The ability to distinguish between details in an image is called sharpness . Sharpness is an important goal in the technical implementation of photography.
- If sharpness is physically present, one speaks of sharpness .
- If sharpness only appears to exist, one speaks of sharpness impression .
In everyday photography, sharpness only plays a secondary role; the impression of sharpness is always the dominant factor in assessing the quality of a photo .
Image information can only be represented through sharpness. If many details can be distinguished, an image is very sharp. Differences between details can be seen in brightness, hue, and color intensity.
- In everyday life, the term sharpness is often mixed up with the term resolution (fine grain or high number of pixels = high sharpness). However, the resolution is only one of many factors that influence the sharpness.
A photo can only convey information if it is sharp. The quality of this information transport is assessed in photography in three ways:
The classic counting measure
The number of lines that can still be distinguished is counted. The result is a numerical value. This method is often used to evaluate lenses, printers, cameras and other technical devices.
The artistic picture statement
In art, sharpness is a special contrast medium. The additional image information is only transmitted through the difference between sharp and unsharp. From an aesthetic point of view, less sharpness (= fewer details) can be seen in an image, but this image can nevertheless convey more information.
The subjective impression of sharpness
This sharpness rating dominates our daily viewing habits. Depending on the specific circumstances (media resolution, image size, viewing distance and time, image message and your own expectations), an individual degree of sharpness is created.
- An example: In the upper focus pattern 2 (excerpt from Siemens star) there is a shrinking font. If this font is compared with the small font on a printed product, the smallest font on the printed product can be seen more clearly. Nevertheless, the monitor image is also perceived as sharp because the monitors themselves light up. This creates a high contrast range, which in turn creates a high impression of sharpness.
This type of sharpness assessment is influenced by a great many subjective things - hence the name subjective sharpness impression .
|Sorting||Influencing factor||Details, examples|
|Admission condition||light||Brightness, color, direction|
|distance||Disturbances in the air, focal length , depth of field , distance measurement ...|
|Move||Camera, subject ( camera shake , motion blur , ...)|
|Weather||Digital everyday cameras often have an ideal working range of up to 40 degrees Celsius - artifacts are increasingly formed above this .|
|Recording technology||Lens system||Quality, construction|
|resolution||Film grain, amount of pixels (physical amount)|
|Image storage||Film quality, file format, ...|
|equipment||Image stabilizer, tripod, ...|
|motive||Differences in detail||Contrast , brightness, color, ...|
|structure||Moiré , ...|
|development||Fixation||chemical development, manipulation by the camera software|
|optimization||Unsharp masking , ...|
|transfer||Exposure, printing, ...|
|evaluation||The right amount||s. a. Sharpness patterns 5 and 6: when is a detail sharp?|
|Type and circumstance||Image projection, printing, room lighting, ...|
In optics , photography and cinematography , this describes a special criterion that can be observed at the edges. The more abrupt the transitions from dark to light, the sharper the image. The extent to which the edges are represented is called acute in photography .
The edge sharpness is a matter of the (light) opening: An infinitely small hole in the camera obscura creates an infinitely sharp (without considering diffraction effects ), but also infinitely dark image. When you finally make the hole large, the image becomes brighter, but also less sharp.
Edge sharpness has nothing to do with resolution .
In addition to suitable technical devices , three requirements are important when measuring sharpness:
The reference (test image)
The test image is reproduced - and compared with the finished image as a reference. The difference between the two measures sharpness. Only a suitable reference offers optimal conditions for measuring sharpness.
Defined boundary conditions
Sharpness is influenced by many factors. Since the sharpness measurement should deliver a comparable result, the boundary conditions must be specified. The result is a numerical value. These idealized conditions have the disadvantage that they are of no great use in everyday photography, because something is constantly changing here - ideal conditions rarely prevail. The change of an essential condition, for example the temperature, often makes the determined numerical value unusable.
The result depends on the selected measurement method (measurement accuracy) and its evaluation.
Focus measurement always needs a comparison. The sharpness of the images above can be compared in several ways:
- Visibility of disturbances (grain) = blurring (so the film would be the loser)
- High contrast at the edge border = sharpness (that would make the film the winner)
Even with sharpness pattern 5 , the result would depend on the measurement accuracy. Since the artifacts can only be seen in the medium image resolution, the measured value can be falsified.
In everyday life, people are always surrounded by a mixture between actual (physically present) sharpness and the appearance of sharpness, caused by the subjective perception of the person. The latter is referred to as the impression of sharpness. The sharpness of photographs can be changed using the impression of sharpness. The techniques used to influence the impression of sharpness are summarized under the term image optimization .
