These might not be the best pictures to illustrate this point but they do. I was on the way down to the central building the other evening to take some pictures of an event. Due to the lighting, the distance between me and the actors, etc. I had to take pictures with fast prime lenses. On the way down I had the 50mm f/1.8 lens mounted on my K-5 and used it to take the first picture above.
On the way back I happened to have the 21mm f/3.2 lens on the K-5 and used it to take the second picture. The 21mm lens was probably the best choice for both pictures but I just used the 50mm for the first picture since that is what was quickly available. It is the nature of those scenes to not last long and I didn’t wish to take the time to change lenses. Would I have been better off to have had a zoom lens mounted and ready? Probably, but since I wasn’t using a zoom for the event I didn’t have one with me. I was traveling light with my camera in hand with the other lens plus extra battery and memory card in vest pockets.
I wasn’t using a zoom lens because my zoom lenses are all slower, bigger, and heavier and I couldn’t use them to get acceptable pictures for the conditions I expected. By acceptable I mean as good as I could get with my 50mm f/1.8 prime lens. I had tested them out and found that it was better for me to use the 50mm lens and crop-zoom to get pictures without digital noise in the poor lighting. The zoom lenses all required a higher ISO which resulted in less detail in the pictures after I removed the noise.
There is another reason that I didn’t switch lenses for the first picture. I try not to change lenses any more than necessary to lessen the chances of getting dirt in the camera. I also find that it isn’t the easiest thing to do, change lenses, when I’m walking. I prefer to do it while sitting next to a flat clean surface to set the lens on.
My world of photography seems to revolve around the issue of what lens should I use. When going out to take particular pictures under a known environment, I usually can pick the right one. If the circumstances are unknown I will usually use a general purpose zoom lens, either the 18 – 135mm or the 55 – 300mm depending upon the likelihood of particular subjects … but, not always. I don’t like the combined weight of a zoom lens on the K-5, so I often just go for a walk with the 21 or 50mm lens on it, or with the NEX-6 with a 35mm lens, which is much lighter, and just photograph what is appropriate for the lens. This means I will be photographing landscapes and if a fox runs across the field, or a colorful bird lands in a tree, it isn’t photographed.
I really prefer a prime lens due to better speed, higher image quality, quicker response time, and lighter weight.