"Neutral Density" filters vs. "Polarizing" filters

Aaron Silverman · 07-07-2004, 04:47 PM

Forgive me if this is a silly question. . .

I've read that a Neutral Density filter will help reduce the purple fringing common in high-contrast digital shots. Is that the same thing as a Polarizing filter?

Tom Meyer · 07-07-2004, 11:18 PM

not sure about the purple fringing thing but ... a ND filter is used to reduce the overall exposure uniformly without affecting the color (either bw or color). It is commonly used in situations such as ..

- if it is very bright out and using a wide aperture to get a shallow depth of field would result in overexposure

- if you want to slow your shutter speed to get a more "liquid" effect when shooting water or overall motion in general.

- split-density ND filters are used when you have a very contrasty scene where 1 part of the frame would be overexposed while other parts would be properly metered. They have 1 half w/ the ND filter and the other is clear and can be rotated over potentially overexposed frame

A (usually circular) polarizing filter also generally reduces exposure but is used to do things like enhancing colors, skies, etc.. or reducing glare or reflections off water or glass. You use them by standing a right angle to the sun and rotating it until you get the desired effect.

Man-Fai Wong · 07-08-2004, 11:24 AM

Purple fringing is usually the result of sensor blooming, so to the extent that you avoid blooming, both ND and polarizing filters would probably help, particularly polarizers. Polarizers would help prevent blooming caused by reflections in certain parts of the scene, which can happen more often than one assumes. For example, trees, vegetation, etc. often collect some amount of water that can lead to such reflections in landscapes. Cutting down the glare in such instances would probably help.

Also, some lenses are designed to help reduce purple fringing, so that's something to consider when buying a lens for your DSLR -- if that's what you have.

_Man_

Aaron Silverman · 07-08-2004, 05:36 PM

Thanks for the info, guys!

Man-Fai, I am still shopping for a new camera (you helped a lot in my other thread on the subject!). I am leaning strongly towards a Canon G5, but can't find a local store that carries it in stock so I can check it out. I'm about ready to resign myself to just watching the online prices and ordering one.

Xie xie ni!

JohnRice · 07-15-2004, 02:04 AM

Aaron, a polarizer does actually also work as a roughly 1 2/3 stop ND filter, while also doing other things. The truth is, it is very difficult to explain what all a polarizer can do and most people are wrong, or incomplete in how they explain them.

There are so many ways polarizers can be used to improve outdoor photos they are almost too many to list. the funny thing is, I rarely use them for the single thing most people use them for, which is darkening the sky. Or, if I do, I only polarize partially, so I don't get too artificial a result.

**HTF Ads**

Scott Kimball · 07-15-2004, 08:25 AM

The best thing to avoid fringing is to be very careful about your exposure. If your camera has a histogram feature, use it.

While, as others pointed out, a ND filter can help reduce fringing, you can do better by adjusting your exposure compensation to -.5 or -1 (when needed), or by bracketing your shots, and checking the histogram before you move on. The curve on the histogram should approach and diminish toward the right-hand side of the graph, but not be abruptly cut off at the end. Don't judge exposure by the image on the LCD - they are lousy for that. The LCD is okay for checking composition and focus, but not exposure.

Personally, if I can avoid using a filter of any kind, I do. I usually carry a circular polarizer, a UV filter, and a ND filter - but they don't live on my lens... they only come out for the specific circumstances for which they are needed.

-Scott

Man-Fai Wong · 07-15-2004, 10:37 AM

For more of what I meant about polarizers, see here:

http://dpfwiw.com/filters.htm#polarizers

The site seems to be a very good resource in general, and I found their info on polarizers particularly helpful.

But no, it's not a cure-all kind of thing.

For one thing, it's not likely to help prevent PF around bright lights in night scenes or any other direct light sources or reflected light off metal surfaces.

_Man_

Aaron Silverman · 07-15-2004, 06:51 PM

Thanks again, guys!

The G5 apparently has a built-in adjustable ND filter, so if I do end up with one, I can play around with that.

JohnRice · 07-15-2004, 07:41 PM

Aaron, you might want to check on that ND filter. It may be some marketing BS and not actually a physical ND filter.