The technique of being able to change the subjective impression of sharpness of an image is more than a hundred years old. Using fine brushes and special paint, the image details that are important for the impression of sharpness (mostly eyes and contours) were delicately drawn. This method was so successful that it was used by most professional photographers for such purposes by the late 20th century. Even today, with the appropriate software, images can be retouched in such a way that their impression of sharpness is improved.
The second, also very old method is that of changing the contrast . Using different developer chemistry and paper types as well as targeted re- exposure , the contrast could be manipulated globally in the entire image. In the second half of the 20th century, the change in contrast received a decisive leap in quality: unsharp masking (the contrast is only intensified locally at the edges within an image). Both techniques were able to assert themselves successfully in the digital photo age.
At the beginning of the 1990s a new era in photography was ushered in (very unspectacularly). At this point, 95% of all photos were made in large laboratories. At that time, the large laboratories introduced the technology of automatic image optimization . In a split second, every photo (then analog) was analyzed and manipulated. This technology was able to dodge, re-expose, unsharp masking of each image in tenths of a second , etc. The software on which this process was based required years of fine adjustment (it had to be “trained”). As a result, over time there was a gradual transition to completely different viewing habits. The best way to see this change is by comparing photos from the 1980s and 1990s. Although the technology of the 35mm film (used by most photographers) has not changed significantly during this period, you can clearly see the difference in the photos from this time.
In the years of the millennium, the mass spread of digital photography began. In order to conceal the serious deficiencies of the digital technology at the time, the entire image production line was adapted to the user profiles . A typical example of this is the profile of the “clipper” : Vacation and family motifs should be printed out for the photo album in a size of 10 cm × 15 cm. The printers were set up so that they increased the amount of black much more than was necessary. As a result, the image motifs looked very strong and rich in contrast (impression of sharpness). This concealed the weakness of the printer that it was very difficult to print delicate colors. With the restriction to 10 cm × 15 cm, the physical lack of sharpness of digital images was barely visible.
Since our viewing habits, as mentioned above, have now adapted to the omnipresent, automated image optimization , the actual sharpness of an image only plays a secondary role - the impression of sharpness is primarily important.
Influence on the impression of sharpness
1. The image optimization has the most important influence on the impression of sharpness .
- It is impossible to show the raw version of a photo with any imaging technique . In every imaging process, there is a change in the image.
- All methods and associated devices (printers, monitors, imagesetter ...) that can display photos are called imaging processes.
- The most basic form of seeing a photo in its raw version is only offered by the slide . But even here, changing the color temperature with which the slide is viewed is enough to change the image impression.
- Since a photo is changed in every processing step, one tries to control this change (to optimize the images ).
- In professional photography, one tries to keep the effects of these changes low by coordinating all image-changing things (calibration, profiles, ...) . But here too the change is there.
2. The second most important influence is physical sharpness.
- This means the sharpness of the raw version of a photo ( film according to standardized development or RAW file format of a digital camera). Up to this point in time, it is mainly optical, mechanical and physical effects of the image generation (chemical or photoelectric ) that determine the impression of sharpness.
3. The third most important influence is the resolution .
- The most common assumption is: many pixels = a lot of sharpness. However, sharpness is mainly perceived by the differences in brightness, color and saturation.
- Of course, the number of pixels also plays a role, but this is third-rate.
Assessment of the impression of sharpness
Tests are necessary to assess the impression of sharpness. If these are carried out taking into account the typical problem areas, the defects become visible, which have a serious effect on the reduction in the impression of sharpness.
- Example test 1: In the sharpness pattern 10, two image motifs can be seen on the left and in the middle, which can serve as a pattern for a picture with your own camera.
- Very dark and very light image details are important, combined with a typical mixed color (skin).
- A photo of the finished image must be at least 20 cm × 30 cm in size. This photo must be held next to the original (the model) .
- How many image details are still visible in the dark and light parts of the image? Is the skin tone right? Is there a color cast?
- Sample test 2: A photo of this test file must be at least 20 cm × 30 cm in size and compared with the monitor image.
There is no imaging procedure without flaws.
- All methods and associated devices (printers, monitors, imagesetters, ...) that can display photos are called imaging processes.
A drawing refers to the recognizability of details and structures in local areas of an image. If there is no longer any drawing, the image will only contain low-contrast areas in these areas instead of the structures. Drawing is a term for the subjective (sharpness) evaluation of a photo.
What comes closest to this ideal (100% sharpness) is the vector graphic . However, only lines and edges can be displayed with it.
