Exposure
Exposure are production processes for paper products from film and photo material. Various raster processes are used to fix analog material or digitally rasterized image data. Films ( negative or positive ) or files are used as templates .
Basic principle
In most cases, exposure takes place on photo paper (not to be confused with printer paper, which is usually called "photo paper" and is printed using a raster process ).
In principle, however, other materials are also possible:
- Negative or slide film (a conventional negative or slide is created from a file )
- Printing film (all colors to be printed - usually four - are each exposed on a film and required as a template in the further printing process)
- Archival film (a file is usually exposed in high contrast to a film with a special format)
The need for prints on photo paper is growing every year. The proportion of exposures on other materials is constantly falling.
Exposures are always raster-free, as the image pixels are not rasterized and because the individual pixels "merge" with one another due to various physical causes during exposure. In (rare and) unfavorable cases this can lead to a reduction in the impression of sharpness .
The following two methods are described here as examples:
Film on photo paper
Light is sent through a film in such a way that the photo paper behind it is exposed in the right way ( brightness , sharpness , ...). After exposure, the photo paper is developed in a conventional (chemical) way. This procedure is grid-free.
- The brightness that hits the photo paper is controlled by the duration of the exposure.
- The color mixing is controlled by the use of color filters that are located in the light channel between the film and the photo paper.
- The sharpness is regulated by a lens
In the case of imagesetters that offer image optimization , there is also a special transparent pane in the light shaft. This transparent pane can be partially blackened by electricity. This means that the most important optimization techniques (change in contrast , unsharp masking , post-exposure , dodging ) can be carried out in fractions of a second.
File on photo paper
Each pixel is converted into a specific pulse of light. The hue and brightness are controlled . This exposes the photo paper . After exposure, the photo paper is developed in a conventional (chemical) way. Tube exposure units, laser exposure units or LCD exposure units are used. This procedure is grid-free.
In order to compensate for the lack of quality in this data, images often go through an automatic optimization process (change in contrast , unsharp masking , ...).
Aspect ratio
In addition to the classic photo formats (aspect ratio of a 35mm film 1: 1.5) there are countless special formats. Most special formats arise from digital photography and are due to the design of the sensors (frequent aspect ratio of 4: 3) or post-processing .
Usually rolled photo paper is used. The exposure follows the aspect ratio provided by the file , (but mostly) up to the classic aspect ratio of 1: 1.5. After the photo paper has been developed , it is cut. Each photo is cut according to its individual length.
Panorama formats pose a problem for most imagesetters . Although the images are exposed on roll paper, the outer edge of the light source is - for structural reasons - usually made with an aspect ratio of 1: 1.5. In principle, the panorama format does not pose a problem when it comes to exposure.
The currently largest exposure format in the amateur sector is 76 cm × 300 cm.
Original size
The size of an original (see also image resolution ) is one of several factors that significantly influence the quality impression of the reproduction.
The majority of all exposures on photo paper are made as amateur work. The values given in the table for the original size are orientation values in order to be able to use the full print resolution.
| Photo dimensions in cm × cm | 10 × 15 | 20 × 30 | 30 × 45 | 40 × 60 | 50 × 75 | 60 × 90 | 70 × 100 |
|---|---|---|---|---|---|---|---|
| Image size in megapixels | approx. 2.2 | approx. 8.7 | approx. 19.5 | approx. 24.4 | approx. 39.0 | approx. 55.6 | approx. 72.4 |
In the professional sector, higher, but often similar, image sizes are used for exposure. The greatest difference in quality is the better contrast differentiation in professional exposures .
The second, essential difference between amateur and professional exposures is the image optimization.
- In the amateur sector, the production process is based on data of low and medium quality - the optimization is correspondingly high. Typical for this is the increase in contrast and a sharpness correction ( unsharp masking ).
- In the professional sector, high quality data is assumed. The image optimization is correspondingly lower. Since this type of production lacks the contrast enhancement typical for amateur work, professional exposures often appear "dull" at first glance. This low contrast is only intended to prevent loss of sharpness.
The resolutions of the exposure units are different; machines from 150 to 400 ppi are common for photo paper . Economic reasons are usually decisive for this. An example: If a 50 cm × 75 cm photo were to be exposed at 400 ppi, a raster graphic size of approx. 270 megabytes would be required. Larger data packets mean a longer computing time, which means an increase in a significant cost factor. In the course of the current development of computing power, a gradual adjustment process is taking place. As a comparison: Daily newspapers are usually produced with a print resolution of 150 ppi, while the currently best photo imagesetters (for photo paper) achieve a resolution of 400 ppi.
