Mako:

6 uncoated filters!!!  You did get that “vintage look”.

We, and others, put a top layer of Silicon Dioxide (SiO2) on our multi-layer AR coatings.

It’s as hard as quartz. You can scratch it, but you would have scratched the glass as well.

The rental houses have always told me that they’d buy what was demanded by customers.

So I think it’s up to those of you that rent filters . . . to ask for coated glass.

Dwight

Dwight Lindsey

President

Lindsey Optics, LLC

+1 661-522-7101 X102 (Office)

+1 818-634-1503 (Mobile)

www.lindseyoptics.com

I have the hydrophilic filter from Tokina an PV size.

It’s brilliant!
I posted about it when I got it, I had intended to post test results but wet, we know what that looks like, and clear, how do you know that I’ve even got the filter on?

They need to be exposed to UV light after 8-10 hours of use.

Cheers

Geoff Boyle

DP

Netherlands

From: cml-glass@... [mailto:cml-glass@...] On Behalf Of Mako Koiwai

We DO also have those filters now that have hydrophilic coatings … that shed water. I thought there were some now available in Motion Picture sizes?

A well manufactured filter will be very flat. We just measured one of our filters at 1/10 of wavelength of light (RMS) and most of that deviation from flatness is where the filter rolls at the very edge.  The clear aperture is even flatter.

Uncoated glass will add stray light.

A filter that’s not flat will cause a loss of focus.  An irregular surface will affect sharpness differently in different areas of the image.  If the filter has power, a slightly spherical shape, then it’ll shift focus slightly.

4mm of very flat glass with a good AR coating shouldn’t have any measurable effect on image quality.

Dwight

In my experience, anti-reflection (AR) coated 4mm-thick PV size filters have been provided for evaluation and each time rental houses were not interested. That was a while ago, and there may today be a different level of acceptance. And rental houses, although obviously an important market segment, are not the whole market. Let’s consider some key issues:

1. Cost—clearly, there is a cost to AR (and other) coating. A case can be made that coated filters can sell at a similar price as competitive uncoated filters if you charge less for the filter part. As the buyer, you should then further compare the specs for the filter itself, including optical surface and transmitted wavefront, as well as spectral transmission for light-attenuating filters.
2. Durability—more care WILL be taken with an expensive lens than a filter. Some filters, notably clear “protective” ones, are deliberately considered “expendable.” For lenses, housings can look well-used yet still work to spec and individual optical and mechanical components can be replaced as needed, so that overall functionality can be maintained without a complete replacement. For filters, a single scratch is all it takes to render a filter unrentable. And an otherwise acceptable unscratched filter with a blemished coating will (usually) not be accepted by a renter.
3. Refurbishing—scratched filters can be refinished. But that also removes what is left of any original coating. It should be noted that primarily for economies of scale the refinishing of filters by rental houses appears to be important only in the US.
4. Benefits of AR coating, part 1—coatings are typically of greater value to overall transmission when the optical element is clear which is why such coatings are usually offered for clear protective filters and for close-up lenses. For light-attenuating filters that already absorb light by their function, such as ND filters, the small percentage of light lost to reflection is usually figured into their design.
5. Benefits of AR coating, part 2—a real advantage to coatings on filters, including light-attenuating, is in the reduction of internal reflections and in minimizing resultant flare. Since this is usually when combining bright, especially “point” light sources in the scene with multiple filter stacking, it relates to a modest percentage of overall situations. But use two or more filters at night with streetlamps or car headlights and you will see a clear benefit to AR coatings. An alternative solution for internal reflections when stacking uncoated filters is to use a matte box with a tilting filter stage.

Things do change, and I am all for keeping up with what works best today. Dwight is taking a new, no less valid approach to his competition, combining certain price, coating and transmission features that offer a viable option to what is also out there. The best part is that it’s your choice and you have more to choose from…

Ira Tiffen

PA, USA

Ira is undoubtedly the foremost expert regarding motion picture filters.

Because of his excellent and detailed message, I’ve quoted the entire post below, for easy reference.

