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This article shows the use of a cheap 3-segment macro extension tube I got off eBay.
The lens is a Pentax-M 50/F1.7, which is MF & ME, used on a Pentax K-5. MF is not a problem in macro photography.
The lens should be capable of manually setting the aperture. This is an ideal application for sharp old primes.


Here is the macro extension tube.



The 1st segment incorporates the K-mount lugs. This can not be unscrewed.

The 4th part is the K-mount socket which will hold the lens. The socket is attached to whatever ends up as the last segment.
You can get various lengths by using segments 1, 1+2, 1+3, 1+2+3.


Here are segments 1+2 with the lens mounted.




First off, just the lens on the camera, at the closest focus distance.
The 23.6mm x 15.6mm rectangle on a piece of paper represents the size of an APS-C sensor.
(The line across the middle of the rectangle has marks every 5mm from the left.)


The width of the rectangle is 15% of the image width, so the magnification is 0.15x,
i.e. the image width on the sensor (approx. 3.5mm) is 0.15x of its original width.


Now we use the lens with segments 1+2 (combined length of these segments = 32.5mm).



The magnification is about 0.76x. At this magnification, a 10mm-wide object would be 7.6mm wide on the sensor.

The formula is:
Magnification = ext. length / lens FL

Magnification = 32.5/50 = 0.65x

The discrepancy in the measured magnification compared to the formula in this and the next example seems to indicate
that the actual FL of this "50mm" lens is probably 43-45mm.


Here is the result using segments 1+2+3 (length = 60.5mm).



The object width that now fills the image is 17.4mm, so this is a magnification of 23.6/17.4 = 1.35x.
Therefore, a 10mm-wide object would occupy 13.5mm of the sensor width.

Note: these shots were done hand-held and it's hard to stay steady and at the correct focussing distance this way.
You really need a suitable tripod.

Dan.

[photoleet]

Wow nice guide. Just what I needed to get started.

Few questions.

Where can I buy these segments ? How is the picture maginified so much with just using 1+2 (like does the tube zoom in on the center or something) ?

Shouldn't the final picture be sharper ?

[dosdan]

Where can I buy these segments ? How is the picture maginified so much with just using 1+2 (like does the tube zoom in on the center or something) ?

Just do a search on eBay for: pentax macro extension tube. You'll find plenty of results.

The tubes are empty. (That's why they're so cheap.) They distance the lens further from the camera while preventing light getting in. It's like the magnification you see when you move a magnifying glass away from you. More of the scene falls outside the sensor periphery as you move the lens further away so the part that does fall on the sensor is now bigger, in relation to the width of the sensor, then it would be if the lens was mounted directly on the camera. And dimmer - same number of photons from the object of interest falling over a bigger area.

I usually use a tripod when I do macro photography. This demonstration was hand-held for speed. I really just wanted to show the effect.

Dan.

[photoleet]

Just do a search on eBay for: pentax macro extension tube. You'll find plenty of results.

The tubes are empty. (That's why they're so cheap.) They distance the lens further from the camera while preventing light getting in. It's like the magnification you see when you move a magnifying glass away from you. More of the scene falls outside the sensor periphery as you move the lens further away so the part that does fall on the sensor is now bigger, in relation to the width of the sensor (and dimmer - same number of photons from the object of interest falling over a bigger area), then it would be if the lens was mounted directly on the camera.

I usually use a tripod when I use do macro photography. This demonstration was hand-held for speed. I really just wanted to show the effect.

Dan.

cool!

So something worth $7 like this 3 Ring Macro Extension Tube f Pentax K100D K10D K200D | eBay

works ?

So does this allow 1:1 magnification like a true macro lens ? And are there any downsides to using them like decreasing picture quality ? And how do I use automatic focus lenses with them, like 18-55 kit lens ?

[dosdan]

cool!

So something worth $7 like this 3 Ring Macro Extension Tube f Pentax K100D K10D K200D | eBay

works ?

