This post was continued with some tests: https://www.dreamstime.com/thread_29668
Test finished (at least, a preview).
Test subject: a small part of acrylic on canvas covered with varnish (lake protection). For next test I will use a towel (thanks Afagundes).
Test conditions: Nikon D90 body, Nikkor 35mm f/1.8 AF-S DX, f/1.8 - f/22, Giottos MT9361 tripod, exposure delay mode ON (anti-vibration).
Tethered shoot (aperture changes was made from PC so the body was not moved).
Original Jpeg from camera (Standard, ISO 200, Noise reduction OFF, Long exposure noise reduction OFF). Click on images for bigger (but not full!) resolution; I uploaded the max/fine size images but Google blogger resized them to 1600 x 1063 pixels. Please wait for a center and margins crop or give me an idea how to upload on Google blog full res jpeg and RAW files.
Comments are welcome. Thanks to all from this post: https://www.dreamstime.com/forumm_29668_pg5 for helping me.
UPDATE:Another test with a towel and center CROP images uploaded: http://alveraphoto.blogspot.ro/2012/08/diffraction-on-smaller-aperture-values_14.html