A photograph's detail or lack of resolution and detail is an integral part of its appeal. Many photographers spend a great deal of time, energy and money acquiring equipment to make sharp images, but what is a sharp image?
The ability of the eye to resolve detail is known as "visual acuity." The normal human eye can distinguish patterns of alternating black and white lines with a feature size as small as one minute of an arc (1/60 degree or π/(60*180) = 0.000291 radians). That, incidentally, is the definition of 20-20 vision. A few exceptional eyes may be able to distinguish features half this size. But for most of us, a pattern of higher spatial frequency will appear nearly pure gray. Low contrast patterns at the maximum spatial frequency will also appear grey.
Why you can't trust lens tests
Lots of magazines and a few newsletters publish the results of lens tests. Some individuals post results of lens tests or photography websites and forums. The big question is just how good are these tests and how much can you depend on the numbers they give?
First let's look at what you would need to do a really good, scientifically accurate, lens test even before doing the testing.
You would need selection of randomly purchased lenses of the same type not just one.
You would need to obtain several lenses in order to see what the average quality was and make sure that there isn't a large lens to lens variation. The lenses should be from different batches to obtain as random a sample as possible.
I don't think anyone does this this. It would be expensive and greatly increase the amount of time and work you'd have to do when testing a lens. Popular Photography tests just one lens - they even list the serial number in the test. Statistically, unless you are sure that lens to lens variation is small (and how do you know this if you only test one lens), the results of testing a single sample are unreliable as a predictor of what you might expect if you bought a similar lens.
My eyesight is not the same as yours. Unless you have some idea of what the standards of the person making the comment are, claims that the optical performance any given lens is "good", "bad", "fair" etc. don't carry much weight.
For this reason I publish images with the lenses not my lens tests on this site, you can then make your own decisions with your own eyes. I do carry out lens tests for my own comparative reasons, but do not publish them as your vintage lens could be very different to mine and I do not want to mislead anyone.
Next what tests to carry out?
Back in my film days, lens and film detailing power was measured in lines (or line pairs) per millimetre (lp/mm)— easy to understand, but poorly standardised. It was obtained by photographing a chart and looking for the highest resolution pattern where detail was visible. This is still the way most internet sites review lenses. But because perception and judgment are involved, measurements of the same camera or lens are highly inconsistent. The sharpness of a photographic imaging system or of a component of the system (lens, image sensor, scanner, etc.) or in the case of a digital camera-based imaging systems consist of a lens, digital image sensor, de-mosaicking program, image editor, and printer requires something with a bit less subjective judgment if real detail measurements are required.
Each of these components has a characteristic frequency response; MTF (Modulation Transfer Function). This is merely the measurements name in photography. The beauty of working in frequency domain is that the response of the entire system (or group of components) can be calculated by multiplying the responses of each component.is characterised by a parameter called Modulation Transfer Function (MTF), also known as spatial frequency response.
Most of us are familiar with the frequency of sound, which is perceived as pitch and measured in cycles per second, called Hertz. Audio components— amplifiers, loudspeakers, etc.— are all characterised by frequency response curves. MTF is also a frequency response, except that it involves spatial frequency— cycles (line pairs) per distance (millimetres or inches) instead of time.
The target is composed of 199 pairs of dark and light lines of progressively decreasing widths. The line widths are usually on a log base 10 scale, but I use a linear scale. As the detail in a photographic subject becomes smaller, the lens begins to resolve this information with decreasing amounts of contrast. This is seen in images of the target as the lines in the target grow thinner. In this case, the dark lines become progressively lighter and the light lines become darker (i.e., a decrease in contrast) until they merge into a gray zone which shows no detail. By identifying the thickness of pairs of lines that first show a 50 and then a 10% decrease of the maximum amount of contrast measured in the image .
Of course thesis far more complicated than we have so far stated. So far we've only considered images in exact focus. That's all you need if you only photograph distant landscapes or two-dimensional objects like paintings. But most subjects are three-dimensional and you want to capture objects clearly over some range of distance from near to far; hence you need to be concerned with depth of field (DOF). The basics of DOF are well known: The more you stop down a lens (the larger the f-stop number), the larger the DOF. Wide angle lenses appear to have much larger DOF than telephotos. Telephotos are often used to intentionally limit DOF, for example in portraits where you want the subject to be in focus, but you want a distracting foreground or background to be out of focus. But telephotos don't actually have less DOF.
Most 35mm and medium format prime lenses and some zooms have depth of field (DOF) scales. Your camera's instruction manual states that if you stop down your lens, for example to f/8, everything at distances between the two f/8 DOF marks will appear to be "in focus." Of course, not exactly in focus. You may therefore ask the question, "How sharp is the image (what is its MTF?) at the DOF limit?"
Diffraction Light bends when it passes near a boundary. "Near" is defined as a few wavelengths of light, where the wavelength ω at the middle of the visible spectrum— green to yellow-green— is 0.0005 to 0.000555 mm (500 to 555 nanometers). The eye is most sensitive at 0.00055 mm, but 0.0005 may be more characteristic of daylight situations. This bending, called diffraction, is an unavoidable physical effect that happens regardless of lens quality.
The smaller the aperture— the larger the f-stop ( N )— the more the image is degraded by diffraction.