I use the KOOD -9 stop ND, it's multi coated which helps cut down on flare, but keeping it clean is not easy, it smears!
Bought in England for £60, it came mounted in a kind of toothed wheel design for a Cokin square filter holder, I didn't like this method and found it mounted nicely in a 67mm filter ring.
Colour bias is there, but seems to be variable depending on the scene.
Quote:Can you please clarify?
that would be correct if you will shoot one scene and not set a profile for the filter. you shoot the grey card with the filter mounted set balance then shoot, however you can do better and profile your filter.
in fact every camera needs calibration itself and if you are a color accuracy fanatic every camera/lens/filter combination needs calibration , You shoot a color target with the filter and set the color shifts using software you save the profile , this way when using the filter you set the new camera profile and everything is automatically corrected
here's the hardware and software to do it, it's not expensive but time consuming
http://xritephoto.com/ph_product_overview.aspx?ID=1257
Quote:I confirm, they DO have a strong cast, even the high quality ones. I'm using them too rarely, and this is a fault of mine, but I've used them enough to tell you that they need some work to get a good colour picture (as you said, B&W is easier).
First I tried a medium quality stopper, and was appalled at seeing the strong colour cast - so I thought it made sense to spend more money, and bought a Lee Big Stopper (the smaller, for mirrorless, but same glass). It still has a strong cast, as you'll see by the images I'm going to attach (curiously, I did something with the Big Stopper just the past Saturday).
With Lightroom, it was a big problem: while most of the cast can be corrected with white balance, it's not enough. Lightroom allows colour channel correction, but it's fixed. With Capture One... it's much easier 
because you can pick the colours that need correction and selectively fix their hue.
Another approach would be to shoot a reference with the ColorChecker Passport and create a specific profile for the shot. This works, but it requires more work, the most annoying stuff being that you need a clamp to keep the color target in a fixed position.
So, my current approach is: shoot without the filter, to get a visual reference for what the final rendering should be; first apply strong white balance correction to the photo with the filter, and then selectively adjust the remaining colours that aren't good yet. For my limited experience, this is the fastest way.
I'm going to attach: a colour reference (not exactly from the same perspective, but it works), the post-processed final photo, and the unprocessed photo straight from the camera.
Sorry this is exactly what I was trying to say when I replied
Quote:Do you think this ND would do what they are claiming
Honest answer? No. Unless the filter convinces me of the opposite, which I consider a possibility. I was hoping some of us already have experience with this brand, it's unknown to me.
Unknown brand + a mouthful of promises = waking up my sceptical parts ^_^
Quote:that would be correct if you will shoot one scene and not set a profile for the filter. you shoot the grey card with the filter mounted set balance then shoot, however you can do better and profile your filter.
in fact every camera needs calibration itself and if you are a color accuracy fanatic every camera/lens/filter combination needs calibration , You shoot a color target with the filter and set the color shifts using software you save the profile , this way when using the filter you set the new camera profile and everything is automatically corrected
here's the hardware and software to do it, it's not expensive but time consuming
http://xritephoto.com/ph_product_overview.aspx?ID=1257
And you have to do this with cloudy weather, suinny weather, indoor, fluorescent. Oh, and repeatedly every couple of weeks. And for every lens. And temperature... in theory and for a given studio situation tis kind of calibration can save time inpost production. Walking around and getting all light and weather conditions during a day, shooting a grey card or the colorchecker is good enough to correct color drift.
Quote:Can you please clarify?
The cast from an (ND) filter is nothing different as a light source which is not "white" to our vision. So, just like with setting custom WB because of coloured light, just set custom WB because of the filter? Of course, that does require you to use a RAW converter which can handle the custom WB setting.
Here is what the Hoya HMC ND400 (9 stops) filter does, colourcast wise (top without filter (1/200th sec), bottom with filter (2 sec)):
[ATTACHMENT NOT FOUND]
Quote:just set custom WB because of the filter
I'm doing already in the post-processor. This actually fixes a lot - but with the Big Stopper it is not enough, some minor per-colour fix seems to be needed.