I've collected quite a few lenses (and cameras) in the last year, and it's about time to grab more Variable ND filters instead of swapping them and sharing them during the workflow. Variable ND filters or 'Fader Filters' are Neutral Density filters used to block out light for better exposure. You will find ND filters built in to professional video cameras, but DSLR's lack this and it's an important filter to use when shooting video. Especially if you're trying to achieve a more film like look with your camera, this will help control your shutter speed on bright days. Variable ND filters are great because they are adjustable to perform like several different ND filters wrapped up in one, but if you've got time to add and swap you can use standard ND filters too. There's three big names that come to mind when shopping for these filters which are Singh-Ray most expensive, Genus, and LightCraft Workshop. Here's what you should think about when shopping for your Variable ND Fader Filter.
Depending on where you shop, the Genus and LightCraft might be very close in price, but the Singh-Ray is the most expensive by an additional $200+ dollars. Some things to keep in mind when choosing a Variable ND are the quality of the glass used, color temperature change, and vignetting when mounted. These three brands have really good reviews, but I haven't tested them side by side to do a full pixel by pixel comparison.
Quality of Glass: For video use on wide lenses, you may not immediately notice a difference, but when using some type of magnified lens (macro and some zooms) you could find loss of sharpness. Especially if used in high megapixel photography, there will definitely be a difference. This is because you are adding an additional 2 pieces of polarized glass over your lens, and the quality of those 2 pieces will affect final image.
Color Temperature Change: Without getting too technical, as you turn the filter it changes the amount of light that is allowed through. Some of these filters could have slight color changes which means you will have to correct your color balance each time. It could be very marginal and even done in post without most people noticing.
Vignetting: The newer versions of these Variable ND filters are built over sized to prevent vignetting. LightCraft calls this their 'Mark II' and Singh-Ray calls this their 'Thin Mount'. So a 77mm lens will end up having something like an 82mm Filter at the tip. This means you will lose the ability to place your stock lens cap and also use of any stock lens hoods. These filters are great for controlling exposure, but shading the front of the lens with a hood or matte box is just as important to maintain contrast and color. Keep in mind about those extra items you'll still need to shop for. If you use the older versions, you should be fine with the lens cap, but most likely not the hood unless you're using an aftermarket mount like the ones in this article: http://cheesycam.com/hoods-covers-caps-and-pouches/. I would definitely think about going with the newer filters with the larger glass, but just beware if you think you've found a cheaper deal, it might be the older version.
List of Variable ND's at eBay:Variable Neutral Density Filters
List of Variable ND's at BHPhotoVideo: Variable Neutral Density Filters
List of Variable ND's at Amazon: Variable Neutral Density Filters
If you're just experimenting with ND variable filters, there's also a DIY article I posted here where you can make your own. Of course you'll probably suffer some softness, color changes, and vignetting, but for less than $10 dollars it's a great project to jump into. Here's the DIY Link: http://cheesycam.com/diy-fader-nd-variable-nd-filter/
If you're not ready for Variable ND filters, at least grab a set of basic ND filters. Having more than one will allow you to stack them to block more light, or unstack them to allow more light. Just be careful of that vignetting. Click here for a list of Standard Neutral Density Filters.
#####New Product Alert####
Quick Note: There is a new brand of these Variable ND filters showing up online for 'HALF' the price. This is the first time i'm seeing them online, but it looks exactly like one of the brands above. If anyone has tried these, send in some demo videos. I'm going to give a try on one of these to see how it works out. Here's the link: New 'Cheaper' Variable ND Filters
click image for pricing