So I shot a series of pics trying to focus on the same chalice mouth using first a single 36mm extension tube, then stacking a 20 mm on top then added the 12mm (so total of 68mm on the final shot. Then tried to crop the first and last shot the same. It was really hard to focus that last shot (lighting was pretty low, mostly blues). Kept the settings the same on all shots (f/7.1 , ISO 200, with aperture priority setting).
First with 36mm
peelx36mm by Peter Young, on Flickr
Then with both 36mm and 20mm
peelx36x20mm by Peter Young, on Flickr
Then with all 3
peelx36x20x12mm by Peter Young, on Flickr
Here is the crop of the first shot
peelx36mmcrop by Peter Young, on Flickr
And the same (sort of) of the last shot
peelx36x20x12mmcrop by Peter Young, on Flickr
And here's what the camera looks like (and how close to the glass I was on the final shot).
Untitled by Peter Young, on Flickr
First with 36mm
peelx36mm by Peter Young, on Flickr
Then with both 36mm and 20mm
peelx36x20mm by Peter Young, on Flickr
Then with all 3
peelx36x20x12mm by Peter Young, on Flickr
Here is the crop of the first shot
peelx36mmcrop by Peter Young, on Flickr
And the same (sort of) of the last shot
peelx36x20x12mmcrop by Peter Young, on Flickr
And here's what the camera looks like (and how close to the glass I was on the final shot).
Untitled by Peter Young, on Flickr