Today, when very many cameras and lenses have stabilization systems, one can ask why then would want filters that prolong the exposure. Shots with extremely long exposure times are a very nice effect with clouds, water and the like and can serve the image composition. But as is often the case, there are different ones here.
At the beginning of my photo cells there were already filters with very large extension factors, but because of B&W or Schneider were quite expensive.Hats the situation has changed and there are different. manufacturers at the start, such as Haida, Urth or K&F. Hats you also get a complete set of ND filters for the same money as the single filter back then.
Today, however, the same manufacturers offer variable ND filters that allow a stepless extension of the exposure in a certain range. So which filters should you use?
There is the so-called exposure triangle, which describes the 3 parameters for a given incidence of light to get a correctly exposed image. First there is the ISO setting, which of course should not be unnecessarily high, even if modern cameras can handle more. The second factor to be mentioned here is the aperture, which you also cannot change at will, since this can also change the image effect via the depth of field.
The last factor is the exposure time. Provided that there are no moving objects in the image, you can change the exposure time almost at will, and you will hardly see any difference in a photo.
With moving objects, such as clouds or water, I might want a long exposure time so that the water or clouds are blurred in the later photo. With a landscape I can stop down quite a bit, but not too much, because otherwise the overall sharpness of the image suffers (diffraction blur). As a 2nd parameter from the exposure triangle I can set the ISO low, e.g. to ISO 200 and some cameras have a low setting where sometimes ISO 50 is possible.
The effect then depends on the proper movement of the object and of course the exposure time. But even with ISO 50 and f 11 or even F16 I sometimes don’t get more than 1 second.
If I want to achieve longer exposure times even in sunshine, there is no getting around ND filters. They are available in different strengths and the extension factor is given as a multiplier. In practice, a set with an 8x, 64x and 1000x ND filter has proven itself to me. With it one is equipped for all cases. Whether one decides at this point for the screw filters or a filter system with filter holder and rectangular filters, is then again a completely different problem.
You should pay attention to quality here and not take cheap filters, because they sometimes significantly worsen the optical quality of the overall system. I have a set from Haida that has a good quality. And of course you buy the filters with the largest diameter of your lens park. Step-down rings do the rest.
Now the question arises, if the variable ND filters are not better then and can replace a filter set as recommended by me?
The answer is not so sweeping and simple. In principle, a variable ND filter can certainly replace the extension factors 8x and 64x. A 1000x filter, however, usually not, because on the one hand most of the variable ND filters do not really achieve the 1000x extension and with the extreme position often the X-effect is added, that in the picture a brighter or darker X is to be seen. However, there are also ND filters that can’t be rotated that far at all and don’t get beyond a Max position.
Nevertheless, variable ND filters are of course useful, especially in video. In video, the shutter speed parameter is practically omitted if you set the shutter speed to 2 times the frame rate according to the 180 rule. Here you can only adjust the exposure with such a ND filter, which can be adjusted continuously in a certain range.
ND filters as well as their variable versions are more than useful additions to photographic equipment. I prefer the ND filters with fixed extension, but it happens often enough that I don’t have the others with me and then use the variablene filter. If you trust a quality brand, it shouldn’t make any difference.
Which filters do you use?