I found so much obsession with sharpness on the photographic forums on the Internet that I feel I have to add my two cents to the bunch.
I thought I would start by defining sharpness of a photograph, but that’s actually not as easy as I initially thought, and I will rather list all the factors that detract from image sharpness.
A photograph is not sharp when there is camera movement during exposure, subject movement during exposure relative to the film/sensor, when the desired point of interest is not in focus, when depth of field is insufficient to encompass the subject of the image, when diffraction reduces the optical resolution, when film is not perfectly flat, when the lens doesn’t project image of sufficient resolution, or when film/sensor has resolution insufficient for the desired enlargement.
Those causes can be further summarised into technical errors, equpment defficiencies, and subjective assessments.
So, if you caused camera shake during exposure, that’s a technical error. If you have subject movement during exposure because you failed to set short exposure to compensate for it (because, at whatever speed something is moving, that movement will be twice as short in half the time, until at some point movement falls beneath pixel size of your sensor, or at least beneath the pixel size on the print of the size you are happy with), that’s a technical error. So, if your failure to keep the camera steady or keep exposure short enough or focus on the right place or set the depth of field properly or you stopped the lens down to the point where diffraction degraded image sharpness, is the cause of the perceived lack of sharpness, those are technical errors, or, in other words, user errors, because it’s the user’s technique that is at fault. Removing those errors will remove causes of the perceived lack of sharpness. Those errors make up the vast majority of all issues with sharpness, and the only way to resolve them is by developing a good, meticulous photographic technique – basically, user training.
The second cause of the lack of sharpness are the equipment defficiencies. This means that the equipment itself caused the lack of sharpness even though the photographer made no technical errors. For instance, if a camera sensor introduces strong banding and noise, this will degrade image sharpness; if a lens produces an image whose sharpness falls strongly toward the corners, and you want the image corners sharp, that’s also a problem. If a lens produces strong coma in the corners wide open, and you want to use it for astrophotography, that’s also an equipment defficiency. Anything that makes a lens or a camera unsuitable for the task at hand can be considered an equipment defficiency. This is a very popular subject that is incredibly amplified on the forums, because it makes people think they can solve their photographic problems with money. However, this was always a less important cause of overall issues than people thought, and today it is even more so, to the point of being practically negligible, except in few areas of photography where you can literally spend your way out of the problem, such as photographing insects and birds in flight, where quality of autofocus is critical, and the role of equipment in the overall picture taking ability is remarkably high.
The third cause of the perceived lack of sharpness is subjective assessment. This means that for some people an image is “sharp” if it can be enlarged to the desired print size and the details that are in focus are perceived as sharp enough. For others, who inspect image files, an image is sharp if everything is sharp on the 100% magnification. Also, for some an image is sharp if absolutely everything on the image is so sharp that circle of confusion across the image falls below the pixel size of the output medium, while for others an image is sharp if the point of focus is sharp, and enough of the motive is within the depth of field, and the rest of image is blurred out. For instance, those two images are sharp:
On the first image, the motive is critically sharp and the rest is blurred out, while on the second image everything is sharp. This makes sharpness a function of subjective assessment and photographic genre, and not an absolute category.
So, essentially, with sharpness, a proper answer to “how sharp do you want it” can be either “just the right parts”, or “yes”. If the motive is the whole scene, then the whole scene has to be reproduced in crisp detail. However, if the motive is a person, a detail or atmosphere, the answers will be all over the place. In fact, if a camera cannot reduce the depth of field enough to allow for the subject isolation, the way early digicams with small sensors used to, I will reject it as inadequate, because excessive sharpness, of things that should be blurred out, can very much detract from the quality of the image, and in fact often requires expensive equipment to remedy.
This image, for instance, would be completely ruined if depth of field was increased to the point where everything in the background is sharp, because it’s exactly the reduced depth of field, or selective sharpness (great where you want it and not at all where you don’t want it), that makes the image. On the other hand, with landscapes you often want the whole scene to be sharp:
However, notice how even here, where the image is critically sharp from corner to corner, and from front to back, the water was intentionally blurred out by long exposure, because that adds to the calm atmosphere of the image, where crisp appearance of water movement stopped by short exposure would look nasty and amateurish. This means that sharpness is merely an aspect of photography that needs to be controlled, so that you get it when you want it, and selectively remove it when you don’t, and the problem arises only when you can’t do something due to either equipment deficiencies or flawed photographic technique, or you inspect a photograph in ways that defeat its purpose.
There’s also the matter of lens sharpness and sensor resolution, that causes unending debate. Yes, there are sensors that have insufficient resolution and lenses that have insufficient sharpness. With sensors, it’s merely a function of magnification – how far can you enlarge it on screen and in print, and still retain subjectively crisp detail on the desired viewing distance. This was a big problem when resolutions were 3-5 megapixels, less of a problem at 8-12 megapixels, and ceased to be a problem for almost all purposes at 24 megapixels. With lenses, it’s a more complicated matter, because a very bad lens can create all sorts of optical mischief even at quite modest magnifications. This is mostly the case in low end smartphone cameras today, but historically speaking, lenses that had poor sharpness and produced inadequate resolution and color at any aperture did in fact exist. Old Soviet cameras such as Smena or Lubitel, or toy cameras such as Holga or Diana, and also lenses such as “Lensbaby”, are created or sought for that “artistic effect”. However, with modern lenses it is mostly a problem at wide apertures, where even the point of focus isn’t “crisp”, or the lens doesn’t produce enough resolution, measurable in line pairs per inch/centimeter, to resolve fine detail on desired levels of magnification. This means that a certain lens will be sharp on an A3 sized print, and unsharp at an A1 sized print. Also, a lens might be critically sharp at f/8, and lacking wide open, at whatever its maximal aperture. Depending on what you intend to do with it, this may or may not be a problem. If you are shooting sharp landscapes at f/11, a certain lens will be perfectly adequate. If you want to get crisp detail in a sea of blur, you will often need to purchase a specialty lens that is designed for extreme sharpness and resolution wide open.
So, as I said in the beginning, sharpness is what happens when the user doesn’t mess up, when equipment is up to the task, and the result is evaluated as intended. Fortunately, for most types of photography, 20 year old digital cameras and lenses are capable of producing perfect sharpness on very large enlargements, such as A2/B2 print size, which are as large as a normal wall would bear. Anything even barely modern, with resolutions around 20 megapixels, will enlarge so well, one should not worry about it. Also, any reasonably modern lens, such as the Canon EF 17-40mm f/4L, will project sharp enough image on such a sensor as to allow for meter-wide enlargements. This means you can get sharp images on huge enlargements with inexpensive gear, if you’re shooting landscapes, portraits or details. Wildlife, birds and bugs in flight and sports are a different matter and quite a money pit, so I’ll leave those out for now, but for most of the things people want to shoot, a Canon EF 100mm f/2.8 USM macro, EF 85mm f/1.8, EF 50mm f/1.8 STM, EF 70-200mm f/4L, EF 17-40mm f/4L and EF 24-105mm f/4L will produce images of extreme sharpness and clarity on the modern sensors, if you know what you’re doing, technically. Most people don’t, and then they blame the equipment. The lenses I mentioned are mostly quite inexpensive today, so it’s not a pay-to-win sport. However, any amount of user error will reduce the image quality of a 24 megapixel camera to something far below what a 5 megapixel camera can do, if used with perfect technique. Essentially, cameras and lenses can fuck up, but compared to how much photographers can fuck up, that’s nothing.
(The newest of the cameras used to produce the illustrations in this article is 11 years old. The oldest lens used is a 38 years old design, and it’s also the sharpest lens used.)