So does this allow 1:1 magnification like a true macro lens ? And are there any downsides to using them like decreasing picture quality ? And how do I use automatic focus lenses with them, like 18-55 kit lens ?

A true macro lens, not a telephoto zoom with "macro" in its model name, is designed differently. It is very sharp and has flat curvature-of-field: on a standards lens, the edges can be slightly out of focus compared to the centre - you won't notice this normally, but it can become visible with magnification. In the example I presented, we went from 0.15x to 1.35x using a 60.5mm extension - that's 9 times greater/closer than the lens was designed for.

For the same reason, other lens imperfections like chromatic aberrations are also magnified. That's why true macro lens are more expensive - they have to be of very good quality.

You can't use your kit lens. It's a DA lens - it does not have an aperture ring on the lens that you rotate to control the aperture. That's where now-cheap old manual lenses, like the Pentax-K & Pentax-M series, are a good proposition. The other thing is that a prime (non-zoom) lens usually has significant better quality than most zoom lenses so you will end up using it for more than just macro work.

Dan.

[dosdan]

Based on the magnification of a subject of known size, I said that the true FL of this Pentax-M 50/F1.7 lens is about 43-45mm. I was wrong. The FL appears smaller when the subject is close.

Based on the angular separation of two far away subjects, the stars Betelgeuse & Rigel in the constellation of Orion, and using the method described here:

https://www.pentaxforums.com/forums/pentax-lens-articles/169225-using-2-stars...-distance.html

I've determined that the real FL of this 50mm lens is 52.7mm. This is probably accurate to about 0.5% or better.

Dan.

[pacerr]

Well done -- a practical demonstration of lens extension effects, Dan. It might also be noted that any method of extending the lens achieves the same effect -- bellows, tubes, (napkin rings?) etc; and that TTL metering pretty much solves the exposure compensation issues.

I find it useful to suggest to noobies that just hand-holding the lens in front of the body will provide a crude demonstration of the effect -- and that cutting a cylinder from a paper tube about equal to half the FL of the lens offers a more stable experiment at no cost.


That's an interesting use of Betelgeuse & Rigel for calibration that I hadn't seen before, too; I'd note that's a much easier task than bringin' 'em down to a watery horizon one at a time with a sextant at sea!

H2

[Jools]

Nice demonstration of the extension tubes.
Thanks for posting.
This pretty much my setup times 2.
These cheap Ebay tubes works wonders.
And yes, a good tripod sure helps. I also use a 4-way macro focusing rail (also from Ebay)

[jlaubza]

I have the same tubes and lens! The curvature of field is more extreme than Dosdan makes out. This reduces the effectiveness of this set up for extreme closeups - one is limited to a small portion of the center field. The effectiveness of this setup is likely to improve if one uses a longer focal length prime lens but I dont know what is out there in the old but good category.

But the lens is wonderfully sharp - it is a pity that the images are hand held - the end result at modest magnification can be gratifying. A small tripod is very handy for studio shots.

Regarding the use of kit lenses, this can be done! Simply stick the aperture lever back with some tape while the lens is off the camera.

[Jan67]

Theory and praxis of magnification with macro rings

There is an interesting issue of variable magnification of the lens+macro ring according to focusing distance.

According to macro math, for example: www.peterforsell.com
the magnification formula is: M= ((mL x F) + E) / F
where
M = magnification
mL = the magnification of the lens alone (0.15 for A50/1.4)
F = the lens focal length in millimeters
E = added extension in millimeters

Magnification in this formula should be constant for given F and E.
I made test with A 50/1.4 and 20mm extension ring.

1) Picture of a ruler, with focusing ring set up on the shortest distance gave 43mm (the sensor wight 23.7mm). So the magnification is 23.7/43=0.55, what is the same as formula

2) Picture of a ruler, but this time with focusing ring set up on infinity gave 60mm, so the magnification is 60/23.7= 0.40

3) I also made test with the DA 50-135+20mm ring. The diference of magnification was much smaller, ca 0.40 vers 0.42. The reason might be internal focusing.


