Neutral Density vs. Dual Polarizers

Anyone have any experience using two circular polarizers as a variable ND filter? Are there any drawbacks with this method? It would certainly be less expensive than buying 3 or 4 different ND filters for 67mm.

-dave-

Dave,

That is an interesting concept I had never thought of.

Only downside I can see is that you will be ending up with 4 pieces of glass (2 per filter) stuck on the end of the lens. I would hope that you wouldn't get degraded image quality like that.

If you are looking at NDs or even graduated ones, you may want to look at the Cokin filter system that uses square (rectangular for grads) resin filters. I use these to great effect for landscape work.

Cheers

Ian

You should know that when you use a Polarizing filter the filter factor is the same whether you've dialed in a lot of Polarizing effect of not. This comes as a surprise to most people and you will need to prove it to yourself - as I did - to see that it's true.

Your testing results will be that you've lost about two stops with the P filter no matter how it's rotated. So two Polarizing filters will provide a ND filtration of 4 stops, but will place four (!) pieces of glass in front of your lens and NOT be variable.

I'd suggest a real 4 stop ND filter to do the same thing with one only piece of glass.

The concept of using two polarizers as a variable ND filter is pretty old. I even remember my physics class where it was demonstrated that two polarizers with one tilted 90° vs. the other gives complete darkness.

BUT this works ONLY with LINEAR polarizers!

As far as I remember, these circular polarizers replaced linear ones with the appearance of autofocus because AF won't work with linear polarizers. Normally you would use ND filters in landscape photography for instance to smoothen running water and for this you don't need AF anyway.

Conclusion: should work out great with two linear polarizers and manual focus!

--Jens--

Dual polarizers will not render a nuetral density. As a matter of fact people have succesfully used them to do near UV photography with 90 degree alignment.

I did not say it will RENDER ND, I just said it would WORK. BTW: Have you tried it?

near UV photography? 90 degree vs. what? OT?

cowman,

Why risk messing with your image with neutral filters, polarisers or any other bits of glass which might dumb down your lens, when you can add any level of neutral filter you wish at the processing stage.

The advantages of applying neutral filters AFTER capture (other than eliminating the extra layers of glass problems) are that :-
1. Its easy and fast,
2. You can adjust the filter any way you want until the amount, tilt, depth etc. it exactly to your liking.

It really is very easy. In PS, set foreground to black, set the gradient tool to linear, mode to colour burn and amount to 20%.

Pull the gradient tool down the image and bingo, a great neutral filter applied directly to the shot (leaving all those puffy little clouds bright and white).

Don't like it? Want a bit more 'Northern sky' tint? Then delete the gradient and apply a new one drawing the gradient from one top corner toward the center of the image.

Want a inverse neutral filter to enhance foreground colours? Then drag the gradient tool from the bottom to the middle, and there you have exactly the effect you would have achieved with a glass filter, without the downside effects of extra layers of glass and the result is in a form that is totally editable in the darkroom.

Don't forget, that the creation of a final image used to be half in the field and half in the darkroom. Today, your darkroom happens to be about as powerful as your new 10D is. Photographers who no longer use their darkroom out of some sense of 'pureism' are foregoing half of their image creation potential.

Derek

I guess the only reservation that I'd make about using ND filters in the field, compared to faking it in the computer later, is when you WANT to use a very slow shutterspeed.

For example I like to use an eight second exposure when I'm photographing a waterfall or a stream. The water becomes silky and I like the effect. With ISO 100 there is almost always too much light - even on an overcast day - to do this shot without a ND filter. You certainly can't overexpose and work that out later. Underexpose maybe but not overexpose!

I come from a large format background where it's not unusual for lenses to stop down to f64 (my LF 300 mm Nikkor f9 stops down to f128!) and even that doesn't get it done sometimes.

I agree with everything else you say but sometimes an ND in the field is what you need.

Derek, you're absolutely right in your suggestion, but ND is for increasing shutter speeds, and unfortunately, that's not something we can do in photoshop.

I was looking for a cheap way out to get great, slow exposures of running water, specifically.

I think I'll consider a cokin system or the like.

-dave-

Derek,

That is all true (and I do it myself) but in some situations (sunsets especially) the 10D simply doesn't have the latitude to capture the full range of information needed. We are talking 5 or 6 stops difference between highlight and shadow.

If you expose for the highlight you end up with very dark shadows. Recovery of these is fairly straightforward in most situations but for sunsets the shadows have almost no detail left and when boosted are full of noise.

There is still a place for a Cokin (or Lee these days) kit with various ND grads in my camera bag

Cheers

Ian

iwatkins wrote:
Derek,

That is all true (and I do it myself) but in some situations (sunsets especially) the 10D simply doesn't have the latitude to capture the full range of information needed. We are talking 5 or 6 stops difference between highlight and shadow.

If you expose for the highlight you end up with very dark shadows. Recovery of these is fairly straightforward in most situations but for sunsets the shadows have almost no detail left and when boosted are full of noise.

There is still a place for a Cokin (or Lee these days) kit with various ND grads in my camera bag

Cheers
Ian

But an ND filter will not give you a greater exposure latitude?? You should do exposure bracketing and merge the exposures on the computer if you want that. Be sure to put the camera on a tripod and use the same aperture (so that the DOF stays the same) etc...

Here is an example what ND filters are normally used for:

sunny, f16, landscape, waterfall, tripod.

Your camera suggests 1/100 sec for exposure. But you want to smoothen the water and not freeze the motion Therefore you need for instance 1/10 sec exposure. Your camera can't provide that because your aperture is at upper limit (f16).
An 8x ND filter does the trick: set to f16 and the camera will expose approx. 1/10 sec and provide the slower shutter speed without changing anything else.

--Jens--

Fee Fie Foe Fum, I smell the post of a PURIST.

'FAKE IT. Fake it on the computer' you say. I think this calls for a new thread to discuss what is fake and what is real.

Derek

Apologies, I don't think I was making myself clear. I was talking about ND *grads* that reduce the brightness of the sky without affecting the brightness (or lack of) of the ground. I.e. reducing the contrast between brightest and darkest part to something the sensor can reliably capture fully.

Yes, you can of course bracket then merge later but I still prefer to get the shot in one hit, if you like.

Anyway, seems the emphasis for this thread is on full coverage NDs rather than grads.

Cheers

Ian

On the issue of speed, I have to agree. Once you have used the slowest ISO, the smallest (appropriate) aperture, and as much of the dynamic range as possible, then apart from using some optical impediment such as a piece of smoky glass, you are nearly buggered.

The ISO setting on the 10D is in reality just a gain setting on the imaging chip. Hopefully Canon will now realise that there is a real value in having access to gain or ISO settings much power than 100.

So PEKKA, is there any way to forward this need to Canon? If it is possible to simply tweak the system firmware, could we all have an upgrade to access 2,3 even four stops additional control via ISO settings of 50, 25 and 12?

Could we petition Canon to produce (for sale) new firmware that upgrades the power of my 10D. Just think Mr. Canon, sell the hardware once and then sell software upgrades every few months just like Mr. Gates does. That's a bit like selling the camera over and over again.

Once you have access to new features you drive your photography to use them when they can give an edge. I rarely used high ISO film, but I now find myself regularly using ISO 1600 for indoor shots where flash is too intrusive. ISO 1600 is now an invaluable part of my toolkit. I am sure that by the same token very low ISO settings would also be of value were they to be made available.


Derek