Stephane Savard wrote:I read that a DC-150 must be mounted with 208mm of extension, and the DC-250 with ~125mm of extension from the sensor
That word "must" is way too strong.
To explain...
When you move the tube lens away from its infinity focus position, you both 1) change the magnification and 2) introduce a certain amount of aberration, which
may or may not visibly degrade the image quality.
The amount of added aberration depends
strongly on the NA of the objective -- in fact it varies as something like NA raised to the fourth power. As a result, a change in extension that would be a serious problem at 20X NA 0.42 will be utterly insignificant at 5X NA 0.14, since the 3:1 ratio of NA's corresponds to an 81:1 ratio of aberrations.
With a low magnification objective, like that 5X NA 0.14 that I mentioned, you can change the extension almost however much you want, and added aberrations will not be a problem.
Also, what are the limitations of attempting the change magnification?
If you're looking for numbers, I suggest consulting the graph at
http://www.science-info.net/docs/etc/Tube-Length-na.gif . Line A reflects the amount of change that could be made in one experimental setup without visibly affecting the image quality due to added aberrations. The left-most point on the graph is at NA 0.25 and about 220 mm, reflecting an increase in the tube length of a 10X NA 0.25 objective from 150 mm to about 370 mm, more than doubling the magnification. In contrast, the middle point is at NA 0.50 and about 14 mm, which would be less than 10% change in magnification.
Another limitation is field width coverage. On APS-C, you can push Mitutoyo objectives down to about half their rated magnification. Beyond that, the corners are liable to go soft.
--Rik