While shooting sunsets or landscape photos where the upper part of the horizon is very bright and the lower section is dark. Using a Graduated ND filter, you can avoid overexposed skies by partially reducing the light coming from that part of the scene.
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The 0.3/0.6/0.9 and 1/2/3-stop ratings are both logarithmic scales, so you can convert between them easily just by multiplying or dividing by 0.3. The 2/4/8 scale is exponential, meaning it's the inverse of a logarithmic scale - in fact, 10 raised to the power 0.3 is (roughly) 2, 10 raised to the power 0.6 is (roughly) 4, and so on. Similarly, "stops" are logarithms to the base 2 - 2 to the power of 1 is 2, 2 raised to the power of 2 is 4, 2 raised to the power of 3 is 8, and so on, so that's how you convert between stops and ND-2/4/8 numbers. FInally, the percentage light transmission is based on the reciprocal of the neutral density - it's probably easiest to think of it related to stops. 1 stop gives you half the light, so 50% gets through. 2 stops gives you one quarter of the light, so it blocks 75% of the light, and 3 stops gives you 1/8 of the light, so it blocks 87.5% of the light, and so on.
Sounds like a conversion spreadsheet or a little Javascript calculator would come in handy ...