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Body reversed 50mm macro lens magnification

 
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LordV



Joined: 22 Nov 2007
Posts: 1568
Location: UK

PostPosted: Wed Mar 31, 2010 2:07 pm    Post subject: Body reversed 50mm macro lens magnification Reply with quote

A question that came up on another forum had me wondering what magnification do you get if you reverse a 50mm 1:1 macro lens onto the camera body and use it at minimum focus ?

No idea if there is a calculation for working out rough mags of body reversed lenses but I know from other forums you get around 1:1 magnification with a normal 50mm lens body reversed but would you get 2:1 with a macro 50mm lens ?

Brian v.
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canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Wed Mar 31, 2010 5:17 pm    Post subject: Reply with quote

Don't know about a 50 mm.

For amusement, I tried reversing a Sigma 105 mm. With the ring twisted to what would be 1:1 in normal orientation, the lens needed a little bit of extension simply to focus at infinity -- magnification zero. Twisting it to any lesser magnification setting required still more extension to focus at infinity.

It all depends on where the front principal plane is located.

With those old fixed-focal-length lenses like we used to use on 35 mm SLRs, the front plane was located way behind the front of the lens, actually back close to the mounting flange. Reverse the lens and add the thickness of the reversing ring, suddenly that plane is a couple of FL away from the sensor plane, and there's your 1:1.

With the 105 that I just tested, the front plane is only a couple of cm in back of the front of the lens. Reverse that lens and the plane is not even one FL away from the sensor, which is why more extension has to be added just to focus on infinity. Twisting the magnification ring on the lens probably moves its front plane a little bit, but more importantly it also shortens the FL, which is why less additional extension is needed when the lens is set to high magnification.

I have very little idea where the principal planes might be located on a 50 mm macro lens, so I'll be interested to hear if somebody else runs any experiments.

--Rik
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LordV



Joined: 22 Nov 2007
Posts: 1568
Location: UK

PostPosted: Thu Apr 01, 2010 12:33 am    Post subject: Reply with quote

Did wonder where the focus point might be at min focus presumably there is a chance it might work reversed onto extension tubes ?
The original question I was asked was the likely magnification of a reversed 50mm macro lens on to 68mm of extension tubes - I reckoned on between 2.5:1 to 3.5:1 depending on whether focus could actually be achieved with the lens at min focus.

Brian v.
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mgoodm3



Joined: 08 Sep 2008
Posts: 273
Location: Southern OR

PostPosted: Thu Apr 01, 2010 9:35 am    Post subject: Reply with quote

The mag that you get varies with the lens. how far back are the elements from the front, how long is the lens?

55/2.8 micro - 0.9:1
50/1.4 - 0.97:1
El-nikkor 50/2.8 - 0.54:1

55/2.8 with 50 mm extension - 1.8:1
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rjlittlefield
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Joined: 01 Aug 2006
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PostPosted: Thu Apr 01, 2010 9:51 am    Post subject: Reply with quote

mgoodm3 wrote:
55/2.8 with 50 mm extension - 1.8:1

Is this front-forward with an extension, or reversed with an extension?

I am presuming the others are reversed, with no additional extension.

--Rik
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mgoodm3



Joined: 08 Sep 2008
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PostPosted: Thu Apr 01, 2010 10:00 am    Post subject: Reply with quote

All that I listed are reversed. No extension unless specified.
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rjlittlefield
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PostPosted: Thu Apr 01, 2010 11:06 am    Post subject: Reply with quote

What is the maximum magnification of the 55/2.8 in its normal orientation with 50 mm of extension? (Somewhat over 2:1, I would expect, assuming that the lens goes to 1:1 just by turning the focus ring.)

--Rik
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mgoodm3



Joined: 08 Sep 2008
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Location: Southern OR

PostPosted: Thu Apr 01, 2010 11:46 am    Post subject: Reply with quote

Normally mounted, the 55/2.8 gives you 1.43:1 with 50 mm of extension.

It only focuses to 1:2. It's one of the older MF ones.
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