The real deal about faster lenses
In photography, speed of lenses has little to do with speed. Not directly anyhow. Instead, it refers to the maximum aperture. The theory goes something like this: The more you can open up a lens, the more light hits the sensor. Then you can shoot with a faster SS. So that’s where speed comes in. But fastest possible SS is like a sports car. Most of the time, you don’t need it. It’s just nice to have … in case.
Does that mean a F/1.4 lens is a waste of money, considering it is typically 3 times as expensive as the F/1.8 equivalent? Well, it all depends on what you would like to do.
The biggest advantage of wider aperture is the increased amount of light. More light often gives you a better picture. You are pushing the histogram to the right. More details. Higher signal to noise. Wider aperture also yields a shallower depth of focus, which affords greater artistic versatility.
F/1.4 lenses often come with better design and build quality than the F/1.8 counterparts. It’s akin to auto makers adding more luxury features to sports cars.
A photographer must use wide aperture with caveats. Many lenses are not at their optical best when wide open. It may be softer. There may be more chromatic aberration. More importantly, your margin of error, when it comes to nailing focus, approaches paper thinness as the F-stop number drops.
All to say — Shoot fast, with care.