Extension factor

The term elongation factor is used in several different contexts in photography . The extension factor can relate to the exposure time when taking the picture, to conditions that deviate from the standard during film development, or to the focal length of lenses or lens attachments.

Extension factor for the exposure

filter

When using filters there is an extension factor for the exposure time. In the case of TTL measurement (through the lens), this factor is also measured and therefore automatically taken into account, provided that extreme color changes (e.g. with the red filter) do not result in incorrect measurements of the sensor built into the camera , which also points to a certain Color spectrum is sensitized.

Optical filters in black and white digital photography

The use of classic, optical filters in digital photography is subject to a special feature, since CCD or CMOS sensors are not panchromatic . The extension factor taken into account by the TTL measurement must be compensated with a corresponding minus correction of the exposure taking into account the basic transparency of the filter. If this does not happen, the extension factor leads to the imposition of the dominant color in the corresponding color channel, which is shown in areas without drawing after conversion to black and white. In contrast to filtering by manipulating the color channels on the software level (channel mixer), optical filters influence specific wavelengths of light and thus enable not only the traditional, finely graduated, creative effect but also the selection of wavelength ranges in analytical photography.

Narrow-band filters in the red and blue spectral ranges are subject to restrictions on digital cameras, since they lead to loss of resolution due to the construction of the Bayer pattern . Broadband filters in both areas, which also partially address the green channel (e.g. orange and blue-green filters as well as low-density blue filters), are not critical in their application and enable the full resolution to be used.

Extension factor for close-ups using a pull-out extension

With TTL measurement (through the lens), the extension factor is also measured and therefore automatically taken into account. However, if an external (hand) light meter is used, it must be taken into account that the greater the distance between the lens and the film plane, the less light that hits the film plane. The aperture values , which are basically calibrated for an infinite distance on the lens, would no longer be correct, an exposure correction must be made.

The extension factor can be calculated from:

${\ displaystyle {\ text {factor}} = {\ frac {({\ text {excerpt or image width}}) ^ {2}} {{\ text {focal length}} ^ {2}}} = \ left ({ \ frac {\ text {Extension}} {\ text {Focal length}}} + 1 \ right) ^ {2}}$

The aperture can be determined from the factor as follows:

${\ displaystyle {\ text {Aperture steps}} = \ log _ {2} {({\ text {Factor}})} = {\ frac {\ log {({\ text {Factor}})}} {\ log {(2)}}} \ approx {\ frac {\ log {({\ text {factor}})}} {0 {,} 3}}}$

Among image distance is understood as the distance between that of an optical lens or a mirror image formed and the image-side principal plane of the optics along the optical axis . If the subject is at an infinite distance from the camera, the image distance is the same as the focal length .

Extreme exposure times

Films with underexposures react to both extremely long exposure times ( Schwarzschild effect ) and very short exposure times ( short-term effect ) . In the case of color films, the colors are usually falsified , as the effects on the three to four light-sensitive layers have different effects. While the required extension of the exposure times can be managed quite easily, correct filtering out of color casts requires a lot of experience.

Extension factor for the focal length of auxiliary lenses

The focal length of a lens , together with the format of the film used, is a measure of the angle of view that a lens takes. It is a fixed property of the lens (with varifocal lenses you can change the focal length within certain limits). However, by using special assemblies (so-called converters), the intended focal length of lenses can be varied. The extension factor indicates the number by which the focal length printed on the lens has to be multiplied in order to obtain the resulting focal length. With the wide-angle converter the factor is smaller, with the teleconverter it is greater than one.

Digital cameras

In the case of digital cameras, the term extension factor (also: focal length extension factor ) is often incorrectly used in German for the format factor , a conversion factor between the recording formats of digital single-lens reflex cameras (DSLR) and their film-based 35mm forerunners.

The DSLR image sensors on many models are smaller than the 35mm format. Often, however, existing lenses from previous film-based models can still be used. For example, if you use a normal 50 mm focal length lens with a digital SLR with a smaller sensor, the resulting image looks like it was taken with a light telephoto lens , with a format factor of 1.6 as if you had an 80 with a 35mm camera mm lens used. In fact, of course, the focal length does not change and consequently nothing is lengthened, the effect is created solely by the crop and the resulting smaller angle of view .

Film development

Aside from the largely standardized processes involved in the industrial processing of photographic films , different processes are used in individual development to achieve special effects or to compensate for special recording conditions. These include in particular the so-called push development to achieve a higher than nominal film speed or the use of more dilute developer solutions in order to achieve a development that reduces the contrast compared to the standard. The extension factor for the development time in such processes is generally not calculable, but is determined experimentally and is given in the data sheets of the photochemistry used.