Agora engraçado, sei que ninguém gosta do Kenrockwell, mas por pura coincidência, acabei de ler um artigo em sua página sobre definição (sharpness) em uma foto. Vou colocar as partes que achei mais interessantes (inglês)
http://www.kenrockwell.com/tech/lens-sharpness.htm:
Photographic lenses, used properly, have always been sharp, even at the dawn of photography in the 1840s. Optical design is a much older science than photography.
The reasons some photos aren't sharp rarely have anything to do with the lens."Any good modern lens is corrected for maximum definition at the larger stops. Using a small stop only increases depth..." Ansel Adams, June 3, 1937, in a reply to Edward Weston's request for lens suggestions, page 244 of Ansel's autobiography.
Ansel was telling him to stop worrying about lens sharpness, since all the ones he was considering were sharp. This was the 1930s. Today even crappy lenses, including plastic lenses on most disposable cameras, are sharp when used properly.
Use the Right tools:
Within any format (35mm, DSLR, 120, 4x5, etc.), the sharpness of any image is limited mostly by the format. No matter how sharp your 35mm Leica might be, even a crummy medium format camera is much sharper.
If sharpness is your concern, be sure you're shooting the biggest format you can bear. If you shoot digital, shoot Canon full-frame or Nikon FX, (…).
Don't expect that new lenses are going to make any difference with the same camera you've been using.And Again, Who Cares?
All this has been presuming you want a sharp photo.
Being sharp has little to do with being a good photo, unless you're doing forensic work. Many great photos use deliberate unsharpness to express their points, and if you look at sales and auction prices of photos as art you'll see that the fuzzier ones sell for much more.
Sharp photos are boring.
Photos that are sharp all over are usually amateur attempts, which glaringly show too much detail for many unrelated, confusing and distracting elements. A good photo has impact and a punch line. The fewer things a photo tries to say, the more powerfully it says them. Things need to stand out.
Having everything sharp edge-to-edge rarely makes for a strong photo.You don't put details in your corners. It distracts the viewer and weakens your image.Ele fala muita besteira em alguns assuntos, mas até que concordei com ele em vários pontos. Realmente, a foto não precisa ser totalmente definida de fora a fora, mas pra isso vc isola o objeto com um bokeh. Não sou um profissional da área, mas gostaria da opinião dos mais entendidos no assunto e o que o mercado demanda por "aquela" foto.