Summary:
Above mentioned formula is not valid for different positions of the lens focusing ring (and focusing distance) . In case of A50/1.4 the formula was valid just for the shortest focusing distance (0.45m).
The reason for that (correct me if I am wrong) could be that lens with macro ring changes its focal length according the position of the focusing ring.

[Stone G.]

Theory and praxis of magnification with macro rings

There is an interesting issue of variable magnification of the lens+macro ring according to focusing distance.

According to macro math, for example: www.peterforsell.com
the magnification formula is: M= ((mL x F) + E) / F
where
M = magnification
mL = the magnification of the lens alone (0.15 for A50/1.4)
F = the lens focal length in millimeters
E = added extension in millimeters

Magnification in this formula should be constant for given F and E.
I made test with A 50/1.4 and 20mm extension ring.

1) Picture of a ruler, with focusing ring set up on the shortest distance gave 43mm (the sensor wight 23.7mm). So the magnification is 23.7/43=0.55, what is the same as formula

2) Picture of a ruler, but this time with focusing ring set up on infinity gave 60mm, so the magnification is 60/23.7= 0.40

3) I also made test with the DA 50-135+20mm ring. The diference of magnification was much smaller, ca 0.40 vers 0.42. The reason might be internal focusing.


Summary:
Above mentioned formula is not valid for different positions of the lens focusing ring (and focusing distance) . In case of A50/1.4 the formula was valid just for the shortest focusing distance (0.45m).
The reason for that (correct me if I am wrong) could be that lens with macro ring changes its focal length according the position of the focusing ring.

It will actually often be a bit simpler than that. What you call "magnification of the lens alone" mL is not a constant, but a reproduction ratio that varies with focusing distance. If you focus on a truly remote object, say a bird one kilometer away - or better: A giant star several hundred light years away - the image size of that bird or that star wil effectively be zero and mL will be zero too! Insert that in your formula above and see, what happens.

The term "magnification", when mL is stated as a specific constant, for example in a data sheet, only holds for the closest focusing distance, which will depend upon the actual, mechanical and optical design of a specific lens.

For the general situation somewhere between infinity and closest focusing distance, you can estimate the reproduction ratio quite accurately from the simple lens formula. With your nomenclature above it will read:

With focusing distance object-sensor = L; distance from object to optical center of lens = s; distance from optical center of lens to sensor = b you have:

1/F = 1/s + 1/b
L = s + b
mL = b/s

Finally, just to complicate matters a bit, the focal lenght can indeed vary dependent upon focusing distance. Any lens with internal focus and/or lens groups moving relatively to each other with varying focusing distances will show that behaviour. But tha's not the case with your 50/1.4 lens.

[Jan67]

Thanks for explanation !

[Audionut]

Sorry to bump an old thread.

Extension tubes can be used with auto focus lenses on Canon by setting the aperture, holding the DOF button and slightly releasing the lens.
I assume Pentax has a DOF button!

[dosdan]

Extension tubes can be used with auto focus lenses on Canon by setting the aperture, holding the DOF button and slightly releasing the lens. I assume Pentax has a DOF button!

The power switch lever can be rotated to a Preview position. The preview can be either digital or optical. Optical preview can be used to check DOF because it stops down the lens to the set f-stop. In Pentax cameras, this stopping down is down mechanically, not electrically. Cheap ext. tubes have no connection either mechanical or electrical, so MF & ME. Fancier ext. tubes for a Pentax can have an electrical connection but this will not set a specific aperture. In this circumstance, the aperture ring (on older Pentax glass) can be moved from an "A" (AE) position to a specific manually-set f-stop.

Some of these concepts are covered here:

https://www.pentaxforums.com/forums/pentax-lens-articles/211619-using-super-t...ax-k-01-a.html

Dan.

[LSR]

I got a cheap set off a Chinese website for $5 including shipping, but they actually seem decent. For those asking it was everbuying.com

This thread has motivated me to get them out of the camera bag and experiment with them, thank you very much!