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  • Daniel Knop

What step size for focus stacking?

Aktualisiert: 25. Apr.

For a high-quality focus stacking shot, the optimal step size of camera movements between the individual shots is extremely important. Here we explain how you can calculate them in a very simple way.



Eine Hornisse steht und schaut in die Kamera
How long should the individual steps be for focus stacking?


Focus stacking shots take place in numerous individual steps. If your camera sits on a motorized linear stage so that you are working with a programmable control device, then after entering the start and end points of the shooting path you have two options:


- either you specify the total number of individual shots and let the control unit automatically calculate the step length


- or you can instead set the step length and let the control unit calculate the total number of individual shots


Both lead to equally good image results, provided all other conditions are met. How you can easily determine the number of individual recordings as an approximate value was explained in the article “How many individual shots?” This is about the opposite case: you plan to determine the step size. There’s no need to repeat this for every single recording session, but only once for each recording constellation of lens, tube lens and extension, i.e. the distance between the camera sensor and tube lens (I myself always use a bellows device for this distance instead of extension rings or a fixed tube because this gives me enormous flexibility, but more on that in a separate article.)


The principle is very simple and was suggested to me by Rainer Ernst (many thanks for the tip!), but I modified the calculation process. Before starting to shoot a certain object, Rainer recommended to first photograph a micrometer, which has a tenth of a millimeter scale. This gives you the image width recorded by the camera with the respective lens configuration used, in tenths of a millimeter. Such micrometers on microscope slides manufactured in Far East can now be obtained quite cheaply through online auction houses.


Carl Zeiss Mikrometer
Zeiss microscope micrometer with 30 millimeters, five of which are in tenths


In this way, Rainer Ernst determines the image width that his respective shooting constellation of lens, tube lens and extension length will take. He divides this value by 100 and subtracts ten percent from this hundredth of the image width for the overlap of the individual focus zones. The value obtained this way is his step size.


I basically adopted this method because it captures all visually effective components, but I divide the image width by ten times the value because the resulting number of steps is more in line with my intuitive gut feeling for this overall constellation. Which divisor you choose here ultimately depends on the quality standards you place on your photos, in particular on the question of how enlarged they should be viewed later.


A test with an HLB Planapo 20x objective and a Raynox DCR150 tube lens at an extension of 210 mm gave the following values:


Image width 1.7 mm

1.7 mm : 100 = 0.017 mm

0.017 - 10 % = 0.0153 mm

Image width 1.7 mm

1.7 mm : 1000 = 0.0017 mm

0.0017 - 10 % = 0.00153 mm


After entering the start and end points of the shooting path, the control unit calculated 20 respectively 204 individual steps.



Focus Stacking Steuergerät Schritteingabe
After you have entered the start and end points of the shooting path in the control device – here a Castel micro from Novoflex – add the calculated step size



Focus Stacking Steuergerät Novoflex Castel micro
The device then automatically calculates the total number of individual shooting steps


The resulting image (wing scales of Chrisiridia rhipheus) shows massive blur stripes (arrows) when 100 was used as divisor. It was easy to see when upscaled.



Schmetterlings-Flügelschuppen, Focus Stacking
Wing scales Chrysiridia rhipheus, taken with the lens configuration listed above, divisor 100, right picture magnification to 400 percent


With divisor 1000, however, these areas of blur are not visible. That's why I would recommend using 1000 as the divisor to determine the step length using this calculation method – for reasonably good image quality. If, on the other hand, you are planning a photo with a particularly high quality, I would recommend using 1500 as a divisor, which would calculate around 300 individual steps with a 20 x lens for the above-mentioned constellation and recording distance, and that would correspond to my personal intuitive estimate.


Schmetterlings-Flügelschuppen, Focus Stacking
Wing scales Chrysiridia rhipheus, taken with the lens constellation listed above, divisor 1000, right picture magnification to 400 percent



Example measurements


The image width (full format sensor) with the constellation of Mitutoyo M Plan Apo 5 x lens and Raynox DCR150 tube lens with an extension of 210 mm: 6.8 mm


Micrometer, fotografiert mit Objektiv Mitutoyo M Plan Apo 5x und Tubuslinse Raynox DCR150 bei Auszug 210 mm: 6,8 mm
Micrometer Zeiss



The image width (full format sensor) with the constellation of Mitutoyo M Plan Apo 10 x lens and Raynox DCR150 tube lens with an extension of 210 mm: 3.4 mm


Micrometer, fotografiert mit Objektiv HLB Planapo 20x und Tubuslinse Raynox DCR150 bei Auszug 210 mm: 1,7 mm
Micrometer Zeiss



The image width (full format sensor) with the constellation of HLB Planapo 20 x lens and Raynox DCR150 tube lens with an extension of 210 mm:1.7 mm


Micrometer, fotografiert mit Objektiv HLB Planapo 20x und Tubuslinse Raynox DCR150 bei Auszug 210 mm: 1,7 mm
Micrometer Zeiss


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