No doubt photography is all about your vision and bringing that vision to life! Gear is just the collection of tools that help you accomplish your specific vision. So, these are just a few thoughts on the gear front that might be helpful for you in planning your photo trip to Iceland.
First, let's talk about the non photo gear that was critical. Maps were a huge piece of the puzzle for us. They not only help for hiking and driving but also really gave us a much better picture of where we might want to go while we were planning the trip. We used them constantly on the trip and even though we had GPS, the maps were a key difference maker for us. The maps by Ferdakort and Mal og Menning were invaluable. Not cheap but well worth it.
I think another key ingredient was having the right clothing for all the crazy weather that Iceland can throw at you. This is ultimately a safety issue even if you're just day hiking. The weather can change quite rapidly and a calm day can get a little harsh in the blink of an eye. Think merino wool and waterproof breathable outer layers. I am super pleased with the merino wool base layer (t-shirt, socks, and underwear) from Icebreaker. A little on the expensive side but well worth it. The wool base layer will keep you warm and dry and although it does not dry as fast as synthetic fibers it also stays odor free which is great in a two person tent! From First Ascent I used the their rain pants, awesome 800 fill down vest and their wool pullover cap. And from Mountain Hardware, the rain jacket and waterproof gloves were awesome. And all of the above were needed. We had remarkably good weather by Iceland's standards but had the usual heavy winds and rain showers that were off and on. The temperatures seemed to stay between 35F and 52F. The only time I remember peeling layers was when the sun popped out late on an evening hike in Skaftafell. Probably the only time hiking in shorts and a t-shirt was a possibility.
Ok, on to the camera gear! My interests were primarily in the geothermal areas and the beach below Jokulsarlon. We had Landmannalaugur on the wish list but there was just no way to give it the time needed so we had to move on. And we were hoping to do some Puffin photography but that was more of a second thought, not as much of a priority. I shoot the Canon 5D Mark II and carry the 5D as a backup and it's very critical to take a backup when you're spending some serious resources and travel time to remote regions. Wouldn't think of doing the trip without a backup body! In the end I settled on taking just 3 lenses, my favorites: the 16-35mm, the 90mm tilt shift, and the 70-200mm for tighter shots. That's a great lineup for landscape work and perhaps you'd consider a longer lens if you'll be doing more bird photography which is certainly spectacular. The vast majority of the images were shot with the 16-35mm lens. Iceland is one amazing landscape with these wide open spaces that just blow the mind. I like to use the wide angle lens and the 90mm tilt shift to stitch panoramic images as a way of expressing the wide open, rugged landscape.
I think the other items that really helped included a sturdy tripod that can handle windy conditions and the Singh Ray Neutral density filters. Iceland is a place where the bigger the tripod the better. We arrived to a steady 50 mph wind at the sea cliffs at Latrabjarg. Even the Puffins were a little put off by it. We could hardly stand up much less take a photo that had any hope of being reasonably sharp! So definitely bring the sturdiest tripod your gear bag and budget can handle. I use the Lee filter holder to manage the larger 4x6 inch neutral density filters by Singh Ray. The split neutral density filters really help balance the exposure range and the 4-stop neutral density filter adds a nice blurring effect to moving water and clouds. I used the 2 and 3 stop soft step and the 4 stop solid neutral density filter constantly. Wonderful tools!
The support gear that keeps your electronics going is also important. We really got a lot of use from the car inverter and the Hyperjuice battery from Hypermac. This kept our camera batteries, phone, and laptop charged and ready to go at all times. Didn't run out of juice once! And on the low tech side, I don't think you can bring enough microfiber cloths and zip loc bags. I use zip locs to keep the camera dry in between shots. Not as fancy as a high tech rain cover but seems to work ok for me.