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                This is  an endless issue for photographers.

Most of the time we refer at the sharpness of a lens viewing the images on a computer screen or on print. At that point the lens resolution it's already altered by other factors like camera sensor, post processing or printer and photo paper .The sharpness of a lens needs to be considered not alone but as part of your system due the fact you need more than just the lens to record a image..Your basic formula is 1/RS=(1/RL)+(1/RSEN) where RS is your system resolution, RL it's your lens resolution and RSEN it's your camera sensor resolution. The measure of image sharpness is stated in line pairs per millimeter — abbreviated as lp/mm. In other words the ability of the human eye to discern  the number of high-contrast pairs of lines appearing in the space of a single millimeter. So in Digital world the pixel density on the camera sensor it is an significant factor in distinguish that lp/mm. The photo paper resolution it is somewhere around 100 lp/mm but the human eyes it only can perceive 5 to 10 lp/mm. Other factors to consider when talking about resolution are: aperture used together with depth of the field and optimum aperture of the lens, subject and camera movement and in post process the resolution of the printer and photo paper , magnification used for final print and the amount of sharpness and contrast applied . In the case of a print, too small a size would seem to show the limitations of the photographic paper, too large a size shows the limitations of the camera lens and sensor.  The human eye can distinguish at a viewing distance of 10 inches elements of low contrast spaced 0.2mm apart, equaling a resolution of 5 lp/mm and the further we increase the distance we only sense the lp/mm. On 35mm format we need around 25 lp/mm to have a medium sharp image with lots of detail and we need in theory a 200lp/mm image to get an 8x10 print (this is an equivalent of 8x magnification), and this resolution it's not possible.  When we print the images we have to magnify the physical size of the image captured on the camera sensor and the bigger the print size the less lp/mm we have on photo paper. There is software to upsize the file size but don't expect miracles. With a 1440dpi printer and a 360 dpi file you and up with 16 pixels per mm which translates to 8 lp/mm.  

                 Sharpness is made by resolution and acutance (or contrast) . The resolution on the other hand means how small details a lens can record and acutance is the lowest tone difference a lens can record.  However you can increase contrast after the picture was recorded  in post production but you can't increase the resolution, basically if no detail was recorded when the picture was taken  there is nothing to work with. A high resolution lens but low contrast will produce an image with lots of details but will look flat because there is not enough tone difference and a lens with more acutance will produce images that might look sharper because the detail it's more visible because the increase tone differences. The resolution of the lens it is significantly altered if your photo techniques are not perfected. Depending on the type of photography you are doing handholding a lens , special at wide aperture will create poor quality pictures due the narrow depth of the field and the camera shake. Tripod is crucial for sharpness ,precise focusing and best use of depth of the field also contribute for a successful sharp image.

                     Which one are better, prime or zoom ?  Let's don't start this one. I know of very successful photographers who are using consumer grade lenses yet their images have value due the quality of the moments they capture. A special time or event even captured with a point and shoot camera will be more valuable than a flat subject captured with a high end DSLR. Many portrait, wedding photographers or journalists will use consumer lenses with great success but if you get in landscape or wildlife everybody is pixel peeping your images. So I believe depending on your type of photography you have to make the decision if you need to shed the dow  for that hi-rez lens you want because we don't really need the extra lens , we just want another one. If Canon will take the 24-70mm L lens and just add one more red line on it without changing the optics but increasing the price I bet people will sell their one-red-line 24-70mm L just to "upgrade" to the new one


   What is a sharp lens?