Joined: 01 Aug 2006
Location: Richland, Washington State, USA
|Posted: Sun Nov 12, 2017 9:14 am Post subject:
First, when discussing magnification, it is much better to take pictures of a ruler than pictures of a butterfly whose size could be anything from a Blue to a Monarch.
With the ruler, you can immediately get a good number for magnification: just divide sensor width by the length of ruler shown in the picture.
But as you say, we can certainly see that the stacked lenses give more magnification than the 4X objective.
The reason for that must be "Both on closed focus". Reversing a 50 mm in front of 180 mm will reliably give 3.6X when both lenses are set on infinity focus. But when you set the lenses to focus closer, the magnification changes. Obviously in your case the magnification is getting much higher, something closer to 8X based on feature sizes and assuming your 4X setup is correct.
To get that same magnification with a microscope objective on your 70-200, you will need a 10X objective, with the 70-200 pulled back to around 160 mm focal length. Hopefully it will do that without too much vignetting.