Polarizers
From Digital Image MakerLight has properties of both waves and particles. In the case of polarizing filters it is useful to think of it as a wave. These waves wiggle or vibrate up and down. Light reflected from most normal objects has light rays that are vibrating in all different orientations, some up and down, some left and right and some at all angles in between.
Light reflecting off of some objects, like water, however, has all or most or the light rays vibrating in the same way. The light is said to be polarized. A linear polarizing filter can be thought of as being a railing fence that only lets through light rays that are vibrating in the same orientation as the slots in the fence.
Rotate the polarizing filter and the light is blocked. Even with unpolarized light with light rays of all orientations, when you use a polarizing filter it will block the light not of the correct orientation, hence the general filter factor of a polarizer and the fact that your exposure needs to change by approximately two stops to compensate.
Modern cameras (both film and digital, due to the exposure and autofocus systems) can have issues with linear polarizing filters. So we use a circular polarizer. This comprises two filters, a normal linear polarizer followed by an additional layer that depolarizes the light. Since the polarizing part has done all the work, the result is exactly the same as with a linear polarizer.
A polarizer filter will normally be a little thicker than other types of filter and has one part which screws onto your lens ad another which is rotated to change the plane of polarization that is let through the filter. Circular polarizing filters will normally be a bit more expensive than linear ones. As with all filters you put in front of your nice, expensive camera lenses, it is better to use a good quality polarizer to minimize distortions or aberrations being introduced by the filter.
The parts of a photographic scene whose light may be strongly polarized, and thus be most affected by the use of a polarizing filter, are:
- Reflections off of any non-metallic surface, including water, leaves and glass;
- The blue sky 90 degrees away from the sun.
So depending on the orientation of the polarizer, you can:
- Darken the sky;
- Cut out glare reflections when trying to shoot through a window;
- Reduce or eliminate reflections in a lake or sea;
- By removing all the tiny reflections off of things like leaves and grass, increase the color saturation of a scene.
Mechanical Vignetting
on the Canon EF-S 10-22mm Lens
Mechanical vignetting occurs when light is partially blocked by external objects such as thick or stacked filters, secondary lenses, and improper lens hoods. The corner darkening can be gradual or abrupt, depending on the lens aperture.
On a super-wide angle lens at the widest setting, the field of view is so wide that the metal ring of a filter can vignette the corners of the image.
EF-S 10-22mm without filter
EF-S 10-22mm with filter
The onset and amount of mechanical vignetting depends on how far out the filter ring(s) extends from the front of the lens. 'Stacking' filters obviously increases the likelihood of running into a vignetting issue.
'Thin' Filters
So called 'Thin' filters are designed to minimize the thickness of the filter itself, to reduce the chances of mechanical vignetting. Most brands offer thin versions of the common filters.
Important Note: Some 'thin' filters do not have front threads, and therefore will not accept a typical lens cap.
Another possible way to avoid mechanical vignetting is to use a larger diameter filter combined with a step-up ring. This is probably not practical for the EF-S 10-22 due to its 77mm filter size.
So called 'Thin' filters are designed to minimize the thickness of the filter itself, to reduce the chances of mechanical vignetting. Most brands offer thin versions of the common filters.
Important Note: Some 'thin' filters do not have front threads, and therefore will not accept a typical lens cap.
Another possible way to avoid mechanical vignetting is to use a larger diameter filter combined with a step-up ring. This is probably not practical for the EF-S 10-22 due to its 77mm filter size.
ND Filters
Neutral Density filters come in two basic varieties: solid and graduated. Both function by simply cutting down the light that is transmitted to the film, just like someone turned off the lights. The key difference is between the two types is, of course, the graduation.Solid neutral density filters typically come as traditional screw-on filters in a variety of sizes and strengths. Their main purpose is to allow longer exposures when you can't or don't want to stop the lens down any further. This is usually to afford longer exposure times when photographing moving water to get that silky effect, but can also be used as another means to control depth-of-field. Owners of digital cameras may find these filters especially handy if their slowest film speed is no better than ISO 200.
Like everything that has an impact on exposure, these filters are measured in stops, but manufacturers seem to have a hard time agreeing on how to label them. While some are politely labeled with the number of stops of light they block, others such as Tiffen and Lee use the filter grades 0.3, 0.6, and 0.9 for one, two and three stops. Then there's B+W and Hoya who prefer calling the same thing 2x, 4x, and 8x (known as the filter factor).
The idea with GND's is that one end is clear and the other is neutral density. In between the two ends, the filter transitions through either a soft-edged or a harder edged graduation which allows you minimize the likelihood that the transition will be visible within the picture frame.
The aim is to position the filter within the image by sliding it up and down in order to equalize the brightness across the frame.
The trick to getting the graduation in the right place is practice. By stopping the lens down to your shooting aperture and then moving the filter around, it becomes much easier to see the edge of the graduation. Bracketing your compositions slightly with respect to how the filter is positioned in order to make sure you've gotten it in the right place can help if you are unsure. Don't pay any attention to where the graduation appears to be when you look at the filter sitting there on the end of your lens. It may seem like it covers a third of it, but if the lens is stopped down at all, it may not even cover any of the actual image; it is only what you can see through the stopped-down diaphragm that counts, not the entire lens diameter.
