“Well,” I answered, “That depends on what you are shooting. But I would get a FAST lens.”
“What do you mean by FAST? Isn't the speed determined by the camera?”
“Yes. But a FAST lens lets more light in so that you can take the same picture at a faster shutter speed.”
“Why is that good?” he asked.
“That means that you can take pictures when there is less light, like indoors or at night,” I answered.
“Oh, that's okay,” he said, I have a flash!”
I had a hard time explaining why I would pay more for a lens just so that I wouldn't have to use a flash.
Consider, for example, the 2 lenses in the first picture.
The one on the left will zoom from 18mm to 200mm. That means that you can go from fairly close up, to fairly wide without moving your feet.
The lens on the right does not zoom at all. It is fixed at 85mm. You would have to move a lot to get the same range as the one on the left (and sometimes you can't move, like in a small room).
The lens on the left has a variable aperture that goes from 3.5 to 5.6.
The lens on the right has only one aperture; it is 1.4.
The lens on the left has VR (vibration reduction) which compensates for any shaking of the camera when you take the picture. The one on the right does not.
So why would anyone buy the lens on the right? Price?
No. The one on the right costs almost double the lens on the left.
The 1.4 aperture means that the lens on the right has a larger opening. It lets in more light, it is a faster lens. Don't ask why the smaller number aperture means a bigger opening.
So, why do I think that faster is better? or flash bad? Rather than trying to explain the technical differences or the aesthetic, I decided to take comparison images. I went around with 2 cameras and the 2 lenses pictured above. I shot a picture with each lens from approximately the same place at close to the same time. The image on the left was shot with the slow lens. The image on the right was shot with the fast lens. I let the camera decide when to use a flash.
As a more practical example, I took the final image at a party recently. It was getting dark outside and we were indoors without much additional light. Worse, the main light was coming from behind her, so her face was not well lit. I could have added a flash, but that would have eliminated any of the effect or ambience of the existing light. Since I had originally been attracted to the scene because of the way it looked to me, it didn't make sense to change the way it looked with a flash.
People have complimented me saying that my pictures look like they have so much more depth than theirs. Unfortunately, now you know that it is not because I am such a great photographer, but because I use a faster lens.