Among the myriad of waterfalls in Iceland, Seljalandsfoss is unique in that you can easily walk behind the falls to gain a spectacular view through the water curtain over the adjacent plains.
We visited this site on a (typically) overcast day, and got some good images. The following day was much sunnier, so as we drove back to Reykjavik we made a quick stop, which paid off with this image.
Like the nearby Skogafoss, the challenge for photographers is to create images that reduce the crowds. The best way is to get there early before the tour buses from Reykjavik arrive.
An additional approach is to setup a composition with an angle that minimises crowds, and then pick you moment!.
Another challenge here is the spray from the waterfall. It is intense, so you need to setup your composition, cover the lens and the rip off the cover, shoot, wipe and repeat. Needless to say, plenty of microfibre cloths are essential.
This is one location where a very wide lens helps get the full scene, and exposure blending techniques will be useful to balance the very dark to very bright, especially on a sunny day.