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02-09-2021, 08:01 AM   #1
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macro 1:1 extension tubes question

Using the Asahi Pentax M 100mm macro 1:4 lens that takes 1:2 macro photos.


Which extension tube or combination of tubes would take a 1:1 macro photo?

From the web: The Asahi Pentax Auto Extension Tubes are a three piece set consisting of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26mm. Combined the three tubes have a total extension of 57mm.

02-09-2021, 08:50 AM   #2
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You read my mind! I have the same question!
02-09-2021, 08:54 AM - 1 Like   #3
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A lens needs total extension (including that provided by the focus helicoid) equal to its focal length to achieve 1:1 magnification. Your lens at closest focus already gives you 50mm of extension (hence 1:2 magnification), so adding 50mm gets you to 1:1.
02-09-2021, 09:43 AM   #4
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ok thanks!

I wonder what logic was behind the choice of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26m

02-09-2021, 10:14 AM   #5
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QuoteOriginally posted by Michael Piziak Quote
ok thanks!

I wonder what logic was behind the choice of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26m
I'm not sure what specific change would be more logical. Remember these are for use on any lens not just 50mm.
02-09-2021, 10:26 AM   #6
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QuoteOriginally posted by UncleVanya Quote
I'm not sure what specific change would be more logical. Remember these are for use on any lens not just 50mm.
You have a point. I was just thinking that even numbers would be more logical, like 10mm, 20mm, & 30mm, but the people that made them are much smarter than me.
02-09-2021, 10:32 AM - 1 Like   #7
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QuoteOriginally posted by Michael Piziak Quote
ok thanks!

I wonder what logic was behind the choice of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26m
They want to mess with your mind.

I have two sets of extension tubes, one from Soligor (12mm, 24mm, 32mm) and one from Vivitar (12mm, 20mm, and 36mm). Both total to 68mm. If it were 42 (the ultimate answer to life, the universe, and everything), it might make sense, but 68? I have had many sleepless nights due to this issue.

Sleepless nights aside, probably the easiest way to attain 1:1 with your lens is with a bellows. Leave the lens focused to infinity, rack the bellows out 90mm or so, and fine tune the magnification using the focus ring or bellows nobs to attain 1:1 (focus on a millimeter scale to a frame width of 36mm or 23.5mm depending on camera).


Steve
02-09-2021, 10:48 AM   #8
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QuoteOriginally posted by stevebrot Quote
They want to mess with your mind.

I have two sets of extension tubes, one from Soligor (12mm, 24mm, 32mm) and one from Vivitar (12mm, 20mm, and 36mm). Both total to 68mm. If it were 42 (the ultimate answer to life, the universe, and everything), it might make sense, but 68? I have had many sleepless nights due to this issue.

Sleepless nights aside, probably the easiest way to attain 1:1 with your lens is with a bellows. Leave the lens focused to infinity, rack the bellows out 90mm or so, and fine tune the magnification using the focus ring or bellows nobs to attain 1:1 (focus on a millimeter scale to a frame width of 36mm or 23.5mm depending on camera).


Steve
Big Smiles!

Well, achieving a perfect 1:1 doesn't bother me that much, and I know the extension tubes weren't made specifically for *my* lens.

02-09-2021, 11:22 AM   #9
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QuoteOriginally posted by Michael Piziak Quote
Which extension tube or combination of tubes would take a 1:1 macro photo?
I found a definitive answer! The Auto Extension Tube K (100mm)!

Reference found at: Pentax Close-up Accessories | The K-Mount Page
There is also an Auto Extension Tube K (50mm) if that option is preferred.


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02-09-2021, 11:28 AM   #10
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I think the minimum has something to do with the K-mount specifications. On all the Pentax lenses I checked the flange to tip of the aperture actuator lever is 12mm. So it would probably follow to use even numbers for the other tubes. My Pentax Auto Extension tubes measure out at 12, 20 and 26mm.

Edit: Ah yes, I forgot about the 100 and 50mm tubes. And the Dental version comes with a set of close-up lenses to get 1:1+ reproduction. I guess they thought an even longer lens would be intimidating to patients.

Last edited by Not a Number; 02-09-2021 at 11:38 AM.
02-09-2021, 11:38 AM   #11
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Adding the Raynox dcr-150 gives slightly more than 1.2:1, the dcr-250 gives 1.7:1 without loss of light. This is the same approach used by Codina with their 100mm f3.5 that came with a dedicated 1:1 closeup lens.

On an f2.8 macro adding tubes isnít too bad for focus and exposure. The f4 lens will get very dark very quickly as you add extension. Both methods are viable however.
02-09-2021, 01:54 PM - 1 Like   #12
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QuoteOriginally posted by Michael Piziak Quote
I wonder what logic was behind the choice of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26m
QuoteOriginally posted by Michael Piziak Quote
even numbers would be more logical, like 10mm, 20mm, & 30mm,
Well, 10 and 20 would combine to make 30 - Which you would already have!

So, a mixture of lengths that don't add up to each other in any combination gives you the widest range of possibilities. 12 / 19 / 26 gives you length options of 12, 19, 26, 31, 38, 45, and 57.
02-09-2021, 03:33 PM   #13
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QuoteOriginally posted by AstroDave Quote
Well, 10 and 20 would combine to make 30 - Which you would already have!

So, a mixture of lengths that don't add up to each other in any combination gives you the widest range of possibilities. 12 / 19 / 26 gives you length options of 12, 19, 26, 31, 38, 45, and 57.
Yes, but 30+20 = 50mm, which is what it would take to achieve 1:1 macro
02-09-2021, 03:37 PM   #14
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If you are using a macro lens you can adjust the extension on the lens.
02-09-2021, 03:57 PM   #15
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QuoteOriginally posted by Not a Number Quote
If you are using a macro lens you can adjust the extension on the lens.
Right.
I'm not that worried about it. Just was curious.
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