What is the Best Aperture to Use?
I'm sometimes asked what the best aperture is for a given lens. The problem is though that lenses tend to have adjustable apertures for a reason. Not every situation calls for the same settings. The lens itself is but one factor.
After ascertaining what a questioner asking about aperture really means, it turns out most aren't considering anything beyond the lens itself — not what it will be used for. There are a number of publications and websites that publish lens reviews and detailed specifications and clearly some lenses are better than others. In order to get the best lens for their money, it makes good sense to pay attention to such information, but sometimes people can get lost in the statistics.
Most lenses are sharpest at mid-aperture, say, around f/8. When set to either wide open or completely stopped down, they'll naturally be somewhat softer since both ends of the aperture scale push the design parameters the lens was built on. Exactly which aperture is sharpest depends on the design too, and it is this that many people tend to pay attention to.
But the whole reason for having a lens is to take pictures, and if they are of any significance, pictures are of something — they have a subject. Especially when shooting outdoors where not everything about the environment can be controlled, not all subjects under all conditions yield optimal results with the same aperture. Sharp useless pictures are rarely the true goal. What most photographers are after is the best pictures they can manage of the subjects they are trying to shoot.
Sometimes circumstances call for a short shutter speed to get the shot at all. When the wind is blowing for instance it can be maddeningly difficult to get a sharp shot at whatever aperture the lens is truly at its best. Digital cameras make adjusting the ISO easy, but there are limits. It is often necessary to sacrifice depth of field by choosing a wide aperture and focus just on the most important elements in a scene. Shooting at the limits of twilight can also benefit from a reasonably wide aperture to avoid excessively long shutter speeds. To get useful images, these sorts of considerations tend to factor into the choice of aperture more than do published specifications and test results.
Sometimes people think a lens is sharpest when it is stopped all the way down since this will produce greater depth of field than a lens would with a more open aperture setting. Landscape photographers learn this early but sometimes resort to simply stopping the lens down as far as it will go before considering if they really need to. At some point, diffraction will start to work counter to what they are trying to achieve and actually create softer results than they would get by backing off even one stop. The closer you can get to a more moderate aperture and still get the depth of field needed for the subject the better off you will be. A bit of checking with the depth of field preview can be time well spent if sharpness is truly important. If you don't need to stop all the way down, don't.
A good lens is still very important of course. A lens that is razor sharp at f/8 is probably still almost as sharp at most moderate apertures. It will probably also do better at extremely large or small apertures than would a lens that was already somewhat soft at f/8. As such, it still makes sense to buy the best lens you can justify. Just don't let this quest tempt you into losing sight of why you wanted to buy a lens in the first place. Once you have it, pick your aperture based on your subject, not just your lens. The best aperture for one situation is unlikely to be the best for every other. The old adage of "f/8 and be there" (or any other aperture based on published specification) rarely works in nature photography. Good images require careful thought and consideration of all the variables.