Linear polarizing filters seem to have a greater effect on the final image than circular polarizers do, and in spite of many misconceptions, you can use either type of filter on any camera. But there is a catch: if you use a linear polarizer on a modern camera, you may not be able to use your built-in metering, or autofocus, or possibly both.
Modern autofocus systems use mirrors that act as beam-splitters: most of the light is reflected to the viewfinder for metering and viewing, while the rest (typically 25%) is transmitted and then reflected by a secondary mirror to the autofocus sensor, which is in the camera body. With a polarizing filter attached, the ratio of reflected/transmitted light (fixed for non-polarized light) varies with the polarization plane orientation of polarized light, relative to the beam-splitter, causing the amount of light reaching the autofocus sensor to vary. This effect can be particularly troublesome if your front element moves with the lens, as it would cause the amount of light reaching the sensor to change while the lens focuses.
While you can see (rather easily) when your autofocus system has been confused by a linear polarizer, there isn't such an easy way of knowing whether or not the internal light metering system has been stumped. The same loss of light principles that affect the autofocus system can also affect a camera's built-in meter, and obviously this is never good. What use is a light meter if it receives the wrong amount of light?
In both situations, a circular polarizer corrects the problem. Having removed the effect of linear polarization, the light functions normally inside the camera, in the beam splitter, and reaching the autofocus and metering sensors properly. Therefore, for your convenience, you should use a circular polarizer if your camera has autofocus or built-in light metering features. However, if you focus manually, and use a hand-held light meter, you can use a linear polarizer on any camera you wish.
Resta saber se a d50 tem ou não esse sistema.
