Question About Extension Tubes

[neuroanatomist]

So, as I ponder the last line----where did the .18 come from? If the native magnification is .12 x and the added magnification with the 25 mm ext tube is .05 x---shouldn't the end result be .17x?---Is this just a rounding error or adjustment, or is there something I am missing?

The 0.05x addition assumes that the 25mm extension tube is actually adding 25mm of extension. In fact, the EF25 is nominally 25mm in length, but actually provides 27.25mm of extension, meaning 0.1745x final calculated magnifiction (still not quite to 0.18x even with rounding). But, that calculation is based on the specified magnification, which is associated with a specified MFD of 177.2". Most lenses can manually focus closer than the specified MFD, so perhaps Bryan is actually empirically determining the magnifications (his tested MFD is closer than the specified MFD, meaning a slightly higher magnification).

[bob williams]

The 0.05x addition assumes that the 25mm extension tube is actually adding 25mm of extension. In fact, the EF25 is nominally 25mm in length, but actually provides 27.25mm of extension, meaning 0.1745x final calculated magnifiction (still not quite to 0.18x even with rounding). But, that calculation is based on the specified magnification, which is associated with a specified MFD of 177.2". Most lenses can manually focus closer than the specified MFD, so perhaps Bryan is actually empirically determining the magnifications (his tested MFD is closer than the specified MFD, meaning a slightly higher magnification).

Thanks John, That makes sense to me. I just wanted to make sure I wasn't missing something here.


Thanks


Bob