GEHMANN 548 Square Outline Iris can be adjusted to appear as a square or rectangular box. Side walls can be set by simply twisting either of the two calibrated external rings. Adjustment range for both top & bottom or sides is the same. 18MM thread standard. 548A: 2.4-4.4MM. 548B: 4.0-6.0MM. GE-538 collar required for fitting to 22MM foresight.