The resolutions of the photo exposers are often set depending on the format. B. exposed at 300 ppi, larger ones at 200 ppi. Much higher resolutions are often used for exposures on other materials . Note: The unit “ppi” must not be confused with “dpi”. While “ppi” refers to the actual photo pixels, “dpi” only refers to the print points. During the print preparation, several print points (usually four print points per pixel) are calculated from each pixel. In purely mathematical terms, a 300 ppi printout would be equivalent to a 1200 dpi print (but the quality of the printout is better because it is a grid-free process).
Quality criteria
The essential quality criteria of an exposure are determined by the quality of the original image and the technologies used. The technology consists of data conversion , image optimization and the accuracy of chemical development .
Since the exposure is a grid-free process, it can be used to display the image data without the interpolation loss of a printing process. In addition to the very high durability (archiving ability and resistance to environmental influences), the following advantages come into play:
Film on photo paper
- In the centuries of development of photochemical technologies, all the parameters that make a good photo have been fine-tuned to each other.
- Compared to prints, exposures have a larger range of contrast, finer contrast differentiation and clearer colors (since some of the colors are exchanged for black during printing ).
File on photo paper
- The use of the RGB color space for the exposure guarantees a high level of color fastness, since no color separation is necessary.
- Since exposures are screen-free processes, no additional artifacts (such as moiré ) arise .
- Compared to prints, exposures have a larger range of contrast, finer contrast differentiation and clearer colors (since some of the colors are exchanged for black during printing).
In principle, any size of exposure can be produced from any original quality. There are no technical restrictions here - only subjective quality criteria can set limits. The impression of sharpness can be named as an example:
- 100% sharpness would be the ideal condition for a sharp photo - but this is impossible. Every photo contains blur.
- What comes closest to the ideal of 100% sharpness is a vector graphic , which is converted into a high-resolution pixel graphic for exposure .
The essentials 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”).
The following is therefore decisive for the impression of high sharpness:
- the application of the exposure (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%)
All these factors do not result in a clear limit value from which resolution a photo is perceived as sharp, but criteria for optimizing the quality for the desired exposure size.
Other influencing factors
Analog starting material
The sharpness, contrast differentiation and grain determine the possible image quality in photographic film. These parameters are initially influenced by the film speed. Highly sensitive films generally have a coarser grain size than films with low sensitivity. The grain size limits the possible resolution , but a coarser grain also reduces the light diffusion within the emulsion, which improves the resolution. Furthermore, contrast rendition and resolution are influenced by various chemical and physical effects during development. For example, edge effects such as the Eberhard effect increase the contrast at the borders of more and less exposed areas and the Kostinsky effect increases the distance between adjacent exposed points. Such neighboring effects are used in particular by fine-grain developers for processing black and white films to improve image quality.
The common everyday films with a sensitivity of 100 ASA (= 21 DIN = 100/21 ° ISO) meet all normal requirements in terms of grain, contrast and sharpness. Exposure of 35mm films up to a size of 50 cm × 75 cm is usually not a problem. However, there are clear differences to enlargements of 6 × 7 medium format in 50 cm × 70 cm.
Slide films with low sensitivity (25 to 50 ASA) offer a resolution of up to 400lp / mm, which corresponds to a digital image of around 40 megapixels for 35mm film. Since, in addition to the film properties, the optics of the camera also have a qualitative effect, the effectively usable resolution is usually much lower.
Digital source material
Important influences on the physical sharpness of digital recording systems:
- the optical performance of the lens
- the image processing electronics and algorithms: noise reduction reduces the sharpness of the image
differentiation
The colors that can be displayed are u. a. determined by the number of colors used (in the camera there are 3 "RGB" colors). In the printing process - compared to exposure - i. d. Usually more than 3 colors used. The larger amount of colors results in a different amount of representable colors. For example, many printing processes can display colors that many cameras cannot deliver, on the other hand, certain colors supplied by the camera are not displayed by printing processes.
Exposures use the RGB colors and thus represent the camera data with greater similarity than printing processes can.
Hybrid process
With the Agfa Digiprint process, a slide is not optically exposed, but first digitized. Then the exposure takes place with the help of a film exposure unit.
The Agfa Digiprint process was introduced to the amateur sector in the 1990s, but is hardly relevant today. The low demand for exposures from slides nowadays is usually handled with normal exposure - combined with special photo paper.