SpikeW wrote:
I had this feeling that I would Like to try more long exposure photos. I understand that use ND filters can help. I think I remember that the variable filters are not much but i see they are made by companies that have been recommended by mu UHH group. IF they are good the price and the ease of using just one filter sounds good. Give me some recommendions.
Variable ND filters are essentially two polarizers, stacked. If you are familiar with the "uneven affect" that can occur with polarizers, it's even worse with Variable ND. A big "X" will appear in many of your images, which is virtually impossible to correct well in post-processing.
If you only want to make still shots with longer exposures, that's pretty simple and one or two standard ND filters of fixed strength will work much better than a Variable ND filter (which almost always cost more, too).
With a fixed strength ND there is no risk of the uneven exposure problems. This type of filter is simply dyed a neutral gray.
For really long exposures, you may need a 9 or 10 stop ND filter. For more moderate length, around 6 stop ND is generally fine. Or for use in lower light conditions (deep in the woods, early or late in the day), you may need a weaker 3 stop ND.
I recommend a 6 stop as the best "all around" choice. Or, also get a 3 stop, then you can stack the two if you need stronger, or have the alternative of a weaker ND if needed. But some landscape shooters prefer one 9 or 10 stop. It's up to you. If you buy top quality, stacking two should be fine unless you're using the filter on an ultrawide lens where that would cause vignetting in the corners.
For still photography one or two ND filters is usually all you need. Variable ND are more likely to be needed for videography, where there is less control over exposure in other ways and finer control with the filter may be needed. Ideally a videographer would also use fixed strength ND too, for best video quality. However that might mean buying and carrying around a stack of filters. A Variable ND is simply more practical for video.
But for still photography one or two fixed strength ND can usually handle all your needs. With still photography, you just need to "shift" your exposure range fairly strongly, and can then fine tune it with other controls (ISO, aperture and shutter speed).
One way that variable ND might be helpful is that they can quickly be set to a minimum while composing and focusing, then turned to the strength you want to make the actual exposure. It can be a problem with a strong filter installed to see to compose or to focus. A lot depends upon the other gear you're using and the light conditions you're dealing with. For example, mirrorless cameras that can preview exposure, brightening up what you see on their electronic viewfinders to help you compose, as well as aiding manual focusing. Some mirrorless camera AF systems are also able to work in considerably lower light conditions than the typical DSLR, too, so AF might be usable with them, while AF fails when there's a strong ND on a DLSR's lens.
There are other possible solutions for this "problem", besides Variable ND: You can temporarily remove the filter from the lens while composing and focusing, only installing it when ready to make the exposure. Or, Live View on the rear LCD of a DSLR (or a mirrorless, for that matter) might assist with composing and, if necessary, manual focusing while the filter is installed. If using a DSLR, check if it can do "exposure simulation" in Live View. Also check if it has manual focus assist features, such as "focus peaking" (which many mirrorless have, but not all DSLRs do... However, in some cases focus peaking can be added, such as with the Magic Lantern app that's available to use with some Canon DSLRs).
Besides allowing for longer exposures that create deliberate motion blur effects in images, another use of ND filters is to allow especially large lens apertures to be used for shallow Depth of Field effects in brighter lighting conditions (I'm getting one to use on an f/1.1 lens I've got, which I have to stop down to a middle aperture in brighter daylight conditions... Without an ND filter, setting it's lowest ISO and using my camera's fastest shutter speed isn't sufficient to allow larger apertures to be used, which is sort of the point of buying a "fast" lens).