Stacking drop-in filters?

[fotofool]

Lately I've been shooting motorsports with my 200 f/2. I've found that, to get the look I want, I need to use a polarizer (to manage glare) and a slow shutter speed (for motion blur of the background and spinning wheels).

So, what I want is both a CPL and an ND filter.

The problem is, there doesn't appear to be room in the slot for drop in filters to stack filters. And the Canon drop in CPL doesn't have threads to hold another filter anyway.

Does anyone know of a solution to achieve both polarizing filtering and additional ND filtering in a Canon super-tele?

Thanks,
Jeff

 

[Jim Saunders]

The two logical options are putting ND film on the front element or the back element; I don't know about the reality of accomplishing that though. A CPL holder with provision for an ND internally seems like the elegant fix, but I have a feeling doing that would make the lens look like a bargain. :-/

Jim

 

[Loswr]

I wonder if there's a way to affix a cut gelatin ND of the desired strength to the DI CPL?

 

[chromophore]

Even if you could physically manage to stack filters in the drop-in slot, it is a bad idea optically for the same reason that it is not recommended to shoot with the slot completely empty--the lens is *designed* to have a flat piece of glass with a specific thickness at that position. If it's not there, you will notice a slight loss of imaging performance. It will still focus, and the center will remain sharp, but the image periphery would not be as sharp as it would with the glass there.

If you insert additional glass, this again will have an impact on the image quality.

Why do front filters not have this problem? They don't because the light passing through it hasn't been focused yet.

 

[florian]

I have the 300 II IS and the 600 II IS and I got myself for both the screw in filter holder to get rid of the gelatin filter holder that does have a smal piece of glass. And I see a little improvment in the pictures. I have the CPL to and only put it in when I need it because of the picture quality. ND I only usefor filming with my 1D-C when I need them.
And for Fotofool : I would try to work with an ND gelatin and see how it works.
might be interesting to know if it does impact the picture if you let the light pass the ND before or after the CPL.

 

[privatebydesign]

Even if you could physically manage to stack filters in the drop-in slot, it is a bad idea optically for the same reason that it is not recommended to shoot with the slot completely empty--the lens is *designed* to have a flat piece of glass with a specific thickness at that position. If it's not there, you will notice a slight loss of imaging performance. It will still focus, and the center will remain sharp, but the image periphery would not be as sharp as it would with the glass there.

If you insert additional glass, this again will have an impact on the image quality.

Why do front filters not have this problem? They don't because the light passing through it hasn't been focused yet.

And what about lenses with rear gel holders like the 17-40, 16-35 MkI and MkII etc? Marks on the rear element are much more image impact inducing than marks on the front element.

 

[fotofool]

I wonder if there's a way to affix a cut gelatin ND of the desired strength to the DI CPL?

I guess this sounds like the least bad option. Thanks for your thoughts everyone.

Interestingly, I found Tiffen makes "combo" polarizer+ND filters in several steps of ND. But they are linear polarizers and only come in 138mm! :-\

 

[Loswr]

Interestingly, I found Tiffen makes "combo" polarizer+ND filters in several steps of ND. But they are linear polarizers and only come in 138mm! :-\

Even if they made it in 52mm, there would be no way to rotate the CPL using the drop-in holder...

 

[fotofool]

Interestingly, I found Tiffen makes "combo" polarizer+ND filters in several steps of ND. But they are linear polarizers and only come in 138mm! :-\

Even if they made it in 52mm, there would be no way to rotate the CPL using the drop-in holder...

True... Which would be annoying but not a show stopper for my purpose.

 

[chromophore]

And what about lenses with rear gel holders like the 17-40, 16-35 MkI and MkII etc? Marks on the rear element are much more image impact inducing than marks on the front element.

I'm not speaking about image degradation caused by marks or debris on internal surfaces. I'm talking about the loss of imaging resolution due to the fact that an optically flat piece of glass will alter the path of a light ray due to refraction at two air-glass interfaces that are separated by a non-negligible thickness. If a lens is designed to take this into account--as all lenses that are made with a drop-in filter slot are so designed--then the removal or modification of that piece of glass can have consequences on image quality. The impact on the central portion of the image is minimal, but for oblique rays hitting the periphery of the image circle, especially when the lens is shot wide open, the impact is more evident.

Any piece of glass of nontrivial thickness that you put between the lens and the sensor will cause some kind of refraction that is proportional to the thickness of that glass. A gel filter is about one order of magnitude thinner than a glass filter, and thus the refractive error is also similarly smaller.