Some truths are self-evident. For my part I mistrust the ones that are self-serving. Especially if they are large servings.
The photographic fact that a lens with a large maximum aperture will focus faster and more surely than a lens with a smaller aperture is one of these assertions that it is good to mistrust. I undertook to treat it with suspicion in my studio when I compared three lenses of similar focal length as they struggled to focus on an indeterminate subject.
Focusing upon a stark black and white laboratory target is generally easy, and is accomplished quickly by most lenses when there is enough light available. But as photographers we always seem to be going to dim places and viewing fuzzy subjects, and the lenses we buy are apt to either turn out indeterminate images or refuse to focus at all. And when they baulk, the camera won’t fire.
So I gave the three candidates; a zoom lens, a macro lens, and a fast normal lens – a matt-painted model tank as a target. With a diffused light from the modelling lamps of the studio strobes all three lenses functioned well. The fast lens was fast, the macro lens was slow, and the zoom lens was middling…but they all got there in the end.
Turn the modelling lamps off, however, and the macro lens failed to find a sharp focus and the zoom lens failed to focus or fire at all. The fast standard lens carried right on with no interruption.
A surprise? No. But now I have a benchmark level of illumination that I know I must achieve if I am to use the lenses I do have. I no longer have any interest in the proprietary macro lens and I do have an increased appreciation for the rather expensive fast standard. Pretty good result for a holiday afternoon and no monetary outlay.