ND filters or neutral density filters are used to reduce the amount of incoming light. They are designed in such a way, that the intensity of the whole visible light spectrum is equally reduced, to avoid changes of specific wavelength of the light resulting in color cast. Basically there are two different types: graduated and non-graduated filters. In a graduated filter the density changes from top to bottom or top to middle, and in a non-graduated filter the density is constant over the whole filter area. A special type of ND filter is the so called variable neutral density filter. In this filter two polarizing filters are rotated against each other and reducing the amount of incoming light from almost 100% to 0% transmittance.
graduated ND filter (Cokin A121 S, ND8)
ND filter (B+W F-PRO, ND8)
In photography ND filters are quantified by their optical density or equivalent of f-stop reduction. Unfortunately there are different ways for ND filter notation and it can be confusing to compare filters from different manufacturers. There are tables available where the different notations are listed. Below is an example for the different notation systems:
|ND 1## notation||ND #.# notation||ND# notation||f-stop reduction||% transmittance|
|ND 101||ND 0.3||ND2||1||50 %|
|ND 102||ND 0.6||ND4||2||25%|
|ND 103||ND 0.9||ND8||3||12.5 %|
|ND 104||ND 1.2||ND16||4||6.25 %|
|ND 105||ND 1.5||ND32||5||3.13 %|
|ND 106||ND 1.8||ND64||6||1.56 %|
|ND 2.0||ND100||6 2/3||1 %|
|ND 107||ND 2.1||ND128||7||0.78 %|
|ND 108||ND 2.4||ND256||8||0.39 %|
|ND400||8 2/3||0.25 %|
|ND 109||ND 2.7||ND512||9||0.20 %|
|ND 110||ND 3.0||ND1024 or ND1000||10||0.10 %|
|ND 111||ND 3.3||ND2048||11||0.05 %|
|ND 112||ND 3.6||ND4096||12||0.02 %|
|ND 113||ND 3.9||ND8192||13||0.01 %|
But now enough of the introduction and I will write about my experience. Recently I started to play around with ND filters, because I like the effect of motion blur in some situations. The first filter I bought was a ND 0.9 (ND8) filter, but noticed quickly that this one doesn’t reduce the amount of incoming light enough. So I got then a ND 1.8 (ND64) and a ND 3.0 (ND1000) filter. With these three filters and the combination of them, I was able to control the amount of incoming light and the resulting shutter speed.
Very ambitious and naive I started to take my first photos and was very disappointed. What happened? Firstly the images were not really sharp, and secondly they had a very warm tint. In some situations it looked not bad, but for most photos it was just a weird color. Luckily solving these issues was not too difficult.
When using ND filters on my camera, the auto-focus has some problems. Actually the camera focuses quickly and indicates it is sharp, but later the image appears still a tiny bit out of focus. My approach now is, that I focus first without the filter, then set the camera to manual focus mode, screw on the filter and then I take the shot. This works pretty well for me, and now I get very sharp and clear images. Of course it is not the fastest workflow, but I think that you are slow anyway when using a tripod.
The problem of the warm tint in the images seems to come from the white balance setting. Actually I am a bit surprised that the filters create a warm tint, because they are supposed to reduce the amount of light equally for the whole wavelength spectrum of the visible light. Apparently this is not the case and it seems they block short wavelengths more efficiently than long wavelengths. But the warm tint is not too strong and appears only in specific situations. At the moment I adjust the white balance manually in the RAW developer to correct for the warm tint. I also tried to use the custom white balance setting of the camera. But this didn’t worked well and I will try this again later. The custom white balance would be actually perfect when using ND filters.
Another thing which I noticed is, that when using strong ND filters and very long exposure times, there is a tendency for underexposure. This can be easily corrected when shooting or if the underexposure is not too strong, it can be fixed in the RAW developer.
Below are some example images I took at the north coast of Taiwan. There are still many things I will try, so that I can improve my skills for taking long exposure photos.
The image below was taken without ND filter. ISO 100, f/11 and 1/25 sec shutter speed
This is the same image as above, but now with ND filter. ISO 100, f/8 and 17.3 sec shutter speed
You will notice that the colors now are darker and the rocks have a warm color cast. It doesn’t look too bad, the colors appear much more saturated, but in my opinion they don’t reflect the real colors.
And now the image after some adjustments in the RAW developer. Following changes were applied:
Exposure Compensation: +0.5 EV, because the image was a bit too dark
Custom White Balance Setting: 4600 K, I did it just by feeling, until the colors match with the image taken without ND filter
Highlights and shadows slightly increased
Here is another example. First shot without ND filter. ISO 100, f/8 and 1/60 sec shutter speed
And here is the same image after using a ND filter. ISO 100, f/11 and 40 sec shutter speed
The water looks now pretty cool, but the image has a quite noticeable yellowish color cast.
After some adjustments in the RAW developer the image looks now like this. Following changes were applied:
Custom White Balance using a grey point, which is the white bucket next to the fishermen.
Highlights and Shadows slightly increased
And here are two more examples from the north coast. The first image without ND filter and the second one with ND filter and after post-processing.
Now it’s 2017 and since publishing this post, I haven’t done much with long exposure using ND filters. After reading my post again, I should go back and do more. The north coast and east coast of Taiwan offers so many good spots, that it’s worth to continue with it. But as always: So many ideas, so less time.