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02-27-2011, 11:16 AM   #1
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why use an extension tube?

what is the purpose of an extension tube? rather than a teleconverter?

02-27-2011, 11:26 AM   #2
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QuoteOriginally posted by dh4412 Quote
what is the purpose of an extension tube? rather than a teleconverter?
macro
02-27-2011, 11:35 AM   #3
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QuoteOriginally posted by dh4412 Quote
what is the purpose of an extension tube? rather than a teleconverter?
An extension tube allows you to focus closer than you'd normally be able to focus with a particular lens. The flip side is you can't focus on things that are far away while the tube is on.

A teleconverter essentially magnifies the lens you are using and narrows the field of view (FOV).
02-27-2011, 11:44 AM   #4
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The glass of a teleconverter will degrade the image, how much will depend on the teleconverter and whether or not this is perceptible enough to matter depends on the individual. No added glass in extension tubes means cheap tubes (provided they have no light leaks) will easily be preferable to a cheap teleconverter.

Also, you can get way more magnification from tubes than a teleconverter. A $10 set of tubes can get you to 1:1 with a 50mm lens. You would need something like 8x worth of stacked teleconvertors to get 1:1 with the FA50/1.4. This would be expensive and darn lousy quality.

02-27-2011, 11:49 AM   #5
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so an extension tube can essentially turn a 50mm 1.7 lens for example into more of a macro lens? but at the same 50mm? sort of like getting the 50mm 2.8 macro ? without the cost? is the image quality unaffected? are they all the same length tube basically? does it make the lens into a manual focus ? guess i would have to understand the undelying optics better ? thanks.
02-27-2011, 12:15 PM   #6
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QuoteOriginally posted by dh4412 Quote
so an extension tube can essentially turn a 50mm 1.7 lens for example into more of a macro lens? but at the same 50mm? sort of like getting the 50mm 2.8 macro ? without the cost? is the image quality unaffected? are they all the same length tube basically? does it make the lens into a manual focus ? guess i would have to understand the undelying optics better ? thanks.
Yes, the extension tube keeps your lens at 50mm but just turns it into a macro. Like others have mentioned, when a lens is on the tubes, you loose ability to focus to infinity.
02-27-2011, 02:15 PM   #7
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thank you.
02-28-2011, 04:47 AM - 1 Like   #8
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There are two ways to get magnification: optics, and extension.

One favored optical trick is to reverse-stack a shorter lens (the SECONDARY) onto a longer lens (the PRIMARY). Magnification is calculated as MAG= PRIMARY / SECONDARY, so reversing a 35mm secondary onto a 105mm primary gives 105/35= 3x magnification. Control light with the secondary's aperture ring; leave the primary wide-open. But working distance is REAL close, under two inches, which can scare-off live bugs etc.

The other way is extension, with tubes and/or bellows. To reach 1:1 magnification, add as much extension as the lens' focal length -- 50mm of tubes for a 50mm lens, 135mm of tubes+ bellows for a 135mm lens, etc. With a 50mm lens, adding 100mm of extension takes you to 2x, 150mm to 3x, etc.

But magnification eats light. High magnifications usually require a tripod or flash. Flash gets tricky with manual tubes and bellows.

Longer lenses let you work further from a subject -- you can't focus closer than a lens' focal length, no matter how much extension you add. So, many cheap macro shooters use a lens around 100-135mm on tubes and/or bellows. The mantids are less likely to fly off when you are a few inches away.

Those of use who are REALLY cheap use sharp enlarger lenses (EL's) on extension. These are often under US$15 each, sometimes four for a dime. And an EL longer than 80mm can usually reach infinity focus on most bellows, and so is good for non-macro shots too. Let's see, should I spend a few hundred bucks on a 100/2.8 macro lens? Or put a US$15 105/3.5 EL on a US$30 bellows, and use it for macros AND portraits AND distant shots? I'm a cheap bastard, so you can probably guess my choice.

But a 50/1.7 lens on a US$8 set of tubes is a good place to start. Have fun!

02-28-2011, 04:54 AM   #9
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QuoteOriginally posted by dh4412 Quote
what is the purpose of an extension tube? rather than a teleconverter?
to decrease the minimum focus distance hence increase magnification ratio for macro shot
03-02-2011, 04:58 PM   #10
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so i just look for a pentax extension tube, on ebay or someplace? b and h?
03-02-2011, 05:21 PM   #11
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I got mine on ebay for something like 6 or 7 bucks. And free shipping!!!
03-02-2011, 06:50 PM   #12
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Drawbacks:

-Basically you could call the use of extension tubes for "analog cropping". By moving the lens further away from the sensor you cathc only the center part of the image circle. With digital cropping, you can also come closer, but lose of course part of the sensor resolution. Somewhat similar to this, with "analog cropping" you loose some of the lens resolution.
-You lose infinity, as someone already mentioned.
-You lose light, also mentioned above.

Advantages:
-inexpensive
-you can reach 1:1 macro
-while loosing some of the lens resolution, you don't downgrade the optical quality of the lens, because you add no extra glass

A bellow unit does the same as tubes, but you are not limited to the tube lengths.

Manual tubes are plenty and cheap.
Currently the only automatic tubes in production for Pentax are those from the German company "Dörr". About 100 Euro. With these you get all exposure modes. There are also used automatic tubes of several brands. Well worth the money if you ask me. These tubes should have 7 contacts that transfer information about the lens to the camera. Note that for some years in the 80's there were automatic tubes with 6 contacts. These only inform the camera about max and min apperture of lens. You are limited to center weight meetering and loose some flash functions and some more, but can still use P, Tv, Av etc.
Fully manual tubes hasn't even an mechanical apperture transfer, so these you have to run all manual. Those with mechanical apperture transfer would have been called automatic before 1983, and with todays cameras you can use the green button to meeter through them.
03-02-2011, 07:30 PM   #13
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Extension tubes or bellows (do the same thing, just some different conveniences) are for macro. Tele-converters are for increasing focal length (2x convert turns 50mm into 100mm).

With a Teleconverter you loose light. With a 2x converter you have lost 2 stops of light intensity. A normally f2 lens is now effectively an f4 lens. If your teleconverter has electronic and mechanical linkages, then just operate your camera normally. Otherwise you need to use stop down metering techniques (see your manual) to set the exposure.

You loose light with extension tubes and bellows also. It is based on the inverse square law of light and is also called "bellows factor". The amount of light lost is based on the focal length of the lens and the distance that lens is from the sensor (image distance). Unless you have the Automatic Extension Tubes (Pentax) which allow the electro-mechanical linkages from the camera body to the lens pass through the tubes, then you will be forced to use stop down metering. Also, you cannot focus to infinity once you attach the lens extension system.

A macro lens works like the extension tubes do, allowing you to focus closer, thereby magnifying the subject, but since it is attached directly to the camera as a lens, all controls should work normally. Also, you can focus to infinity, unless you use a lens extension with it! Macro lenses are better for CU work as they are designed to focus close and should give less optical distortions than a typical lens.

A lens extension does not distort the image as there is nothing added to distort it! If there are distortions perceived, it is due to the lens used, not the lens extension system.
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