To write about the piece of equipment that has made the biggest impact on my photography I have to choose my 50mm lens. I found it in a second hand camera shop and paid the princely sum of £15 for it. It's a Pentax-A 1.7 50mm lens. At the time I knew not the potential of having a 50mm lens as I'd just started photography after a 5 year hiatus post university. I'd had no confidence in my own abilities and so had been modelling or teaching art, generally drifting around and not concentrating on my own artistic endeavours. I purchased my Pentax K-r in January 2011 whilst managing a small camera shop in the Cotswolds and a few months later started taking photos of friends and models. After a difficult break up and losing my job I took the risk of becoming a freelance photographer and retoucher. All my equipment is scavenged, secondhand or gifted as it's not always a profitable path to take but I have perfect faith and conviction in the joy photography brings me.
I currently use a white K-r and it has become my trademark. Along with my 50mm which only leaves my camera when I'm doing beauty shoots in studio (for that I use a second hand 30-85mm at 85mm which I got in a charity shop for £10). The 50mm is a pancake style lens and doesn't take up much space in my camera bag.
The major differences this lens has made with my work is the fact I can shoot using a wide aperture to get perfect portraits. My ideal portrait has a background and/or foreground that is out of focus (often with a bit of bokeh going on) so that the subject is framed beautifully and thus has a strong impact on the viewer. Also because it's manual focus I have greater control over the image. For example I always prefer the eyes to be in focus. For fashion portraits I can shoot through leaves and petals to create a lovely flare which can give a photo just that little bit extra and means I don't have to use tricks in photoshop to make a photo have that dreamy quality (click an image to enlarge and browse).
Essentially the 50mm has resulted in me taking pictures that have depth and texture, which ultimately has led to my trademark style. It means my work has evolved from just snapshots into stand alone portraits with substance and clarity. It has allowed me to experiment with a wider range of techniques and contributed to my photographic knowledge and expertise. Even if I don't win anything from this post at least I can say from the bottom of my heart: Thanks, Pentax, for making your cameras compatible with the older lenses and thus letting someone like me afford such an important and relevant piece of equipment.
Editorial note: see also this member's Pro Pentaxian Profile