Helicon FB Tube – Setting the step

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In order to obtain good focus stacking results and to ensure optimal operation mode of the lens you need to set the right step in the Configuration Utility.

The step is lens and aperture specific. If the step is defined correctly the focused areas will slightly overlap with each shot and with each shift of the focus, thus covering the whole range. If the step is too big the lens will skip some areas that will stay out of focus. In this case you will see blurred out stripes on the resulting image after focus stacking. The step being too small, the lens will make too many shots, though the resulting image will be completely in focus.

To set the right step please follow these instructions:

1. Download and run the Configuration Utility.

2. Choose the suitable subject to focus on - it should have a lot of detail, like heavily textured fabric or a book page with printed text.

3. Select the camera type, the lens and the aperture. If there's a recommended step value for this combination of settings in our data base, it will appear in the Recommended step window automatically. If your lens is not on the list, you need to add it and define the right step value yourself. For that set the step value to 20 (in arbitrary units) and press the 'Proceed' button.

4. Configure the tube as described in the manual.

5. Shoot the stack and repeat the same procedure with the step values 40, 80 and 160.

6. Process four produced stacks in Helicon Focus and choose the resulting image that is fully focused and has no blurred (unfocused) areas. The step value you've set for this stack of images will be the correct step value for current shooting parameters.

7. Enter the correct step value you've defined in the Recommended step window and press the Save button. Once you save the step for the lens, the program will use it for calculation of recommended step values for different apertures. For example, you found the step 100 to be optimal at f4. When you choose f8, the program will show you that the recommended step is 200.

Example of correct and incorrect steps:

As you can see on the image below every part of the image is sharp. This means that the focused areas of the source images overlap and that you've set the correct step value.

Here's another stack with a larger interval between the shots processed in Helicon Focus. If after focus stacking in Helicon Focus you notice that the resulting image contains areas (stripes) that are out of focus, it means that the focused areas do not overlap when all the shots are stacked together, i.e. the step's too big.

 


 

Focusing distances

A macro extension tube increases the distance between the sensor and the lens by 12/13 mm relative to its normal position. This brings the closest and the farthest focusing points of the lens closer. Impact on long focus lenses (200 mm) is not so big, but it’s very significant for short focus ones (24 mm).

Here’s the formula allowing to calculate the maximum focusing distance for your lens when using an extension tube:

1/s + 1/s' = 1/f, wherein s is the distance to the object, f is the focal length of the lens and s’ is f + extension tube length (12/13 mm)

The table below shows the result of practical tests and measurements for some lenses.

For instance, Nikon 105mm macro lens without a tube can focus at distances from 0.3m to infinity, and at 0.14 m to 1 m with the tube.

Depending on the distance to the object, one and the same step size of the tube will produce completely different linear size of the step and DoF.

For example, Nikon 105mm macro lens at the closest focusing distance of 0.14 m will make a linear step of 0.35mm, and 20 mm at the farthest focusing distance. And that with the same tube configuration settings

 

Distance in the lens window  Distance from first lens to focused object Microsteps per frame in   Helicon FB Tube Step between frames with 50% DOF overlap
Sigma 150mm macro 2.8 (@f8) 0,38m 0,17m 35 0,20mm
2,00m 1,00m 35 0,60mm
1000,00m 3,00m 35
Nikon 105mm macro 2.8 (@f8) 0,30m 0,14m 80 0,35mm
1000,00m 1,00m 80 20,00mm
Nikon 24-70 mm 2.8   (@f8, 24mm) 0,38m 0,01m 50 1,00mm
1000,00m 0,02m 50 1,00mm
Nikon 24-70 mm 2.8     (@f8, 70mm) 0,38m 0,11m 50 1,00mm
1000,00m 0,40m 50 7,00mm