If you are shooting portraits and using a plain photography background there really is no benefit in shooting with the lens wide open because there is no background to blur out and separating the subject from the background won’t create any problems.
When using a photography background you are free to use whatever aperture you want however I would always suggest using apertures of f5.6 – f11, depending on the “sweet spot” of the particular lens you are shooting with. Shooting with the sweet spot aperture will result in the sharpest photo with the best image quality, which in many circumstances is what you want. If the portraits are for a corporate website or something like that the photos need to be as sharp as possible, and shooting around the lens’ sweet spot will achieve this.
There are times when you may want a slightly softer focus, when photographing ladies or children for example, and in these situations I would always open the lens to f2.8 – f4. I would never go wider than f2.8 when shooting portraits against a plain photography background because using really wide apertures it is easy to get parts of the subject that are soft. For example, using wide apertures you may find the eyes are sharp but the ears or the point of the nose soft, and neither of these are acceptable.
If you don’t need to shoot at ultra-wide apertures there is no point in spending the extra money on lenses with wide apertures. Okay, we all know that lenses get sharper when you stop them down, which would mean a f1.4 lens stopped down to f2.8 is sharper than a f2.8 lens used wide open at f2.8, but this is not always the case. Besides, once you start stopping down to f4 – f11 the difference in sharpness, if there is any, is non-existent in most cases.
If you’re using photography backgrounds there really is no need to buy ultra-fast f1.2 – f1.8 lenses when cheaper f2 – f2.8 are plenty fast enough.
Keen photographer addicted to cameras, lenses and everything photography related. Feel free to follow me in my photography ramblings, and if you have any thoughts, comments, queries or anything else to add I would love to hear from you.