DIY Vari-ND Questions

So, I was looking at getting one of these things, or the Vari-N-Duo, and in reading some reviews, and other threads, came across some people saying that you could make your own.

So after reading more on that, it looks like I am half way there already to making my own. I currently have a B+W 77mm Circular Polarizer (Multi-Coated), I haven't looked at it lately, but it looks like from the specifications page, it has threads on the front.

Now, according to the reading I've done, I need a linear polarizer to screw on the front of the circular polarizer. The problem is, there are like 3 different types of these filters, so what is the difference, and which would be the best choice?

1. B+W 77mm Top Polarizer (Linear)
2. B+W 77mm Top Polarizer (Linear) MRC (Multi-Resistant Coating)
3. B+W 77mm Kaeseman Polarizer (Linear)

I realize that the better filter between 1 and 2 would be 2, but what is the difference between 1 and 3?

Does this seem like a good option? I can have the same functionallity as the Vari-ND, for around $100 or less, with a filter I already have.

Also, how would adding a linear polarizer to the front of a circular polarizer affect auto-focus? Would I need to focus manually, or before putting the linear filter on, or would it be just fine with the total combination of filters?

geogecko wrote in post #8153436
So, I was looking at getting one of these things, or the Vari-N-Duo, and in reading some reviews, and other threads, came across some people saying that you could make your own.

So after reading more on that, it looks like I am half way there already to making my own. I currently have a B+W 77mm Circular Polarizer (Multi-Coated), I haven't looked at it lately, but it looks like from the specifications page, it has threads on the front.

Now, according to the reading I've done, I need a linear polarizer to screw on the front of the circular polarizer. The problem is, there are like 3 different types of these filters, so what is the difference, and which would be the best choice?

1. B+W 77mm Top Polarizer (Linear)
2. B+W 77mm Top Polarizer (Linear) MRC (Multi-Resistant Coating)
3. B+W 77mm Kaeseman Polarizer (Linear)

I realize that the better filter between 1 and 2 would be 2, but what is the difference between 1 and 3?

Does this seem like a good option? I can have the same functionallity as the Vari-ND, for around $100 or less, with a filter I already have.

Also, how would adding a linear polarizer to the front of a circular polarizer affect auto-focus? Would I need to focus manually, or before putting the linear filter on, or would it be just fine with the total combination of filters?

The edges of a Kaesemann-style filter are edge-sealed to prevent the ingress of any moisture.

TBH, I haven't heard of anyone using a "DIY" variable ND filter with any degree of success. When you factor in the cost of two top-quality polarizers, for not a whole lot more you can get the real deal and be done with it.

Odds are, you will need to pre-focus, depending upon how much ND strength you plan on dialing in, because you will end up with a black viewfinder. I actually use the Vari-N-Duo filter, and I sometimes pre-focus in live view before adjusting the ND portion. Basically, I determine my shutter speed first for the effect that I'm trying to capture...aperture is determined by the hyperfocal distance needed for the shot. Next, I'll dial in the polarizer, compose, and focus in live view. Lastly, I'll dial in the ND until the exposure meter zeroes-out. At this point, I know that the image is properly exposed, then fire the shutter.