1. Cost:  There is obviously a cost for AR Coating, but that does not need to translate into a higher purchase price for the buyer.
2. Durability:  A good coating is durable.  Just clean it with reasonable care and put it back in the padded pouch when not in use.
3. Refurbishing:  Not a great idea.  While I can’t speak for all filter manufacturers, I know for sure that Tiffen, Schneider and Lindsey Optics filters are manufactured using double sided lappers for grinding and polishing.  We all own Laser Interferometers to measure the resulting quality of the filter.
   When a filter is re-polished to remove scratches, it is usually done on a single sided continuous polisher.  In my opinion it’s unlikely that the original filter quality will be maintained.  The filter may well be wedged and may spring to create a slightly spherical surface. After resurfacing, a filter should be checked for flatness and surface quality. 
4. Clear filters and diopters can easily be coated because they are made from solid glass and therefore can be coated with a “hot coating”.  It’s easier to get a good hard coating when the vacuum chamber is hot.  Many ND filters are laminates, requiring a “cold coating”, which is a bit more difficult to do well. ND filters can be produced using solid glass.  It’s a bit more difficult, however the color won’t ever fade and they can be coated with a hot coating. Lindsey Optics IR ND filters are solid glass and of course have an AR coating.
5. In my opinion, the single largest benefit of AR coating, on any glass, lens or filter, is reduction of reflections and stray light (flare). One can use a tilting filter stage to minimize visible reflections of specular sources in the scene, however that doesn’t address the reduction in contrast and color saturation caused by all that stray light bouncing around in the optical system.  And why spend time adjusting out reflections when using coated filters would solve that problem more simply?

It really is quite simple. As light passes through the first polarizer it becomes polarized. By rotating the filter axis closer to perpendicular to the axis of the second polarizer the second polarizer blocks more and more light.

The variable ND is not perfect and may exhibit some odd x shaped vignetting or some yellow/ blue color shifts, when it is nearly closed, but it can be a very effective tool in many situations.

Mike Fecik

Tiffen Co.

Mick, thanks for your kind words…

Yes, variable ND’s are typically two polarizers. When sold as a single unit, as in a self-rotating ring, they use two circular polarizers. Both have the quarter-wave retarder layers facing outward: one facing the camera lens, the other facing the world, with the two linear polarizer layers on the inside with nothing between them. As we know with polarizers, the two linear pol layers will attenuate light transmission according to whether the polarization axes are in parallel (for max transmission) or are perpendicular (for minimum transmission) to each other. If you put a retarder layer between them, you would lose that effect.

The purpose of using two circular pols is to have one retarder layer facing the camera, which performs the function it usually does when using a circular pol, to eliminate potential interference downstream in the optical path should there be any polarization going on in the system. The outer-facing retarder layer is there to effectively un-polarize the light coming in from the scene. Without that, rotating the outermost polarizer would function as a regular pol does, and would have its usual effects on blue skies and surface reflections. That would not be helpful for a dedicated “variable ND.”

Sometimes, using a linear pol facing the world with the circular one mounted as usual on the lens is desirable when you DO want to BOTH control polarization effects as well as include a variable ND. However, you need to be able to rotate the two filters independently: rotating the outer linear pol until it creates the effect you want in the image, and then holding that in position while further rotating the inner circular pol to adjust the light transmission. It can be tricky, but it is possible.

Another consideration is that the polarization film most pols use turn a dark blue tint when you take two at cross-axes (minimum transmission). Since they are usually used by themselves where this blue tint virtually never “shows up” that’s just fine for regular pols. However, to get the maximum transmission range from using two pols in a variable ND, it helps to use a polarization film that minimizes or eliminates the blue tint as it approaches minimum transmission. There are many wonderful variable ND filters out there, each with its own transmission range. The lowest usable transmission is usually defined by the point at which the polarization pair will begin to exhibit the blue tint. The filter assemblies (round threaded ring types) are usually designed to stop the rotation just before that occurs. Typically, there is a range of perhaps five or six stops. I will say that in my experience, the Schneider True-Match Vari-ND® has the ability to reach the darkest minimum transmission without turning blue. It can vary transmission by about 10 stops.

One more concern is that when you approach the actual minimum transmission on any polarizer-based variable ND, in other words when not bound by a mount that restricts rotation, but when you actually approach having the two pols perpendicular to each other, you will see what is referred to as the “Maltese Cross” effect. If you see that, just dial back to the transmission is less dark and the cross goes away.

Hope this helps.

Ira Tiffen

PA,  USA