Canon must be giving us the run around if they say back focus is 44mm.
Or maybe
As you mentioned, something else is going on. It could be the optical thickness is not exactly 1mm, the adaptor or bayonet on the lens itself are all contributing to spacing errors. Either one of them, or all of them by a tiny amount. The focus point might be razor thin? However as you've been doing, trial and error is in order and your findings are helpful.
Quote:
Originally Posted by Ryderscope
Ok, I must apologise. My original calculations were correct and the adapter size should be 12.5mm. My bad and sorry for the confusion  . Your logic is perfectly correct which begs the question then as to why I had to wind back the adapter spacing to 10.5mm before I could get a flat field .
To demonstrate the results, see attached images showing a section of the test subs taken and a copy of the output from the eccentricity measurement script in Pixinsight. Shown in the test images is a section of the test frame that had an open cluster that was useful to gauge the flatness across the field. The image in the left pane is with the spacing at 10.5mm and the image on the right pane is with the spacing at 12.5mm. There is a significant and obvious improvement on the image on the left.
Also shown in the attachment is the output from the FWHMeccentricity script in Pixinsight. A measurement of 0.45 is a typical guide of an acceptable level of eccentricity in an image. Anything below that is a bonus. Again, the right hand pane (12.5mm spacing) shows a classic radial pattern emanating away from the centre of the image. The eccentricity map on the left hand pane (10.5mm spacing) shows an acceptable amount of eccentricity across the field.
Why is it so then that the practical implementation does not match the theory? The only obvious answer is that the optical thickness for my filters does not match specifications and/or the actual set back distance for the Canon lens does not match specifications. I did measure the adapter width very carefully with digital calipers so I am confident that I had that correct.
Thanks for everyone's input so far and it is good to confirm my original calculations. I would appreciate further input if anyone has a good theory to offer as to why the reality does not match the theory in this case.
CS,
Rodney |