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01-10-2010, 07:25 PM   #1
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How to use extension tubes?

I just picked up an amazingly good condition smc Takumar 50mm macro and set of genuine Pentax extension tubes at this awesome camera store I just found in town. I know the lens gets a magnification of 1:2, but I have no clue how to use extension tubes to get 1:1. I also have an old 100mm macro that also does 1:2.


Anyone care to explain how it all works?

01-10-2010, 08:11 PM   #2
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On the 50mm, 50mm of extension should give you 1:1
01-10-2010, 08:22 PM   #3
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Awesome!

Now to measure my tubes to figure out how to get 50mm.


What about the 100mm?
01-13-2010, 10:16 AM   #4
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100mm

Although I never quite understood how it works with lenses of the same focal length that have different minimum focus distances and max magnifications - can anyone explain?

I know for a give FL, when it is the FL mm away, will give 1:1. How does this work if the 50 mm lens already goes to 1:1 when compared to a 50mm with a max magnification of 1:4? The focus ring already moves the lens elements 50mm in the 1:1? How far does it move in the 1:4? So increasing it would bring it to what, 2:1?

01-13-2010, 12:13 PM   #5
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QuoteOriginally posted by Eruditass Quote
100mm

Although I never quite understood how it works with lenses of the same focal length that have different minimum focus distances and max magnifications - can anyone explain?

I know for a give FL, when it is the FL mm away, will give 1:1. How does this work if the 50 mm lens already goes to 1:1 when compared to a 50mm with a max magnification of 1:4? The focus ring already moves the lens elements 50mm in the 1:1? How far does it move in the 1:4? So increasing it would bring it to what, 2:1?
magnification_change = extension/focal_length

If initial magnification is .25X (1:4) and focal length is 50 then an extension of 50 gives:
mag= .25+50/50=1.25X

If initial mag is 1X
01-13-2010, 12:19 PM   #6
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QuoteOriginally posted by Eruditass Quote
100mm

Although I never quite understood how it works with lenses of the same focal length that have different minimum focus distances and max magnifications - can anyone explain?

I know for a give FL, when it is the FL mm away, will give 1:1. How does this work if the 50 mm lens already goes to 1:1 when compared to a 50mm with a max magnification of 1:4? The focus ring already moves the lens elements 50mm in the 1:1? How far does it move in the 1:4? So increasing it would bring it to what, 2:1?
magnification_change = extension/focal_length

If initial magnification is .25X (1:4) and focal length is 50 then an extension of 50 gives:
mag= .25+50/50=1.25X

If initial mag is 1X (1:1) and extension is 30 and focal length is 50 then:
mag= 1 +30/50=1.6X

If the initial mag is zero and focal length is 50 and the final mag is 0.25X then

0.25=extension/50 so extension = 50*.25= 12.5mm
01-13-2010, 01:31 PM   #7
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Cool, t hanks for the info
01-13-2010, 02:43 PM   #8
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QuoteOriginally posted by Eruditass Quote
Cool, t hanks for the info
Don't worry overly about the numbers though (thanks for doing the research Newarts)
The object is to increase magnification and you would use the required number (and sizes) of tubes for the subject to fill the frame (to the extent that you want).

12-17-2010, 03:16 AM   #9
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Just want to check that these will work with my K20D?
I understand that I will have to use these in Manual mode, but I will be using these with a Pentax -M 100mm f4 Macro Lens

Pentax Macro Extension Tube Ring | Virtual Village - UK

Just checking before I buy...
12-17-2010, 03:55 AM   #10
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QuoteOriginally posted by edgedemon Quote
These look like the ones I got from a Chinese ebay seller the other day. They work ok, but the lens needs an aperture ring and that is used to set aperture; there is no aperture coupling, these are simply a set of tube sections with compatible threads and lens + body mount piece. So only M-mode works. (Or strictly speaking Av mode with aperture set by the ring, kind of, works too ...).

Last edited by jolepp; 12-18-2010 at 03:08 AM.
12-18-2010, 01:32 AM   #11
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Those Virtual Village tubes are terribly overpriced. I see the same tubes on eBay for US$7, shipping from China included. I have four (4) sets. They are useful for much more than just macros: impromptu odd-lens mounts, adapters, lens hoods, etc. As mentioned above, such tubes have no aperture automation. Also, they are for PK-mount lenses. You said you bought a Takumar macro lens and tubes -- are they PK bayonet mount, or M42 screwmount? If M42, you will also need a cheap (US$4) safe flanged non-infinity-focus M42-PK adapter. Whew!

Here are some basics of tube use:

To reach 1X magnification, you need total extension that is TWICE the lens focal length. A camera lens (unlike an enlarger lens, often used for macros) has some of that extension already built-in. So for 1X, a 50mm camera lens needs 50mm of tubes, a 100mm lens needs 100mm of tubes, etc. A 50mm lens on 100mm of tubes gives you 2X magnification. NOTE: These numbers are based on the lens being at infinity focus. Focusing closer increases magnification -- on a standard lens, not much -- on a macro lens, quite a bit.

Magnification eats light. Figure your magnification, add one, and that's the number of f-stops of light lost. So a 50mm lens at f/4 on 100mm extension has 2X magnification and loses 3 f-stops, so you're effectively at f/11. But that narrower effective aperture does NOT increase your DOF (depth of field) -- DOF becomes very very shallow when shooting macro.

No non-reversed lens can focus closer than its focal length. No matter how much extension you add to a 50mm lens, it can never focus closer than 50mm. Many people prefer 100mm or 150mm macro lenses because the longer focal length allows (or forces) shooting from further away. A 28mm macro lens is great if you want to get REAL REAL CLOSE; 150mm is better if you don't want to frighten the bugs away.

Tubes are good, cheap tools; bellows cost a bit more but are more flexible in usage. Ask us about bellows sometime, eh?
12-18-2010, 01:36 AM   #12
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Extension tubes, Simple explanation

For any lens, 1:1 magnification is reached when the lens to subject distance is 2 times the focal length, and this is also when the lens to focusing plane is 2 times focal length.

For any lens this means that you need to have an extension tube equal to the focal length to reach 1:1.

A macro lens stating 1:2 means that the focusing helix can move the lens out to about 50% of the focal length, or for a 50mm lens an additional 25mm, so to get focusing between 1:2 and 1:2 use another 25mm. the more you use the higher the magnification
12-18-2010, 04:16 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote
For any lens, 1:1 magnification is reached when the lens to subject distance is 2 times the focal length, and this is also when the lens to focusing plane is 2 times focal length.

For any lens this means that you need to have an extension tube equal to the focal length to reach 1:1.

A macro lens stating 1:2 means that the focusing helix can move the lens out to about 50% of the focal length, or for a 50mm lens an additional 25mm, so to get focusing between 1:2 and 1:2 use another 25mm. the more you use the higher the magnification

OK, so for a 100mm macro lens that does 1:2 magnification I need 50mm tubes to get 1:1?
12-18-2010, 04:50 AM   #14
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QuoteOriginally posted by edgedemon Quote
OK, so for a 100mm macro lens that does 1:2 magnification I need 50mm tubes to get 1:1?
look here: (there's a calculation tool)

Macro Extension Tubes & Close-up Lenses
12-18-2010, 05:52 AM   #15
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QuoteOriginally posted by edgedemon Quote
OK, so for a 100mm macro lens that does 1:2 magnification I need 50mm tubes to get 1:1?
By the above it would seem so: 1/2 + 50/100 = 0.5 + 0.5 = 1 (= <inital mag ratio> + <extension length>/<focal length>).
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