diff --git a/NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md b/NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md
index 2cabf9c..0c816c2 100644
--- a/NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md
+++ b/NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md
@@ -232,3 +232,43 @@ It helps, after they are set, to gently tap the lens, so it finds a stable posit
 The lateral movements of the lens, the to-fro movements of the lens and camera, and a tilting of the camera (if necessary) allow the optical printer to be set for exact 1:1 reproduction.
 Then the printed image is the same size and in the same position as the original image.
 If the printer lacks a tilt adjustment the camera may be shimmed.
+
+## AIMFRAME
+
+A special frame is made to guide the exact 1:1 setup.
+To make an "aimframe" use the optical printer camera (though not necessarily with the optical printer lens) to photograph a target which is especially drawn to contain details exactly coinciding, as seen through the camera eyepiece, with details permanently on the groundglass.
+The photograph made while the coincidence is seen is the aimframe.
+
+Every groundglass has some permanent details, even if only its flaws.
+The field edge is a poor choice of detail if the mask is thick or if the eyepiece is aberrated at the edge.
+Two points of detail are enough for a well~aligned printer, three points for a suspect one.
+
+A reticle made on high resolution film may be attached to the groundglass to add details.
+Small patterns of concentric circles and other patterns which self-moiré are ideal. 
+Also the aimframe can be a negative of the fine-patterned reticle.
+
+For exact 1:1 setup, the aimframe film is registered in the printer gate and the printer camera and lens adjusted to achieve that same coincidence of details, as seen through the eyepiece.
+Focusing must be completed before the final adjustment to the aimframe. 
+It is convenient to incorporate a focusing target in the aimframe.
+
+The aimframe has validity only for the camera in which it was made.
+It does not depend on the accuracy of the cameras reflex viewing system, only the stability of the system.
+Whenever there is doubt about the validity of the aimframe, such as after a camera repair or because of wear to the film,
+The old aimframe can be registered in the printer gate, aimed
+on, and photographed to make a newly valid aimframe.
+
+For rotoscoping with primitive contraptions, an aimframe may
+be projected and drawn.
+This drawing is later used to aim the camera (whose aimframe it was) when photographing the rotoscoped drawings.
+
+The 1:1 accuracy of optical printing with aimframe setups is limited by 
+
+1. the precision in the making and then in the use of the aimframe,
+2. the precision in the film registration mechanisms of camera and gate,
+3. only if the two mechanisms are different, the precision in the film dimensions (perforation and slitting).
+
+Step contact printing, such as by bipacking in the optical printer camera, is a convenient method for making exact 1:1 reproductions.
+It must give exposures which are exact 1:1, but there is then some
+shrinkage in processing.
+Optical printing with the aimframe method compensates for processing shrinkage.
+Shrinkage errors are too small to matter with simple printers.
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