To understand the acutance better, we take a look at the resolution first. Depending on the domain, this term declares not always the same – resolution is not necessarily the same as pixel count.
Different meanings of resolution
For cameras, the resolution is a common term for the number of pixels on the sensor, given in megapixels (MP). A camera whose sensor has 4000 x 6000 pixels has a resolution of 24 megapixels, for example.
Also in relation to the test results, resolution is mentioned. Contrary to the indication of the pixel count, in this context it describes how well a recording system can reproduce fine details. The percentage result represents the performance of a certain camera-lens combination based on the theoretical maximum resolution of a recording system – also called limiting resolution. This is determined in relation to the contrast. (see Result interpretation)
Another distinction refers to the output medium. Here, the resolution describes how many pixels are represented on an area of the image. Fundamentally, the resolution of digital images is specified in ppi (pixels per inch).
Take a look at our example graphic. It shows the difference between 15 ppi and 30 ppi. On the left, 15 pixels can be located on the one inch large area. On the right side 30 pixels.
When evaluating a photo, commonly the term sharpness is used. Mistakenly this is often equated with the limiting resolution. There are special test charts available, like the lensTESTER, to evaluate the limiting resolution. With this it is possible to analyze the performance of a camera-lens combination. But high results do not imply a high sharpness impression. Also cameras with lower limiting resolution can deliver good photo prints.
How can that be?
To make a statement about the sharpness, more than the resolution/limiting resolution is needed. Because, how sharp an image is experienced depends on further important factors – the contrast and the viewing condition.
Whether we observe a photo as sharp, among other things depends on the contrast. More specifically on the contrast of the subjects edges. The more clearly the demarcation of two edges to each other, the higher the contrast and in hence the sharpness impression.
Take a look at the adjacent image. The slanted edges image on the left side has a low contrast. The pattern on the right side has a higher contrast and therefore, appears sharper.
However, the viewing condition also plays an important role. To illustrate this, we look at two recording systems (example):
Camera-lens combination 1 reaches a central limiting resolution of 91%.
Camera-lens combination 2 achieves 69% instead
Because of the higher limiting resolution of combination 1, one might assume that this camera-lens system also delivers a better sharpness. But that's not quite right, since we have not taken the reproduction side into account. To make a statement about the sharpness impression, we have to take a look at the output side. That means the way the image is reproduced/viewed - for example on a monitor or as a photo print.
Example: 100% monitor view
Based on the capabilities of the camera systems, combination 1 delivers the higher test results and captures therefore more details. If we choose a 100 percent monitor view, the image of combination 1 has a better sharpness impression than the image of combination 2. The reason is that the images are displayed in original size and by this, details can be examined very closely.
Example: Photo prints
What if we decide to print the images in a smaller size? For optimum results of smaller prints, a pixel density of 300 ppi is preferred. Depending on the viewing distance and the image content 150 ppi (e.g. poster size → larger viewing distance) can also be enough. To illustrate this, we take a look at the enlarged crops of an image in 4 x 6 inch with three different pixel densities:
If we look at a photo print from an appropriate distance, we would notice single pixels at 72 ppi resolution (left image). However, choosing a higher pixel density, like 150 ppi (image in the middle) or 300 ppi (right image), we hardly perceive the pixel structure.
Which resolution do I need for prints?
The needed resolution for a digital image can be determined, depending on the print size and the pixel density. For a photo print of 4 x 6 inch (10 x 15 cm) with 300 ppi, a minimum resolution of approximately 1200 x 1800 pixels (2.2 MP) is required to secure a good sharpness impression:
4 inch with 300 ppi → 4 inch * 300 pixel/inch = 1200 pixel
6 inch with 300 ppi → 6 inch * 300 pixel/inch = 1800 pixel
When printing a poster with the size of 16 x 24 inch and 150 ppi, the resolution should be about 2400 x 3600 pixels.
It is: The higher the print resolution and the image size, the higher the demand for the digital image resolution. Assumed, the image itself is not blurred.
Let’s come back to our two camera-lens combinations. The requirement for a digital image printed in 4 x 6 inch is about 2 megapixel. Although the limiting resolution of combination 2 is with 69% below combination 1 (91%), it could still be sufficient to deliver a sharp 4 x 6 inch photo print.
Besides the limiting resolution, lensTESTER will give you the possibility to evaluate the edge sharpness that is expressed by the so-called acutance. By this, the sharpness impression is mirrored in form of a test result. Thereby, the outcome refers to a predefined viewing condition. In this case it refers to viewing the image center in 100 percent view on a monitor (96 ppi) at a distance of 0.5 meters. To determine the acutance, the result is related to the Contrast Sensitivity Function of the human eye (CSF) for the defined viewing condition. Thus, a statement about the sharpness impression can be made.
For the acutance the ideal value is 100%. The lower it is, the worse is the sharpness impression under the defined monitor display. But, also results above 100% are possible. In this case, a higher result does not mean, that the photo is perceived better. Images with values over 100% seem increasingly sharpened.
Because this is an unnatural effect for us, the overall impression of the photo may be poorer, not better (see image on the right).
Take a look at the following photos - at present size, you will see no big difference in the sharpness.
Click on a single image to enlarge it for monitor view and to reveal the existing and non existing details.