In order to discuss photo sharpness properly, we must first tackle the most common terminologies used today. To define it: sharpness is the quality of details captured in a photograph. Now notice, I said "quality" and not "quantity." I believe there is a definite difference between these two terms. The "quality" of sharpness is more definitive. Regardless of the "quantity," or amount, of objects that are sharp, a high "quality" is most important.
However, the "quantity" of sharpness in an image can vary greatly due to a number of factors. The foremost reason is the subject of the image. Different levels of sharpness alter an image's appearance. Often the scene itself decides how much of an area should be "in focus," another term for sharpness.
Another common word used is to refer to an image as "soft." Whenever a photographer calls an image "soft," he or she is usually talking about a lack of sharpness. Yet a "soft" image isn't always a negative. Given creativity, soft images can be very artistic, almost like paintings, and thus eye pleasing to a viewer. In the image below, the greater portion of the photograph is soft, yet the main object, the center mushroom, is sharp.
So, how to decide what level of sharpness you need? Always start by paying attention to your aperture and shutter speed. Aperture and shutter speed will affect sharpness. A slower shutter speed gives more time for external factors, such as camera shake, to introduce "blur," and a smaller aperture (F16) preserves more detail than a larger one (F2.8).
It is most important to avoid camera shake. This is the natural, unavoidable movement of your hands due to your being human. Decide if the lighting for that scene is bright enough for you to hand hold the camera. Ask yourself, "Would a larger aperture (letting in more light) enable me to get the shot I desire? Or would the larger aperture lose too many details important to the subject?"
In photography, there is always a trade-off because each factor changes the other. Knowing how these changes come about is half the victory to getting the best photo for each scene. Then, your knowing enables you, the photographer, to make better decisions, and better decisions mean better results.
Another factor is the selection of lens. This is obviously more important to the DSLR photographer who can choose his lens. However, even in a fixed lens camera, you decide between wide angle or telephoto before you take the shot. Each camera has its own amount of flaw where sharpness is concerned and different lenses have a higher or lower quality rate (thus the increase in price)! It really pays to know your gear. I repeat this all the time, but READ YOUR MANUAL. Until you are aware of what your equipment will do, you will be hampered in your ability to produce good photographs.
Sharp or Soft?
Is there such a thing as too much detail? Yes, I think there is. A scene can be way too busy and thus harsh on the eyes. But there is too little detail as well. The question remains then, how much do you really need? A good rule of thumb is the wider the angle, the more detail that is important to the scene. However, detail alone is not enough. The details need to be sharp and in focus.
Here is what I do. If I am photographing a landscape, I use a general aperture of F16. As long as the lighting allows it, F16 will give me a good amount of detail. I also look at my shutter speed. The lowest shutter speed I like to hand hold is 1/20 seconds. I have done less, but it is difficult. If I look through my lens and the meter says 1/20 or slower, I widen my aperture to increase my shutter speed. (Never forget you can also increase your ISO. ISO also affects sharpness by introducing noise.)
In macro photography, there is a place for using a smaller aperture. In extreme close-ups, such as the damselflies above, there is little depth of field, so I used a smaller aperture to increase the quantity of sharpness. The lighting was midday. Therefore, I knew the slower shutter speed, which the smaller aperture would give me, would not be harmful. However, in the next damselfly photograph, I opted for a larger aperture because I didn't like the additional background detail in the stones that a smaller aperture gave me.
As I stated earlier, there is always an exchange, and that is where a seasoned photographer is able to view the situation and make correct decisions.
Sharpness from Post-Edit
I cannot speak about sharpness without addressing the question of adding it through post-editing procedures. The truth is, though some correcting can be done, you cannot take a truly blurry photograph and get something sharp out of it. You cannot create what is not there to begin with.
This is why it is so important to pay attention when you take the original photograph. Often I will take a shot, and if I have any doubts about its sharpness, use my camera's preview feature to zoom in tighter on the subject. Many times, the photo can look sharp as a thumbnail, but not actually be sharp at full size. Knowing this, I can then recompose the photo, change any settings that will help achieve more sharpness, and retake the photograph. I did this numerous times with the damselfly photographs.
All of that said, there are certain things you can do to correct minor sharpness problems, but I do have one important rule of thumb - do as little as necessary at all times. Most commonly, I use unsharp mask. If the image is more artistic, as in the mushroom photo, I might use a sharpening brush on only the areas in focus. This avoids altering the pixels where not needed.
In the end, sharpness is an issue that every photographer
must address. The more you understand its role in photography, the greater
quantity of good photographs you will come away with and the less frustration
you will have. Trust me. I have been there, where the entire shoot turned out
wrong due to my mistakes, and that is not a good feeling. However, by taking
time to learn and by paying attention, you can avoid all the headaches and come
away knowing you understood photography enough to make your best photographs.