CP filter??

lalalala<3life · Aug 6, 2009

I have few quick questions. What is a CP filter? and what are they mainly used for? Also what do they run$$$$???? And what would be a good one for a nikon D90?

Also another question what are gels? or gel filters? I've heard them mentioned on here and I dont know what they are.

Thank you

c.cloudwalker · Aug 6, 2009

Circular polarizer = Polarizing filter for digital cameras.

Here's the explanation from Wiki: "Polarizer
A polarizing filter, used both in color and black and white photography, filters out light polarized perpendicularly to the axis of the filter. This has two applications in photography: it reduces reflections from non-metallic surfaces, and can darken the sky.
Light reflected from a non-metallic surface becomes polarized; this effect is maximum at Brewster's angle, about 56° from the vertical (light reflected from metal is not polarized, due to the electromagnetic nature of light). A polarizer rotated to pass only light polarized in the direction perpendicular to the reflected light will absorb much of it. This absorption allows glare reflected from, for example, a body of water or a road to be much reduced. Reflections from shiny surfaces of vegetation are also reduced. Reflections from a window into a dark interior can be much reduced, allowing it to be seen through. (The same effects are available for vision by using polarizing sunglasses.)
Much of the light from the sky is polarized (bees use this phenomenon for navigation). Use of a polarizing filter will filter out the polarized component of skylight, darkening the sky; the landscape below it, and clouds, will be less affected, giving a photograph with a darker and more dramatic sky, and emphasizing the clouds.
The benefits of polarizing filters are largely unaffected by the move to digital photography: while software post-processing can simulate many other types of filter, a photograph does not record the degree of polarization, so the optical effects of controlling polarization at the time of exposure cannot be replicated in software."

A gel is just a piece of colored platic to change the color of your light. Put a blue gel in front of your flash and you get blue light. For one, it is an easy way to change the color of your background.

In film photography I used gels regularly to change the color temperature of neon fixtures. With color film, neon light looks green and by using a gel you can turn it to daylight color temperature which was very useful when shooting in office buildings. But I don't know if there is still a need for that with digital. Too new to the world of digital.

Cheers.

Dwig · Aug 6, 2009

lalalala<3life said:
...what are gels? or gel filters? ...

"Gel filter" is very often a mis-used term. In common inaccurate jargon, it refers to any filter that is made out of some flexible film. In proper technical terms, it is a filter made from dyed gelatin without any other material, plastic or otherwise, as part of the thin film. These are had made in moulds and are rather expensive, and these days, rather rare.

True gelatin filters have an extremely low refractive index and are very thin. This means that they have almost no impact on the optical image regardless of how flat they are mounted or whether they are perpendicular to the light path. The latter is an extreme problem with wide angle lenses where the light forming the image at the corners passes through any flat filter at an extreme angle. If the filter has a high refractive index, like any glass filter, and any noticable thickness, again any glass filter, it will induce significant optical flaws, both chromatic aberation and astigmatism.

Gel-like filters made using acetate or other plastics can be very thin, reducing the optical problems, but have higher refractive indexes making them less "opticall neutral" than real gels. Some, those sold for lighting use only, are so poor optically that they shouldn't be used on lenses.