When I’m photographing, I tend to be standing around and thus generating little body heat. In temperatures below freezing (or even below 45°F with wind), it becomes difficult to stay warm. I also need to pack with minimal bulk and weight, so I can carry stupidly heavy lenses. The solution is very world-class down outerwear.
The Western Mountaineering Flash XR down jacket and Western Mountaineering Flash down pants weigh in total about 18 ounces and pack very compactly. You cannot buy better down quality AFAIK. Western Mountaineering calls it “850 Plus Fill Power Goose Down”. I suspect that it really is 910 fill power, but WM won’t make that claim that outright. Also, claims of fill power tend to be iffy, but WM is the Real Deal.
Fill power is essentially a rating of insulation to weight—typical down garments are 650 fill power (mediocre stuff that I would not consider). Fill power of 700 is good, 800 is uncommon, 850 is rare and excellent, 850+ is almost unobtanium.
Of course I wear two or three base layers. When it gets really cold, I wear a wool jacket over my down jacket, which adds even more dead-air space for insulation. The only issue I have is hands and feet. Feet are not so bad, but hands are a challenge, given the difficulty of operating most cameras with gloves on.
It is critical that outerwear be packable and light so I can remove and stow it while I hike—as shown, this stuff is too hot above about 28°F if/when I am hiking steadily.
It is also critical that if it rains or snows, the down not get wet. While the Flash pants are not suitable for precipitation with something over them, the Flash XR jacket resists rain and snow for quite a long time. If it really comes down hard, I would seek shelter, otherwise the Flash XR jacket is good to go—and a waterproof shell can be used if it is really awfully wet.
Below, Total weight of the jacket and pants shown: 17.5 ounces = 496 grams!