Getting the Sharpest Images
Achieving sharp images can be harder than it seems like it ought to be. And after buying the best camera and lenses you can, you probably want the sharpest images your gear is capable of.
One form the pursuit for sharp images takes is the optimization of shooting aperture. Since friends know I'm a photographer, I'm sometimes asked what aperture is the sharpest. The context of the question sometimes regards a specific lens or even lens/camera combination, but other times it gets asked in a more open-ended way, without regard to any particular equipment or circumstance. Everyone wants sharp focus. They just need to learn how to achieve it. So they ask.
Lacking any specifics though, it's a tough question to reply to. There really is no answer. It's been said that the secret to good photography is "f/8 and be there," but this expression really has a lot more to do with the "being there" aspect than it does to the f/8 bit. For those unfamiliar, the expression recommends paying attention to what's happening in front of you as opposed to obsessing over technical details. It recommends a modest aperture as being good enough. It does not recommend the best aperture or the sharpest one. The truth is, there really is no "sweet spot" aperture that is universally applicable for all lenses. For a question about aperture to be meaningful here, we need to frame the question in the context of a particular lens setup. For zoom lenses, the "sweet spot" varies based on focal length too.
All this seems to leave out the question of depth of field, too. If an image only needs to be sharp at one specific distance, then the primary function left for aperture to control the optimization of the particular lens optics to focus sharply. I don't know about you though, but most of the images I take need to have at least a reasonable degree of sharpness over a range of distances. Evocative, impressionistic images where just a blade of grass is in focus are only possible at wide-open apertures, but other than that, I needs my depth of field, man. We landscape photographers are addicted to depth of field. So, in addition to all the other factors, it seems only reasonable to consider subject matter when assessing the sharpest aperture. Context does matter.
Then there are those who advocate stopping down f/32, or whatever the smallest aperture may be on a given lens. If objects throughout the distances within the available depth of field are, by definition, perceived as acceptably sharp, they argue that the sharpest images can be produced by selecting the greatest depth of field. And the way to achieve that is to stop the lens down. Basic "Photography 101," at least at first glance. But especially on cropped (DX or APSC) sensor cameras, such small apertures will actually tend to yield softer images through diffraction. Inevitably, squeezing a sharp image through that small of a hole ends up conflicting with the physics of light propagation as a wave. Stopping down further than necessary for the task at hand isn't a good idea. And if the wind blows at all where you take pictures, keep in mind that for every stop you can gain back by not closing down the aperture so far, you will be repaid with the privilege of shooting at a faster shutter speed.
Its also worth considering camera movement. It can be quite difficult to get sharp results when hand holding unless you restrict your shutter speeds negate the possibility of movement during the exposure. The old rule of one divided by the shutter speed for hand-holding may not be sufficient for close examination of today's high-resolution images, but the basic concept will always be valid. If the camera moves during the exposure, the results will probably be soft. For any serious outdoor shooting at slower shutter speeds, you have to have a solid tripod support. Sharpness isn't possible without stability.
Next, let's turn our focus to the third major aspect of the quest for sharpness. And if you haven't guessed already, I'm talking about focusing.
Lately, I've noticed that buzzwords like "back focus" and "focus calibration" have become increasingly common. Most modern cameras use some sort of phase detection mechanism for determining focus. Nobody but you will ever know exactly what you want to focus on, but camera auto focus does its best to use the information it does have regarding selected focus points and other factors to do what it can. And with users becoming increasingly demanding in what the expect from modern technology, they get disappointed when they feel camera focus has fallen short (or long) of meeting their expectations.
Sports, wildlife and other subjects have greater demands for rapid focus that does the typical landscape subject, but I think all of us have felt letdown when we find that an image we thought we had nailed turns out to be a tad soft. Another victim of focus problems.
While we wait for Nikon and Canon (and yes, Sony and others) to develop even better focusing technology, you can take matters into your own hands now. Thankfully, the advances of the digital era have given us both the ability to review images immediately after taking them as well as the ability to use Live View for even more immediate feedback. After shooting a potentially tricky image, I often will zoom in on the camera back LCD to check the results. Sometimes it may be exposure I feel a need to verify, other times it may be concern over whether those people in the distance might have wandered into the frame during a long exposure. But it may be focus that I'm unsure of. The old school solution of relying on the depth of field preview button can be difficult, and formulas and distances can be tedious to work out mathematically. Sometimes, there's just no substitute for checking, and then double checking to be sure. If I don't like what I find using the LCD image display, I can hopefully make the necessary adjustments and shoot again.
The best way to get sharp images is to check if your images are sharp. There really is no way to fully know ahead of time. You do your best to be accurate in your focus and aperture selection, you have a good, solid tripod and you pay attention to details, but if you want the sharpest images, you need to take matters into your own hands and look.