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10-31-2008, 07:56 PM   #1
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Calculating magnification-reversed lens

Folks:

While I wait for the arrival of my 50mm Takumar Macro, I decided to break out an old reversal ring and shoot with "poor mans" macro, where you mount two lenses to one another.

I mounted my SMC M 85mm f2 onto my K10D, then using the reversal ring I mounted a SMC M 50mm f2 on the front of that...reversed so the lens mount of the 50 was to the front. I extended both lens' focus rings to make the entire lens assembly as long as possible.

I got some pretty interesting shots of hairs on my knuckles (looked like 20 gauge wire!) and sharpened pencil points that filled my image. Needs lots of light and the dof was probably a millimeter or two, but the magnification was compelling.

I was going to jump on here and ask if anyone knew how to calculate the magnification of my reversed lens set up, but I got off my lazy butt and searched the Internet... and I found it:

To calculate the magnification of two lenses with one reversed:
magnification = focal length of prime lens, divided by the focal length of the reversed lens.
Example: 100mm forward lens\ 50mm reversed lens = 2X or double life size within a 35mm full frame.


Reference link: REVERSED LENS Photo Gallery by Dang at pbase.com

So, in my case, I had an 85\50= 1.7

Okay, so...here's a question: The above formula solves for "a 35mm full frame." What further math do I need to do to take account of the sensor crop?


Also, in the jargon of macro lenses, would my reversed lens setup be a 1:1.7 macro?

If you've not tried this technique, make the small investment in a 49mm reversal ring (under 10 usd) and give it a whirl. You're not going to sneak up on bugs in the wild with this set up (no dof) but the images are nearly always surreal and interesting.

I am attaching some images shot with this combination.

thanks

germar










Last edited by germar; 11-01-2008 at 01:52 PM. Reason: Adding pictures from macro setup
10-31-2008, 10:52 PM   #2
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11-01-2008, 02:10 PM   #3
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Photos added

I've added some photos taken with the 85mm/50mm combination to my original post. As I said, kinda neat.

regards,

germar
11-04-2008, 10:20 AM   #4
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Are they full-frame or crops?

11-04-2008, 10:36 AM   #5
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QuoteOriginally posted by germar Quote
Okay, so...here's a question: The above formula solves for "a 35mm full frame." What further math do I need to do to take account of the sensor crop?
None, the magnification is the same regardless of the size of the recording medium. If you have a 1:1 macro lens, you will be able to focus on objects that are as small as the size of your recording medium. On film you would be able to fill the frame with a bug 36mm long. On a Pentax DSLR you would be able to fill the frame with a bug 24.5mm long. Either way, it's still 1:1.

QuoteQuote:
Also, in the jargon of macro lenses, would my reversed lens setup be a 1:1.7 macro?
No, it would be a 1.7:1, or a 1:0.59.
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