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Image magnification

 
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Arwin



Joined: 13 Aug 2015
Posts: 16
Location: Holland

PostPosted: Sun Nov 12, 2017 5:41 am    Post subject: Image magnification Reply with quote

Hello,

I bussy with a project.

Ive got a question. I making my stacks with a Tamron 180 mm 3.5 with a canon 50 mm 1.4 reversed on it in front. Both on closed focus. And with a Canon 5D Mark III

This is a sample of it:



And i have tested with a cheap 4 x infinity miscroscope lens on my Canon 70 - 200 2.8 L IS on 200mm

Here is a sample of it:



When i calculat it good the tamron combination must give me about 3.6 x magnification ( 180 / 50 = 3.6 x )

And the microscope lens should give me 4x magnification on 200 mm. But in the images you can see the Tamron combination is closer than the 4 x microscope. How is this possible?

I want to go over to use my 70-200 with a miscroscope lens to get bettere IQ. But i want to have about the same maginification as what i have with my Tamron combination!

Greetings Arwin Lubbers
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rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 18250
Location: Richland, Washington State, USA

PostPosted: Sun Nov 12, 2017 9:14 am    Post subject: Reply with quote

Arwin,

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.

--Rik
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