Autofocus or AF is a feature of some optical systems that allows them to obtain and in some systems to also continuously maintain correct focus on a subject, instead of requiring the operator to adjust focus manually.

They both do the same thing, they just go about it in different ways. You can use either type of filter with a manually focusing camera and get the exact same results. Canon EOS Rebel XTi, then you would need a circular polarizer. Circular polarizers, because they convert the light coming through into convert it to circularly polarized light, will not interfere with your metering and AF systems.

The answer below is wrong, sorry to say. When you buy a camera, consider that youre buying in to a lens system. Your cameras lens mount determines what lenses you can use, and that camera brands lenses will be dependent on whether or not the camera brand changes its lens mount. Nikon has not changed its lens mount since 1959 and it is well known in the industry for great lenses with very good glass formulas.

You wont have problems finding lenses in the future. Get the instructions manual; there are many more features that I do not have the time or space to include here. CD made by the lab that processes your film. Youll the have the film for future use, and not have to worry about archival issues, still not unsettled in the digital technology.

It does not have interchangeable lens but it has many good features considering its price range. Nikon D50; or, the Nikon D70S with the kit lens; this camera is still going strong and people are still raving over its many good qualities.

For now, buy good but used cameras in good shape and buy only from reputable places where youll have a certain degree of security. 530 for a Nikon D50 with the kit lens.

Dont fall into this trap as so many others have; get into good habits from the start. There are three camera performance issues to consider in the circumstances you describe.

Second, you want as little shutter lag as possible. Third, you will need a camera with a relatively high shutter speed if you want to stop motion, particularly if you want to be relatively close to the subject.

But unlike a neutral density filter, you lose only extraneous light and not a lot of desired light. The circular polarizing filter has concentric circles of lines. You can see for yourself what a polarizing filter does. Take 2 REGULAR polarizing filters and place one on top of each other. If you twist one of them, you should be able to see clearly through both of them.

Now continue to twist and soon you wont be able to see through at all. This is because one filter is filtering one direction of light and the other is filtering out another. So when they are aligned, you can see through.

If you use 2 circular polarizing filters, you wont get the opacity because theyre always aligned when on top of each other. If you polarize your lights, youre already filtering one direction of the light.

Thats probably why when you use your circular polarizer, less of the extraneous light is being filtered.