As a minimalist (read, "simpleton"), there's something that appeals to me about prime lenses. Unfortunately, the other side of being a minimalist (read, "cheapskate") means that I tend to avoid using gear that doesn't offer flexibility. When I randomly treated myself to Canon's budget, plastic 40mm f/2.8 pancake, little did I realize how much joy it would bring me.
My kit is seriously sparse. I spend around six months of the year on the road and I've lived in four (some would argue six) different countries in the last two years. The less I have to carry, the lighter the gear, the happier I feel. I'm not quite on a par with my friend Thomas (when packing a bag, he cuts the handle from his toothbrush), but I'm close. One body, two lenses. And now three.
What I didn't anticipate is how much more I would use my DSLR as a result of attaching this lens. Previously this always had an L-series lens attached and consequently felt like a lump that was trying to twist itself out of my hand. Suddenly, it now feels almost like a compact point-and-shoot camera. It gets thrown into a bag or a coat pocket when I'm out and about, has proven to be great for photographing friends and family, and replaces my phone's camera for chance encounters. Plus, I get all the benefits of shooting a full-frame DSLR at f/2.8: decent dynamic range, immediate control over exposure, and some lovely shallow depth of field.
40mm is a bit of an odd length that initially put me off. I assumed that it wouldn't quite feel wide enough for street photography and not quite narrow enough for portraiture. As it turns out, I've found that this betwixt-and-between length to be quite useful, making it a handy all-rounder. It's far from perfect for either, but still workable, which is what you want when out and about with nothing but a prime lens.
The two biggest impacts for me are a new love of portraiture (something I've not done much of before), and a new means of documenting the people that are close to me. The dramatic difference in size (compared to my EF 24-70mm) affects both me and my subjects. For the subject, they don't have this huge, potentially intimidating lens being thrust in their faces, allowing them to relax a bit more or, to a degree, forget that I'm there. And for me, I feel less conscious of myself as a photographer, allowing me to relax further and either create an atmosphere that's more laid back, or respond more fluidly and naturally to what my subjects are doing. With the bigger lenses, it can feel like I'm switching from being, say, a friend or an uncle to "a photographer," waving this huge, pretentious piece of equipment around, and then trying to switch back again. With the 40mm attached, I could almost as easily be picking up my iPhone, avoiding this shift in mindset, maintaining a level of intimacy, and reducing this sense of a physical and mental barrier between myself and the people in front of me.
Photographers more knowledgeable than me have written extensively about shooting with prime lenses and how it gives you a stripped-down, authentic feel to how you work. "Authentic" is always one of those words that I'm wary of using; it suggests different things to different people, and it's one of these slippery, almost meaningless words that's often used without any real thought. In this context, to me it means that, while I feel less of a separation from my subject, the limitations of shooting with a prime lens also force me to work a bit harder, triggering an extra, often physical degree of creativity in my approach that then leaks over into other aspects of the image-making process. Perhaps it's this extra physical effort and consequent boost in creativity that makes a prime lens feel like a more "authentic" tool.
Whatever it is, this comparatively cheap lump of plastic certainly makes me pick up my camera more than I would, and to shoot things that I otherwise wouldn't.