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09-20-2010, 08:27 PM   #1
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Sigma 70mm Macro Extension Tubes

I have a Sigma 70mm DG Macro Lens (pentax k mount). I was thinking about adding some extension tubes to it. Which extension tubes will be compatible with this lens? How will I control the apperture or stop down with the extension tubes in place since the Sigma lens doesn't have any manual apprerture ring?

09-20-2010, 10:44 PM   #2
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Get an extension tube with electrical contacts. It may be difficult to find. You can also get a teleconverter with electrical contacts and remove the glass.

In the photo below, the one on the left is a "proper" extension tube and the one on the right is a gutted teleconverter. Either one will work (manual focus only).

Of course you can take an autofocus TC and remove the glass to make an autofocus extension tube. But an autofocus TC tends to cost more and and the advantage of autofocus is small when it comes to macro photography with extension tube.

09-21-2010, 12:05 AM   #3
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thanks for the reply Soldbear. Removing the glass from TC sounds like a good trick but expensive compared to buying simple tubes. I googled around a bit more and came to this site which described another "hack" to set the apperture. I'll post it here in case someone else comes looking with the same question and source the original site. I hope this is okay.
From About macro extension tubes. :
"What i wsa describing was a hack to have control over the aperture setting when using “dumb” extension tubes that don’t connect electronically to the camera. The only way to set the aperture of the lens is with this trick. First, with the lens you want to use with the extension tubes mounted on the camera, you set your desired aperture. Then, with the camera turned on, you press and hold the depth of field preview button while removing the lens from the camera body. This should leave the iris in the lens set to the desired aperture. You can then mount the lens to the tubes and the tubes to the camera, and shoot with your desired aperture. I hope this answers your question!"
09-21-2010, 03:51 AM   #4
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QuoteOriginally posted by Rashedul Quote
I googled around a bit more and came to this site which described another "hack" to set the apperture.
That 'hack' sounds really clumsy. And if an auto lens set on auto is removed from the camera body, why should the iris stay fixed at the desired f-stop? The lens has no memory. I just tested that on my K20D with some A and FA and DA lenses and get this result: whatever f-stop you set the lens to while on-camera, the lens stops down all the way when removed. That 'hack' doesn't work.

Anyway, the main benefit of aperture automation in macro work is: P-TTL FLASH! This won't work right without the camera controlling the aperture. And you lose that control whenever ANY dumb extension is used.

09-21-2010, 04:06 AM   #5
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There are "automatic" extension tubes which control the aperture, but very few which feed lens contacts through. I am not aware of any that work with AF drives

Regarding tubes with contacts, vs. chopped teleconverters there are lots of PKA teleconverterss out there for $10-20 and if you pull out the glass you are left with a hollow tube with contacts and aperture linkage. Two or three of these will cost less possibly than a set of extension tubes
09-21-2010, 07:31 PM   #6
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RioRico thank you for testing the "hack" and letting me know it doesn't work. I was about to invest in some dummy extension tubes because I was convinced that it was going to work. Alright back on the hunt for cheap PKA tc. Maybe checkout the local Henry's outlet where they sell a lot of used gear. Thanks everyone for all the help.
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