I had been looking forward to last Sunday, with the idea of finally being able to do a nice trekking in the mountains of our beautiful Val Grande, when Giove Pluvio put his hand in it, ruining my long-awaited program! Not being able to give up taking some photos, I still took a short walk to photograph the waterfalls of Rio Vassola, just outside the town of Chialamberto.
How to photograph the waterfalls to obtain the classic 'silk effect ' of the water?
It is quite evident that, to get this type of result, you have to use long exposures. The shutter speed will have to last several seconds, sometimes even minutes, to give the desired silky effect to the flow of water.
What do we need to take this type of photo?
First of all a good tripod because, being shots of many seconds, no stabilization of the camera and/or lens will ever replace a tripod. In addition to this we will need a remote control for remote shooting, those with cable cost a few euros and give the possibility to exceed, using the Bulb pose, the limit of 30 seconds of shooting that reflex and mirrorless cameras have.
Practically you hold down the remote control button for remote shooting for the seconds you need, when you release the button the shot will be completed.
Normally for this type of photo is used a sensitivity of 100 ISO and a aperture between f/11 and f/16, depending on whether cameras with aps-c or full frame sensor are used.
It would seem everything were it not that such prolonged shutter speeds would certainly lead, except in particularly poor light conditions, to the final result of very overexposed images!
To solve this problem, it is necessary to use ND (neutral density) filters. These types of filters, characterized by dark lenses, serve to reduce the incoming light, which is what we need to achieve the silky effect.
There are different gradations for ND filters. It starts from the ND8 (3 stops) to get up to the ND32000 (15 stops), as you go up with the stops the filtered light decreases and the shutter speed become more slow.
Personally I use a circular 6-stop ND64 filter with integrated polarizer , which I also used for the photo of the waterfalls at the top of this post, and a ND1000 filter (10 stops) useful for the middle of the day when there is a lot of light.
How do you use these filters?
It is necessary to expose the image first, detect the correct shutter speed and only then mount our ND filter. At this point, just use one of the many free apps that are in circulation to calculate the shutter speed to use.
I use the NISI app (see photo on the side) but honestly I think that more or less all of them are fine.
The long exposure technique with ND filters applies not only to waterfalls but also to sea waves, rivers and clouds.
You just have to indulge yourself with tests and experiments!