Teleconverters or extenders

Hypilein

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Definitely not recommended to use these on a lens that has an extending barrel. The only front-mount teleconverters that are high quality also happen to be very heavy, and so unless you are able to support the barrel all the time while in use, you can potentially damage lenses like the 45-150mm or 100-300mm that telescope when they zoom.
This is correct and also why the 45-175 which is a non-extending lens is the only lens that seems to have been discussed for using front mounted tcs with reasonable results.
 

barry13

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Unless you very carefully take off the little tab that keeps the MC-14 from attaching to other lenses. :)

That may not make it work with other "native" lenses, but it does allow it to work via an adapter on non-native lenses.
Hi, are you referring to 4/3 lenses?

Otherwise, I've found it works without modification on standard SR/MD and m42 adapters/lenses.
 
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are you referring to 4/3 lenses? Otherwise, I've found it works without modification on standard SR/MD and m42 adapters/lenses.
Darn, I can't recall the specific reason I removed the tab… it may well have been that it would not work with a 4/3rds adapter…

You got my curiosity up, and yes, by putting on any "smart" adapter I have (no-name 4/3rds —> µ4/3rds, MMF-2, MMF-3, and no-name EF —> µ4/3rds), I can sight down the adapter and see that the tab would keep it from attaching. On the other hand, all the "dumb" adapters I have (no electrical contacts) look like they would work with the tab intact.

The tab is soft plastic, and can carefully be removed in numerous ways. I cut a disk of cardboard the same size as the teleconverter's front element, taped it in place with low-residue "blue tape," cushioned the MC-14 with rags, clamped it in my drill press's cross-vice, and then used a 1/16" carbide router bit to remove most of the tab, resulting in a clean removal. I imagine one could just as easily do it with tiny pliers, gently gripping and twisting to break the tab away.

Here is my tab removal job:
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And this is what all the "smart" adapters look like when mounted, with the tab removed:
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You can clearly see here that, if the tab were any taller, you would not be able to attach the adapter. I'd like to claim I used a micrometer to determine how much to take off, but it was strictly an eyeball estimate that worked out more due to luck than skill. :)

EDIT 2019-0216: I forgot to mention that all the 4/3rds adapters I tried (MMF-2, MMF-3, and various cheap Chinese ones) have a bit of extra plastic over the contacts that interferes with full insertion of the MC-14.

I used callipers to find a router bit that was the same size as the protrusion of the MC-14, chucked it into the drill press, padded and clamped the adapter into the cross vice, carefully centred it, then slowly and carefully ground out enough of the extra plastic so the adapter fit the MC-14 perfectly.

I used a convex quarter-round router bit, which shaved the plastic off more gently than using a rabbet (straight-edge) bit.
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This was my second adapter mod, and it came out looking more-or-less "professional." The first one, which I modified using a round-edge wood file by hand, worked fine, but didn't look as quite as good.
 
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Petrochemist

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I'm surprised there aren't more 3rd party teleconverters for mirrorless cameras. Aren't they basically extension tubes with a glass element inside?
They make them for c-mount lenses, so it's certainly possible.
The c-mount model I got does extend into the rear of the lens, but I've seen others that don't. Mine boosts the coverage of a few lenses that were noticeably restricted otherwise :)
 

archaeopteryx

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Aren't they basically extension tubes with a glass element inside?
One could put it that way. There are risks to doing so since, for example, teleconverters retain infinity focus but extension tubes do not. I'd suggest Roger Cicala for an introduction to the effects of following a primary lens with a negative or positive rear conversion lens: Teleconverters 101 and Metabones Magic. Manufacturers---including Olympus and Panasonic---tend not to disclose much to explain why their teleconverters cost more than a goodly proportion of lenses do. But some are more informative: Fuji 1.4x and 2x, Nikon 1.4x and 2x. 7-9 elements in 3-5 groups is probably typical of other TC designs as well. Canon, for example, mentions similar numbers.

I'm unaware of a similarly good quality discussion of extension tubes but, to the extent a lens is image space telecentric, moving it away from the focal plane won't change the size of the image circle. In practice, exit pupil distances are finite and corresponding enlargement with effects analogous to those of teleconverters is to be expected. (Since extension tubes lack elements they don't contribute optical aberrations of their own. However, this is often misunderstood to mean adding extension has no impact in image quality.)
 
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I'm unaware of a similarly good quality discussion of extension tubes
One thing that is often not mentioned is that extension tubes reduce the amount of light getting through. I think it's a full stop for every focal-length of extension. Put a 50mm extension on a 50mm ƒ2 lens, and it becomes a 50mm ƒ2.8 lens.

In the bad old days, you had to take this into account when calculating exposure. (Remember "calculating" exposure? I didn't think so!) But modern automatic exposure systems and electronic viewfinders make the darkening caused by extension nearly invisible, unless you pay attention to the exposure triangle as you change your extension amount.
 

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