- The main characteristics of photos are:
- soft gradients between the colors
- Colored areas with natural structure
- Merging of different picture elements (without edges that look like “cut with scissors”).
Every photo contains blur.
- The following is therefore decisive for the impression of high sharpness:
- the application of the photo (purpose, viewing distance, ambient light, ...)
- the expectation of the viewer (technical "perfection", high recall value , "just for fun", ...)
- the right mixture between sharpness and blurring (artistic blurring, motion blurring, ...)
- the quality of the image optimization ( physical sharpness only influences the sharpness impression by 25% )
Contrast in focus
Differences in brightness are the strongest differences that the human eye can perceive in a photo. These differences in brightness can be perceived both globally (affecting the entire picture) and locally (between individual picture elements). These differences are called contrast. With a high contrast, the subjective impression of sharpness can be increased - usually at the expense of physical sharpness .
The greatest difference in contrast in our everyday life is between the dead of night and blazing sunshine. One speaks here of a large contrast range. The human eye adapts to this difference by opening and closing the pupil . Cameras use a similar method by opening and closing the shutter .
A high contrast range provides a high degree of sharpness - however, there is no image system that can completely fix the contrast range of nature in one image. Therefore, the natural range of contrast of a subject must be reduced before the image is saved.
- In everyday photography, an average 35mm negative film can record a range of contrast (motif contrast) of at least eight f-stops (without any noteworthy artifact formation ). To clarify: This corresponds to a difference in brightness of 1: 256.
- Simple digital cameras offer at least 6 f-stops of contrast range (without significant artifact formation ). This corresponds to a difference in brightness of 1:64. Modern high-quality digital system cameras achieve contrast ranges of 12 to 13.5 f-stops, which corresponds to differences in brightness of up to more than 1: 10000.
The contrast range of a photo is defined by the area between the lightest and darkest point. In the technical representation, black and white are used for this.
Depending on the number of differentiation levels, a photo is able to convey image information (= to be sharp). In digital photography, mostly 8 bits (= 256 gradations per color channel) are used. This guarantees a sufficient degree of sharpness in everyday life.
> literal translation: "nature of a photograph"
Photo quality is understood as the totality of all influencing factors that influence the quality of a photo. The type of recording (analog or digital) only has a limited influence on the photo quality. You can take excellent pictures with old analog cameras that have no electronics, and with an expensive digital camera you can take unusable snapshots.
- technical parameters: color, brightness, ...
- subjective parameters: expectations, experience, ...
- Ambient conditions: lighting conditions, form of presentation, ...
Any condition that affects photo quality also affects sharpness. Therefore, the differentiated assessment of sharpness can be considered as an example for the assessment of photo quality.
> literal translation: "press together"
- If the photo from a camera were saved with 10 million pixels without compression, the file would be 30 megabytes in size. This is roughly the equivalent of 20 photos if the same camera were to save the photos with JPEG (high quality) compression. Storage space costs money. Therefore, all manufacturers of imaging systems are trying to reduce the file size. For this reason it is compressed.
A distinction is made between lossless and lossy compression.
- Lossless compressions reduce the file size to half or a third of the original size. The problem: The resulting files are usually not universal - special software is required.
- Lossy compressions can reduce the file size to as little as 5% of the original size. This is made possible by deleting non-dominant image information. The resulting files are universal and can be used by most computer programs.
Lossy compression is always associated with a loss of sharpness.
> in the same way for: improving the quality impression of a photo
- The image information of every digital photo is influenced by:
- elementary basic functions of the camera software such as white balance , gamma correction , compression processes
- unavoidable loss of sharpness in the processing chain, such as
- Interpolation losses between the pixels ( density interpolation )
- Interpolation losses within the pixels ( color interpolation )
- automatic optimization in image production
- manual optimization in image production
The image optimization tries to control each of these changes. The main methods are:
- Emphasize edges and contours:
- Contrast increase:
- Increase the contrast range:
- self-luminous projection (slide, beamer, ...)
- Use paper of high whiteness (more light is reflected) and use black of high purity (less light is reflected)
- Special photo papers are available for inkjet printers. With these the o. G. Requirements met.
- Influence brightness ( post-exposure , tonal value spreading, ...)
- With inkjet printers, more black is generally added than is technically necessary. This serves to increase the impression of sharpness.
- Create high-contrast differences:
- Change the background in the photo
- Change picture frame and presentation location
- and more …
All of the above methods can be used in analog and digital form.
- Christoph Handmann: Camera test with reference to the dynamic range test , Bachelor thesis, 2015, accessed on April 8, 2016