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FORUMS Photography Talk by Genre Macro Talk
Thread started 26 Jan 2018 (Friday) 10:20
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Macro lenses and focal length

 
Lester ­ Wareham
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Jan 29, 2018 00:45 |  #16

Dalantech wrote in post #18549361 (external link)
Does anyone have a good explanation of why macro lenses with internal focusing lose focal length at minimum focus? I know it has something to do with how the lens elements are shifting, and possibly even the aperture assembly moving. Can't find any resource on the web and my "Goolge Fu" is usually pretty good.

There is an explanation in the Canon Eyes of EOS book. Internal focus lenses shorten the lens focal length moving the exit pupil away from the sensor making the lens focus closer. This is obvious on macro lenses due to the higher magnification compared to general lenses.

A side effect of this is the lens pupillary magnification changes slightly making a second order change in the theoretical light loss and depth of field for a given set T-stop.


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Dalantech
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Post has been last edited 25 days ago by Dalantech. 2 edits done in total.
Jan 29, 2018 01:55 |  #17

Wilt wrote in post #18549535 (external link)
I think you may be jumping to a bad a conclusion in only needing 37mm to get to 2:1.
The 60mm focuses to 0.2m and achieves 1:1 at that distance...4*FL rule means the lens is effectively acting like a 50mm lens at 1:1

I photographed a metric ruler at 1x, and then again with 37mm of tubes. The available number of millimeters in the frame dropped in half. Me thinks that lens design is causing a bigger drop in focal length than your math allows.

Edit: Although I've testing the EF-S 60mm multiple times with tubes I'll take another look tonight when I get home.


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Dalantech
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Jan 29, 2018 02:15 |  #18

For Brian and Wilt: The MPE-65mm is built like a reversed lens with a variable length extension tube. There is a "floating" group of lens elements that moves to improve the focus of the lens as the magnification goes up. Although I shoot with it all the time stopped down to F11 (F16 @ 1x) I'm still surprised at how sharp the lens is.


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Wilt
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Post has been last edited 25 days ago by Wilt. 3 edits done in total.
Jan 29, 2018 11:28 |  #19

Dalantech wrote in post #18551487 (external link)
I photographed a metric ruler at 1x, and then again with 37mm of tubes. The available number of millimeters in the frame dropped in half. Me thinks that lens design is causing a bigger drop in focal length than your math allows.

Edit: Although I've testing the EF-S 60mm multiple times with tubes I'll take another look tonight when I get home.

Your additional explanation makes its palusibility more understandable to me. Take a lens from 1:1 to 2:1, ordinarily would entail going from

ExtenA/FL = 1:1 where ExtenA=50mm and FL=50,
and then to ExtenB/FL to 2:1 where ExtenB = 100 (incremental 50mm) and FL =50,

Applying that same methodology, the incremental 37mm would indeed seem to indicate the lens FLb had indeed changed


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Jan 30, 2018 01:42 |  #20

Wilt wrote in post #18551699 (external link)
...Applying that same methodology, the incremental 37mm would indeed seem to indicate the lens FLb had indeed changed

That makes sense.


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Macro lenses and focal length
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