Mike,
Sometimes, it is necessary to just reduce the light in one part of your image without disturbing the other part. For example, you may want to tame the bright sky only while not affecting the exposure of the foreground. So, besides solid neutral density filters, they also have graduated ND filters, which are darker on the top and lighter or clear on the bottom of the filter. They also sell split GND filters that are threaded, however, then you lose the flexibility of where you would like the demarcation area line of the graduated filter to be, since it is fixed to the lens you are unable to move the filter up or down.
This is where the Lee or Cokin Filter Holder system plays a role. These filters are 4X4" or 4X6" and can be adjusted up or down in the filter holder which will adjust the graduation of the exposure within your frame. Also by having a separate filter holder system that attaches to your lens by an adaptor ring and/or step-up ring, you can buy expensive filters and share them between lenses. These filters and filter holders are also wider than most lenses in order to reduce vignetting.
I will also add that another use for ISO 50 or ND filters is for slower shutter "panning shots" in motor sports.
@iND, I'm glad you're happy with your Lee System.
@Jonathan, I'm fairly new to the 4 X 6's, but my observations so far are as follows. I have a Singh-Ray Reverse 3 GND and I compared it to the Lee 3 stop GND hard (sorry, I don't have sample photos). The Sing Ray actually "pretties-up the photo" a bit, which can be a very nice effect when shooting sun-scapes. The Singh-Ray does not appear as neutral as the Lee filter does, the Lee leans more towards the truly neutral grey side.
Just by looking through the filters without any lenses attached and holding them up to the Sky, I can see that the Singh-ray has a "very slight" color cast towards the blue side of grey and the Lee has no color cast at all and appears very neutral. I would imagine that these discrepancies are magnified at longer exposures. It's kind of like trying on different sun glasses.., some are neutral, some are grayish, blueish, or rosy. The Singh-Ray doesn't keep the clouds as white as the Lee, however the sky looks bluer and prettier through the Singh-Ray.
Another example; I was also looking at some cars through both filters by holding them up, and the color of the cars was exactly the same with the Lee Filter, white was white, red was red, blue was blue, whereas the Singh-Ray filter gave the color of the cars a "very slightly" richer color in every instance, very slight more saturation would be a good way to explain it. I would stress the word "slight." They are both top notch filters and the Singh-Ray quality appears to be actually higher to me. I would not hesitate to buy more Singh-Ray filters and I actually plan on doing so.
Last example, I would also say that the white foam of breaking water, looks more white with the Lee filter. The other factor leading to color casts may be the amount of IR let in via the longer exposures of the higher stop filters. I know that the Lee Big Stopper (10 stop) also claims to reduce the amount of IR that enters the image and this appears to be true as well.
Rich
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Originally Posted by neuroanatomist
