Focus stacking

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Practical application in macro photography: on the left the front part of the fly is sharp, in the middle picture the rear part, on the right the result of the merging.

The English term focus stacking , literally translated as 'focus stacking', more rarely also called depth of field expansion or deep focus fusion ( DFF ), describes a combination of photographic recording and digital image processing technology . It is used in particular in digital macro and micro photography to create an image with an exceptionally high depth of field . In metrology, this technology is also called focus variation .

Procedure

Focus stack of a self-tapping screw
The same screw at f / 32 with noticeable blurring due to diffraction

Classically, a large depth of field is usually aimed for by setting a very small aperture (large f-number) or by means of an adjustable camera according to Scheimpflug's rule or a combination of both. Very small apertures lead to blurring due to diffraction , adjustable digital cameras are rare and very expensive. Focus stacking offers a way out.

Example: source images on the left, result on the right

Similar to exposure fusion (engl. Exposure blending ) is Focus stacking the combination of a photographic shooting and a downstream assembly process. However, in this case the series of images is not a series of exposures , but an image sequence in which the images differ essentially only in terms of their focus.

Stacking with changing the recording distance. A control unit (red LED) controls a stepper motor that moves the camera slide

Various techniques with different suitability and advantages and disadvantages are available for generating such an image sequence, depending on the application.

The positions of the subject and the image plane remain constant while the position / setting of the lens is varied
  1. Variation of the distance setting on the lens - here it can be a disadvantage if the focal length of the lens decreases with increasing close focus ( focus breathing ).
    • Manually turning the focus ring
    • Manual variation of the focus point in systems with autofocus
    • Automatic series recording function of corresponding camera systems ( focus bracketing ) or implementation of this function via corresponding software or app control of the camera or via electronically controlled lens adapters
  2. Variation of the front extension of a bellows device
Variation of the distance between subject and camera
  • The camera is moved from picture to picture, e.g. B. by means of an adjustment slide , while the position of the subject remains constant. Corresponding slides can be equipped with manual control or electronic control.
  • The subject is moved, e.g. B. by means of an adjustment slide, while the position of the camera remains constant
  • Photography through a transmitted light microscope with a height-adjustable stage
The positions of the subject and lens remain constant, while the position of the image plane is varied
  • Variation of the rear extension of a bellows device

The recordings are usually made as single images. Especially in the macro and micro range, where short exposure times are often not feasible due to the small effective aperture , care must be taken to avoid vibrations caused by touching or moving camera parts ( mirror , shutter ) during the exposure process, e.g. B. by mirror lock-up or the electronic shutter release mirrorless cameras . In some cases, individual images from film recordings are also used ("video stacking", "turbo stacking").

Since changing the focus can result in a change in the image scale and objects in the image can shift due to a change in perspective, an image transformation must usually be carried out for the best possible overlay of the images before the actual assembly of the individual images. This task is solved automatically by the special programs within certain limits or optionally offered as a preceding procedure.

During the subsequent assembly, the sharpest areas of the images are put together. Digital cameras , such as the Olympus OM-D E-M1 , are increasingly being equipped with functions in which the individual images are automatically combined by the camera firmware .

software

Machining example with Combine ZP

Since the manual assembly of the pictures is difficult and time-consuming, special software is usually used. In addition to several commercially sold solutions such as Aphelion , Helicon , Zerene Stacker or FOCUS projects , free software is available, for example:

  • CombineZ M or Combine ZP (Open Source, Windows)
  • Enfuse (Open Source, Linux / Windows / Mac OS X)
  • MacroFusion (Open Source, Linux, GUI for Enfuse)
  • Picolay (Freeware by Heribert Cypionka , Windows (also Win10))
  • Example (dental bridge)

  • Image 1 (focus point at the very front)

  • Image 2 (focus point in this and the following images a little further back)

  • picture 3

  • Picture 4

  • Pic 5

  • Pic 6

  • Image 7 (focus point at the very back)

  • After focus stacking (everything sharp)

literature

  • Stefan Groß, Olaf Craasmann: Focus Stacking - Macro Photography for Professionals & Beginners! Traumflieger, Hamburg 2019, ISBN 978-3-9820474-1-6 (297 pages).
  • Jürgen Gulbins, Rainer Gulbins: Multishot Techniques in Digital Photography . dpunkt.verlag, Heidelberg 2017, ISBN 978-3-86490-462-2 (348 pages).
  • Kurt Wirz: Focus Stacking . May 13, 2019 (229 p., Focus-stacking.ch [PDF; 11.4 MB ; accessed on April 6, 2020]).
  • Daniel Knop: “Focus Stacking” in Biology - Customized Depth of Field . In: Biology in Our Time . tape 49 , no. 1 , February 2019, p. 48–57 , doi : 10.1002 / biuz.201910667 .

Web links

Commons : Focus stacking  - collection of images, videos and audio files

Individual evidence

  1. ↑ Is it better to use a focus rail or the ring on my lens? , Zerene Systems, accessed December 27, 2019
  2. ↑ Photo tip: Bringing small things into the picture crisply , test.de from November 23, 2016, accessed on November 24, 2016