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neutral density filter vs polarizer

Ron Cook , Mar 26, 2008; 08:53 p.m.

I recently got a 40d (upgrade from 20d) and the 17-55mm f/2.8. I am looking forward to taking some grand nature scenic shots this year. Some of the most inspiring photos i see mention a split ND filter. I have a polarizer.

What is the difference between a neutral density filter and a polarizing filter?

Responses

Colin Southern , Mar 26, 2008; 09:02 p.m.

Hi Ron,

A "split" (or more correctly Graduated Neutral-Density) Filter is a bit like a pair of sunglasses (or the top of the windscreen in your car) where only the top bit cuts the light back a bit.

A polarising filter is just like polarised sunglasses in that they cut glare. Polarisers are not normally used for wide-angle shots that include sky, as you'll get a very uneven sky tone due to the wide range of angles of the light entering the lens.

Cheers,

Colin

JDM von Weinberg , Mar 26, 2008; 09:37 p.m.

Like Colin says. These are different tools, although many have used the polarizers on narrower field lenses to darken the sky. The trouble with using it that way is you have to have the sun in the right place for it to work at all, and it will not have an even effect for very wide angle lenses.

Polarizers are perhaps most useful in getting rid of reflections on many kinds of surfaces, especially water or glass.

You should get a circular polarizer for modern cameras for reasons of potential problems with exposure and autofocus with linear polarizers.

James Dainis , Mar 26, 2008; 11:43 p.m.

Polarizers not only reduce glare on water and glass, they also reduce glare on leaves and grass giving them a richer look.

Derek Thomas , Mar 27, 2008; 12:31 a.m.

Just remember that a polarizer have much affect a certain angles. They work the best at a 90 degree angle from the sun and as you go more or less from this the affect decreases.

Ken Schuster , Mar 27, 2008; 01:54 a.m.

Sorry to pick a nit here, but "A "split" (or more correctly Graduated Neutral-Density)" is incorrect. There are "split" as well as "graduated" filters. A split filter is usually divided in half, with one half being neutral density (and/or color), and the other half is clear. The demarcation between the two is usually blurred. A graduated filter smoothly changes from full neutral density (and/or color) to clear. Then, of course, there is the common N.D. which is the same density over the entire area.

N.D. filters are commonly assigned numerical density in 1/3 stops. For example, "0.1" represents 1/3 stop, "0.2" = 2/3, "0.3" = 3/3 or 1 stop, "1.1" = 1 1/3 stop, etc. Many manufacturers put those numbers on the filter ring. Others use the factor system, such as "2X" which means 1 stop (twice the normal exposure), "4X" which means 2 stops (four times the normal exposure), etc.

Andy Aungthwin , Mar 27, 2008; 03:24 a.m.

You can use a CPL to reduce reflection and put an ND filter in front to get that overdone water effect, like the shot below.

David Henderson , Mar 27, 2008; 06:29 a.m.

"What is the difference between a neutral density filter and a polarizing filter?"

I think your question potentially confuses a neutral density filter- normally a screw in filter giving a uniform reduction in light across the frame- with a Graduated neutral density filter (aka ND grads; split ND, or GND, or even just "grads") which is usually a square or rectangular optical resin filter which reduces light over part but not all of the frame and are usually used in a holder which allows you to set where you want the graduation to start and use it hands free.

These are very different filters used for entirely different reasons. The Graduated Neutral Density filters are much more common in landscape photography and I'm going to assume that thats what you meant in your question

Whilst a polariser and ND Grads can both be used to darken and add detail to skies they work quite differently.

A grad works the same at any angle to the sun whereas a polariser works most strongly at a 90 degree angle to the sun and has an imperceptable effect right into the sun or with the sun right behind. As indicated, a polariser can produce some strange variations in sky tone when used with wide angle lenses. Grads have the same effect across the frame so avoid this.

A polariser does other things but darken a sky. It reduces reflections from most reflective surfaces - apart from metals- to improve saturation and sometimes clarity. Again this effect varies with angle. Meanwhile a grad pretty much only darkens an area of your photograph- which you can vary according to how you slide the filter up and down in the holder- and thats it.

You can replicate the effect of a grad in post processing but you can't do everything a polariser does in Photoshop. Some people prefer to add in a grad effect on the computer though it probably takes longer to do it that way and of course if you've blown out the detail in the original there's nothing you can do to create it afterwards.

Polarisers come in one strength. Grads come in several strengths and with hard and soft graduations. "Which grads do I need?" and "which brand of filters/holders should I buy?" are worthy of a debate all to themselves. Picking up one previous post I don't think most people- and certainly not the manufacturers - avoid the term "grad" or "GND" for hard transition filters. The term "split ND" just seems to be an alternative, colloquial expression used interchangeably. Might be different in movie production, I guess.

Polarisers are glass. Grads from all the usual brands (HiTech, Cokin, Lee, Singh Ray) are made from CR39 optical resin. There are a few glass ones but they are far too expensive for most still photographers and pretty fragile. The resin filters are prone to scratching over time and probably will need replacing after a few years use. As a side issue there are a few glass/screw in grads too. They are of limited usefulness and they are not the way most people choose to go. Again there are lots of debates on here on this.

A serious landscaper will carry a polariser and probably several grads with a holder system that may or may not be the same brand as the grad filters. Their usage can overlap a little but thay are not interchangeable.

Ken Schuster , Mar 27, 2008; 11:15 a.m.

David Henderson's explanation here is excellent.

John W. Wall , Mar 27, 2008; 06:08 p.m.

Get yourself a 2-stop split ND filter from Singh-Ray. Pricey but invaluable for keeping your skies from being blown out when your foreground is properly exposed. Cokin makes a holder for them, but you can hold them in your hand as well. Just be careful at the wide end that your fingers aren't in the picture. Use evaluative metering (assuming that's the functional equivalent of Nikon's matrix metering).

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