It happens when acquiring images of objects with significant volume that, subsequent to the stacking phase, vertical and horizontal lines localized along the edges of the image are observed, as visible in the image below.
The origin of these lines is related to the optical characteristics of the lens, and to the imperfect on-axis positioning of the optical system with respect to the subject. The presence of these lines is less noticeable when higher magnifications are used, and the “thickness” of the subject is not particularly important. The depth of field of the lens used obviously plays a role.
In the following example, I acquired a number of shots (85) that originate from the apical part of the echinoderm, to the outer edge.
As frames are acquired toward the more distal (peripheral) part of the sea urchin, the optical system must move closer to the upper part of the subject.
By developing the stack on Helicon Focus software (www.heliconsoft.com), you can remove these lines simply by selecting the “Crop output automatically” option found in the Preferences –> Autoadjustment menu.
The result corresponds to an image in which the outer parts, which originally had these lines, are removed and the image is “clean.” Take into account that this cropping operation reduces, of course, the size of the image.